Bosch GWH 425 HN User manual

Water heaters for use with Natural and L.P.G.
GWH 425 HN
Flow Modulated with Electronic Ignition
uitable for heating potable water only - Not approved for space heating purposes
(Intended for variable flow applications with steady cold water inlet temperatures)
GWH-425-HN-N
GWH-425-HN-L
6 720 607 026 US (04.11) AL
Warning: If the information in this manual is not
followed exa tly, a fire or explosion may result
ausing property damage, personal injury or death.
Do not store or use gasoline or other flammable
vapor and liquids in the vi inity of this or any other
applian e.
Improper installation, adjustment, alteration,
servi e or maintenan e an ause injury or
property damage. Refer to this manual. For
assistan e or additional information onsult a
qualified installer, servi e agen y or the gas
supplier.
In the Commonwealth of Massa husetts this
produ t must be installed by a li ensed plumber or
gas fitter.
Upon ompletion of the installation, these
instru tions should be handed to the user of the
applian e for future referen e.
What to do if you smell gas
• Close gas valve. Open windows.
• Do not try to light any applian e
• Do not tou h any ele tri al swit h; do not use any
phone in your building
• If you annot rea h your gas supplier, all the fire
department.
• Immediately all your gas supplier from a neighbor’s
phone. Follow the gas supplier’s instru tions.
• Installation and servi e must be performed by a
qualified installer, servi e agen y or the gas supplier.

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WARNING: Improper installation,
adjustment, alteration, service or maintenance can cause
injury or property damage. Refer to this manual. For
assistance or additional information consult a ualified
installer, service agency or the gas supplier.
Upon completion of the installation, these instructions
should be handed to the user of the appliance for future
reference.
In the Commonwealth of Massachusetts this product
must be installed by a licensed plumber or gas fitter.
WARNING
If the information in this manual is not followed exactly, a
fire or explosion may result causing property damage,
personal injury or death.
FOR YOUR SAFETY
Do not store or use gasoline or other flammable,
combustible or corrosive vapors and li uids in the vicinity
of this or any other appliance.
WHAT TO DO IF YOU SMELL GAS
-Do not try to light any appliance.
-Do not touch any electrical switch; do not use any
phone in your building.
-Immediately call your gas supplier from a neighbor’s
phone. Follow the gas supplier’s instructions.
-If you cannot reach your gas supplier, call the fire
department.
-Installation and service must be performed by a
ualified installer, service agency or the gas supplier.
Specifications................................................................ Page 3
Rules fo safe ope ation .............................................. Page 5
Locating the Heate ..................................................... Page 5
Combustion Ai Requi ements ................................... Page 6
Mounting the Heate .................................................... Page 6
Venting the Heate ....................................................... Page 7
Gas Connections ......................................................... Page 9
Gas Piping.................................................................... Page 10
Wate Connections .................................................... Page 11
Safety befo e tu ning on the heate ....................... Page 11
Ope ating inst uctions ............................................... Page 12
Setting wate tempe atu e ........................................Page 12
Maintenance & Se vice ............................................. Page 13
T ouble Shooting ......................................................... Page 13
Elect ical Diag am ...................................................... Page 15
Diag am of 425HN..................................................... Page 17
Inte io components diag am.................................... Page 18
Pa ts List ....................................................................... Page 19
Flowcha t ..................................................................... Page 20
Wa anty ....................................................................... Page 21
TABLE OF CONTENTS

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Principle of Operation:
When a hot wate faucet is opened, the wate flow th ough
the heate causes the gas valve to open. At the same time
the hyd o-gene ato activates the elect onics which sends
a spa k to the pilot. The flame senso confi ms the pilot has
been lighted and allows the fi st two bu ne s to come on.
The flame senso confi ms co ect activation and all bu ne s
come on. The pilot goes off. The heat exchange coils abso b
the heat gene ated by the bu ne s and t ansfe heat to the
wate . When the hot wate faucet is shut off, the gas valve
automatically closes and the bu ne s tu n off. You hot wate
faucet is an ignition key to activate the wate heate , giving
you cont ol ove you hot wate ene gy use. Eve y time you
tu n off you hot wate faucet, the ene gy consumption of
you hot wate etu ns to ze o.
FEATURES
- Elect onic Pilot Ignition d iven by hyd o-gene ato
- High Quality Mate ials fo Long Wo king Life.
- Coppe heating coils fo endless supply of hot wate .
- Bu ne output p opo tional to hot wate flow demand fo
maximum ene gy efficiency.
- Safety flame senso at pilot bu ne .
- Automatic ove heating p otection shut-off senso .
- Flue gas safety device.
- Stainless steel bu ne s with stabilized blue flame.
- Built-in co osion esistant d aft dive te .
- Compact space save : mounts on a wall with two hooks.
- Easily emovable one-piece cove .
- Easy one pe son installation.
- Adjustable wate flow est icto to ensu e that wate flow
demand will not exceed the heating capacity of the heate .
- On/Off Switch to activate system.
- Red LED indicato flashes with low wate p essu e.
- G een LED indicato main bu ne ope ation.
BOSCH is constantly imp oving ou p oducts, the efo e
specifications a e subject to change without p io notice.
425 HN LP and 425 HN NG Specifications
Gas Input max.: 117,000 Btu/h
min.: 28,000 Btu/h
Wate Connection ½” Th ead fitting NPT
H x W x D 29 3/4” x 18 1/4” x 8 3/4"
Vent 5”
Gas Connection 1/2” NPT th ead
Min. Wate P essu e 18 Psi at 4 GPM
Max. Wate P essu e 150 Psi
Shipping Weight 43 LB
Net Weight 40 LB
1.8 GPM at 90° ise
3.7 GPM at 45° ise
Min. Wate Flow 1/2 gal/min
LP GAS Supply P essu e
(befo e 425 HN egulato ) min. 11” W.C.
max. 14” W.C.*
Requi ed LP GAS P essu e at inlet
tap while 425 HN is ope ating: 10.5” W.C.
LP GAS Bu ne Manifold p essu e while 425 HN is
ope ating at maximum input: 9.0” W.C.
Natu al Gas Supply P essu e min.: 7” W.C.
(befo e 425 HN egulato ) max.: 14” W.C.*
Requi ed Natu al Gas P essu e at inlet
tap while 425 HN is ope ating: 5.7” W.C.
Natu al Gas Bu ne Manifold P essu e while 425 HN is
ope ating at maximum input: 4.2” W.C.
* Inlet gas p essu e befo e 425 HN egulato must not exceed
this value. P essu e may need to be adjusted fo high
altitudes, see page 7.
UNPACKING THE 425 HN HEATER
This heate is packed secu ely. The box includes one wate connection fitting, a cont ol knob with colla , a gas p essu e
egulato , a p essu e elief valve, an incandescent pa ticle t ay, two hooks fo hanging the heate , this manual and a
wa anty egist ation ca d. Do not lose this manual, as there is a charge for replacement. Please complete and etu n
the enclosed wa anty egist ation ca d.

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MINIMUM INSTALLATION CLEARANCES FROM COMBUSTIBLE AND NON
COMBUSTIBLE MATERIALS FOR ALCOVE OR CLOSET INSTALLATIONS
FRONT VIEW SIDE VIEW
MODEL 425HN
MODEL 425 HN
TOP (A) 12"
FRONT (B) 4"
BACK 0"
SIDES 4"
FLOOR (C) 12"
VENT DIAMETER 5"

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GENERAL RULES TO FOLLOW
FOR SAFE OPERATION
1. You should follow these inst uctions when you install you
heate . In the United States: The installation must confo m
with local codes o , in the absence of local codes, the
National Fuel Gas Code ANSI Z223.1/NFPA 54.
In Canada: The Installation should confo m with CGA
B149.(1,2) INSTALLATION CODES and /o local installation
codes.
2. Ca efully plan whe e you install the heate . Co ect
combustion ai supply and flue pipe installation a e ve y
impo tant. If not installed co ectly, fatal accidents can be
caused by lack of ai , ca bon monoxide poisoning o fi e.
3. The place whe e you install the heate must have enough
ventilation. The National Fi e Codes do not allow gas fi ed
wate heate installation in bath ooms, bed ooms o any
occupied ooms no mally kept closed. See the section below
on locating the heate .
4. You must vent you heate . See section on VENTING,
Page 6.
5. The appliance must be disconnected f om the gas supply
piping system du ing any p essu e testing at p essu es in
excess of 1/2 Psig (3.5 kPa).
The appliance must be isolated f om the gas supply piping
system by closing its individual manual shutoff valve du ing
any p essu e testing of the gas supply piping system at test
p essu es equal to o mo e than 1/2 Psig (3.5Kpa). The
appliance and its gas connection must be leak tested befo e
placing the appliance in ope ation.
6. Keep wate heate a ea clea and f ee f om combustibles
and flammable liquids. Do not locate the heate ove any
mate ial which might bu n.
7. Correct gas pressure is c itical fo the optimum
ope ation of this heate (see specifications on page 2). Gas
piping must be sized to p ovide the equi ed p essu e at the
maximum output of the heate , while all the othe gas
appliances a e in ope ation. Check with you local gas
supplie , and see the section on connecting the gas supply.
8. Should ove heating occu o the gas supply fail to shut
off, tu n off the gas supply at the manual gas shut off valve
on the gas line.
9. Do not use this appliance if any pa t has been unde wate .
Immediately call a qualified se vice technician to inspect
the appliance and to eplace any pa t of the cont ol system
and any gas cont ol which has been unde wate .
PROPER LOCATION FOR INSTALLING YOUR
HEATER
Ca efully select the location of you new heate . Fo you
safety and fo p ope heate ope ation, you must p ovide an
abundant supply of combustion ai and a p ope venting. Fig. 1 - Water heater drain plug
The heate may still ope ate even when imp ope ly vented.
It will, howeve , be less efficient and could eventually damage
the heate . It could even esult in human sickness o death
due to oxygen dep ivation and ca bon monoxide poisoning.
Follow the guidelines below:
1. Place you heate as close to a vent o chimney as
possible.
2. National building codes equi e that you do not install
this appliance in bath ooms, bed ooms o any occupied
ooms no mally kept closed.
3. Simultaneous ope ation of othe appliances such as
exhaust fans, ventilation systems clothes d ye s, fi eplaces
o wood stoves could c eate a vacuum effect in you home
which could cause dange ous combustion by-p oducts to
spill back into you home athe than venting to the outside
th ough the flue. Confi m that you 425 HN is venting
p ope ly when all these othe appliances a e unning. See
section on venting.
Do not obstruct the flow of combustion and ventilation
air to the appliance. If installed nea a clothes d ye it is
ve y impo tant that the d ye be p ope ly vented. Failu e to
p ope ly vent a d ye could esult in a g adual accumulation
of lint on the wate heate fin coils and bu ne s, leading to a
dange ous condition of vent blockage and poo unsafe
combustion.
4. You hot wate lines should be kept sho t to save ene gy.
It is always best to have hot wate lines insulated.
5. This p oduct is neithe designed no app oved fo outside
installations. This p oduct is not app oved fo manufactu ed
homes (mobile home), ec eational vehicles (RV) o boats.
Refe ence ANSI Z21.10.3.
WARNING: The wate in this wate heate is cold and
always emains cold except fo the times that hot wate is
being used DO NOT INSTALL IN AN AREA WHERE IT
COULD FREEZE.
This heater is neither designed for nor approved for
outside installation.
Drain the heater entirely if freezing temperatures are
anticipated in area where heater is installed by
disconnecting both the inlet and outlet water connections
from the heater (disconnect the outlet flex line from where
it connects to the copper heat exchanger). Additionally
remove the drain plug under the water valve. See Fig. 1.

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WARNING: Flammable materials, gasoline,
pressurized containers, or any other items or articles
that are potentially fire hazards must NOT be placed on
or adjacent to the heater. The appliance area must be
kept free of all combustible materials, gasoline and other
flammable vapors and li uids.
COMBUSTION AIR REQUIREMENTS
The 425 HN wate heate holds cold wate in its coppe
heat exchange and b ass wate valve when not in use.
Because of this, any cold ai that comes down th ough the
unit’s vent pipe is capable of f eezing these components.
This Installation Manual specifies the minimum ve tical vent
pipe and the amount of combustion ai equi ed fo this unit.
When all equi ements a e followed, the unit will ope ate
p ope ly and safely. Howeve , the e may still be a isk of
f eezing due to negative d aft if all the combustion appliances
in the a ea a e not being supplied with a sufficient amount
of make-up ai . A wood stove o fu nace may pull combustible
ai down th ough the 425 HN vent system, leaving the cold
infilt ating ai capable of f eezing the cold wate in the 425
HN heat exchange . Mo e make up ai is the solution. Follow
the inst uctions on venting and checking adequacy of make
up ai . A HVAC specialist should be used to design solutions
fo p oviding mo e make-up ai if necessa y.
Obse ve the following inst uctions conce ning combustion
ai .
Appliances located in unconfined spaces:
a) An unconfined space is one whose volume is g eate
than 50 cubic feet pe 1000 Btu pe hou of the combined
ating of all appliances installed in the space. That would
be 5850 cubic feet fo the 425HN alone.
b) In unconfined spaces in buildings of conventional f ame,
mason y, o metal const uction, infilt ation is no mally
adequate to p ovide ai fo combustion, ventilation, and
dilution of flue gasses.
Appliances located in confined spaces:
The confined space must be p ovided with two pe manent
openings, one commencing within 12 inches of the top and
one commencing within 12 inches of the bottom of the
enclosu e. Each opening must have a minimum f ee a ea of
one squa e inch pe :
- 1000 Btu/h if all ai is taken f om inside the building.
- 2000 Btu/h if all ai is taken f om the outside by ho izontal
ducts.
- 4000 Btu/h if all ai is taken f om the outside by di ect
openings o ve tical ducts.
O the confined space must be p ovided with one pe manent
opening o duct that is within 12 inches of the ceiling of the
enclosu e. This opening must have a minimum f ee a ea of
one squa e inch pe :
- 3000 Btu/h if all ai is taken f om the outside by a di ect
opening o ve tical duct.
Louve s, g ills and sc eens have a blocking effect. If the
effective f ee a ea is not known, inc ease the sizes of you
openings by 75% if you louve s a e wood and by 30% if
you louve s a e metal. Refe to the National Fuel Gas Code
fo complete info mation. In buildings of tight const uction
all ai should be taken f om outside.
CLEARANCES
The 425 HN is design ce tified fo installation on a
combustible wall and fo installation in an alcove o closet
with the minimum clea ances to combustible and non -
combustible const uction listed below
A. Top 12 inches (306 mm)
B. F ont 4 inches (102 mm)
C. Back 0 inches
D. Sides 4 inches (102mm)
E. Bottom 12 inches (306 mm)
Clea ance f om vent is dependent upon the clea ance ating
of the venting mate ial used. Fo example: type B-1 vent is
app oved fo 1 inch clea ance.
Note: Typically, the minimum clea ence to combustible
mate ials should not be less than 6“ fo single wall flue
pipe. Note that this clea ance can be educed if combustible
mate ials a e p otected as pe table VI of the National Fuel
Gas Code o if Type B gas vent is used.
MOUNTING I N STALLATION
The 425HN is design ce tified fo mounting on a wall.
Secu e the two L shaped hooks, which a e p ovided with
heate , to a wall su face. Place them 13 ¼” apa t as shown
in Fig. 2.
Do not install this appliance on a ca peted wall o ove floo
cove ing which is combustible, such as ca pet. The heate
must be mounted on a wall using app op iate ancho ing
mate ials. If wall is a stud wall sheathed with plaste boa d, it
is recommended that support board(s), either 1x4’s or
1/2" (minimum) plywood first be attached across a pair
of studs and then the heater should be attached to the
support boards. See Fig. 2.
Expansion and cont action of piping due to changing wate
tempe atu e in the pipes impa ts movement to the heate
which, if mounted di ectly to a b ittle, f iable boa d, such as
425HN Air Vents
(5 ½ X 5 ½ in. each)
Air Vents
(10 ¾ X 10 ¾ in. each) 425HN

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VENTING
Vent pipe connection.
WARNING: Do not reduce the vent pipe size. See Fig. 4
for high altitude installations
The heate must be vented to the outside following all local
o dinances and specifications fo installing a gas appliance
vent o chimney. The heate must be located as close as
p acticable to a ve tically ising chimney o vent that has a
listed vent cap at its te mination point. The venting system
must be designed and const ucted so as to develop a posi-
tive flow adequate to emove flue gasses to the outdoo s.
Consult the National Fuel Gas Code if the vent will have
elbows or share venting with another natural draft
Roof pitch H (minimum) feet meters
Flat to 6/12 1.0 0.30
6/12 to 7/12 1.25 0.38
Over 7/12 to 8/12 1.5 0.46
Over 8/12 to 9/12 2.0 0.61
Over 9/12 to 10/12 2.5 0.76
Over 10/12 to 11/12 3.25 0.99
Over 11/12 to 12/12 4.0 1.22
Over 12/12 to 14/12 5.0 1.52
Over 14/12 to 16/12 6.0 1.83
Over 16/12 to 18/12 7.0 2.13
Over 18/12 to 20/12 7.5 2.27
Over 20/12 to 21/12 8.0 2.44
GAS VENT TERMINATIONS FOR LISTED VENT CAPS
Listed vent cap
Lowest discha ge
opening
H (minimum) height
f om oof to lowest
discha ge opening
Roof Pitch
IS X/12
12
X
Listed gas vent
Figure A Figure B
Establish a one foot
rise before any elbows
MINIMUM 6 FEET (1.8M)
Listed vent cap
plaste boa d, can cause failu e of mounting.
In ea thquake-p one zones, CEC ecommends that installe s
use a la ge washe and lag sc ew th ough the existing holes
used to hang the heate to affix the uppe thi d of the heate
to the mounting boa d. To affix the lowe thi d of the heate ,
CEC ecommends that two new holes be d illed in the
heate ’s f ame, each one 16 inches below the top two holes,
and that washe s and lag sc ews be used to secu e the
lowe po tion of the heate to a spacing boa d.
Before installing the unit, be certain you have the correct
heater for your type of Gas – Propane or Natural Gas.
Identification labels are found on the shipping box, and on
the rating plate which is located on the right side panel of
the cove . Also, each bu ne o ifice is stamped with a numbe
(79 fo LPG and 120 fo Natu al Gas).
Fig. 2 - Mounting the Heater
WALL STUDS
1” X 4”
SPACE BOARD
SUPPORT
BOARD
13 ¼”
Fig. 3 - Incandescent Particle Tray Illustration
The incandescent pa ticle t ay (shipped loose in the ca ton
with the wate heate ) must be attached at the bottom of
the wate heate f ont cove at the time of installation. Use
the sc ews p ovided. See figu e 3
SCREWS
INCANDESCENT PARTICLE TRAY
appliance. The heater should never be common vented
with a mechanically vented appliance. Single wall vent
pipe is not recommended, if used consult the National
Fuel Gas Code.
Ho izontally venting to a sidewall vent te minato o a
ve tically const ucted vent stack along an outside wall of a
building is not pe missible. A Powe vent (Model AQ-1), with
a p oof-of-d aft safety inte lock device, is equi ed and is
available in o de to sidewall vent. Contact you deale . In
the Commonwealth of Massachusetts: Powervented
applications must utilize proof-of-draft safety interlock
device.
VERTICAL GAS VENT: A 5 inch diamete gas vent const ucted
of double wall Type B gas vent is ecommended. 6 inch is
equi ed in elevations g eate than 2000 feet, see Fig. 4.
Any gas vent section that is g eate than 45 deg ees f om
the ve tical is conside ed ho izontal. Ho izontal sections must
slope upwa ds at least ¼ inch fo eve y foot of its ho izontal
length and be p ope ly suppo ted. Keep the ho izontal section
sho t and avoid too many elbows. The minimum ve tical gas
vent height allowed is 6 feet; ho izontal vent connecto s
and elbows a e not to be conside ed in the total gas vent
height. All gas vent sections must be secu ed to each othe
with sheet metal sc ews and be p ope ly suppo ted.
The gas vent const ucted of double wall Type B gas vent
must te minate above the oof su face with a listed vent
cap at a height that’s in acco dance with Figu e A o B and
thei table, p ovided they a e at least 8 feet (2.4 m) f om a
ve tical wall o simila obst uction. All othe gas vents that
a e not able to te minate within the minimum specified height
allowed must te minate not less than 2 feet (0.6 m) above
the highest point whe e it’s passed th ough the oof and at
least 2 feet (0.6 m) highe than any ve tical wall o simila
obst uction within 10 feet (3.1 m).

86 720 607 026
To check the draft:
Close all doo s and windows to the outside.
Tu n on all appliances that fo ce ai out of the building.
These include all exhaust/ventilation fans, fu naces, clothes
d ye s, wood bu ning stoves, etc.
Open all doo s between the 425 HN and these othe
appliances.
With the cont ol knob set fully clockwise and st ong hot
wate flow ate, un the unit fo at least 10 minutes. The
425 HN is equipped with a Flue Gas Senso ; it’s mounted
on the ight side of the d aft dive te (Flue Gas Senso #34
on page 17). If the main bu ne s shut off du ing this test it
is because the Flue Gas Senso has detected inadequate
venting. This is a se ious health haza d and must be co ected.
Poo venting can esult in soot building up inside the heate ,
ove heating of the heate and f eezing of the heat exchange
in a f eezing envi onment, which is all a esult of negative
ai flow. Additional combustion ai and/o imp oved venting
will be necessa y to co ect this.
WARNING
Note: The bu ne s of an instantaneous “on demand” wate
heate such as the 425 HN a e only on at the time that hot
wate is actually being used, the vent pipe is the efo e cold
except fo the sho t du ations when hot wate is being used,
it is the efo e ve y impo tant that the venting and ai supply
be adequate to p ovide a good positive d aft as soon as the
bu ne s tu n on.
The 425 HN instantaneous wate heate s have built-in d aft
dive te s and a e designed fo indoo installation only. The
MASONRY CHIMNEYS
Gas vent
Vent
connecto
Establish a one foot
rise before any elbows
Listed vent cap
Figure C
MASONRY CHIMNEY: Mason y chimneys shall be built and
installed in acco dance with NFPS 211 o local codes. A
minimum 5” diamete gas vent pipe (metal double wall Type
B), o an app oved clay flue line o a listed chimney lining
system must be used when venting into a natu ally d afting,
inte nal mason y chimney. 6 inch is equi ed in elevations
g eate than 2000 feet, see Fig. 4. Local codes may equi e
the use of both gas vent and an app oved lining system
when venting into a mason y chimney. The Commonwealth
of Massachusetts equi es the use of a listed line . Lining
systems include app oved clay flue lining, a listed chimney
lining system o othe app oved mate ial that will esist
co osion, e osion, softening, o c acking f om exhaust flue
gases at tempe atu es up to 1800 deg ees F. The lining
system must be listed fo use with natu ally d afting, d aft
hood equipped gas appliances. Follow local codes and efe
to NFGC 54 and NFPA 58.
When connecting the water heater to a masonry chimney
the following connector guidelines must be followed for
safe and proper operation: An app oved gas vent connecto
must be attached to the top of the wate heate and ise
ve tically at least 12” befo e ente ing into an app oved gas
vent connecto elbow. Any gas vent section that is g eate
than 45 deg ees f om the ve tical is conside ed ho izontal.
If a ho izontal vent connecto is to be used to connect the
ve tical gas vent connecto on the top of the wate heate
to the mason y chimney, that app oved ho izontal gas vent
connecto must be kept as sho t as possible and must be
sloped upwa ds at least ¼” pe foot of its length. This
connecto must be suppo ted th oughout its ho izontal length.
This ho izontal gas vent connection may be no g eate than
75% of the total ve tical gas flue vent within the chimney.
Also, an app oved thimble o colla must be used when
penet ating a mason y chimney.
A) Existing INTERIOR Masonry Chimney
The metal gas vent pipe should be pe manently mounted
inside the mason y chimney. Double wall Type B gas vent is
ecommended. The mason y chimney may have to be tile
o metal lined befo e the inse tion of the gas vent pipe;
check local codes fo cla ification. The lining mate ial must
be listed fo use only with natu ally d afting, d aft hood
equipped gas appliances. Follow manufactu es inst uctions
fo installation of listed lining mate ial. You may not vent any
othe fuel bu ning appliances into any f ee space emaining
in the chimney. The minimum ve tical gas vent length within
the mason y chimney should be no less than 5 ft (1.5 m);
the vent te minato should extend at least 3 feet (0.9 m)
above whe e the chimney meets the oofline and at least 2
feet (0.6 m) highe than any ve tical wall o simila obst uction
within 10 feet (3.1 m). The top of the gas vent should have
an app oved vent te minato . See Figu e C.
B) Tile Lined INTERIOR Masonry Chimney
The mason y chimney must have an app oved line , ce amic
tile, clay o metal. The mason y chimney must be able to
accommodate the spent fuel gasses; consult a venting
HVAC cont acto fo chimney capacity. A common venting
like-fuel appliance into this lined mason y chimney is
pe missible. The chimney’s diamete must be la ge enough
to adequately d aft the spent fuel gasses. A venting HVAC
cont acto will be equi ed to spec a chimney system. The
minimum ve tical chimney length should be no less than 5
ft (1.5 m); the te mination point should extend at least 3
feet (0.9 m) above whe e the chimney meets the oofline
and at least 2 feet (0.6 m) highe than any ve tical wall o
simila obst uction within 10 feet (3.1 m). See Figu e C.
C) EXTERIOR masonry Chimney
Refe to the National Fuel Gas Code and consult a local
venting HVAC cont acto .

96 720 607 026
GAS CONNECTIONS
NOTE: The 425 HN is supplied with a gas pressure
regulator that must be installed on the heater before
attaching the gas supply line. See fig. 5. Failure to install
the gas regulator as shown in figure 5 will be a violation
of CSA certification of the unit. The regulator supplied
with the heater is preset for the gas shown on the rating
plate to the correct pressure. It is an appliance level
regulator designed for (low inlet) pressure (less than 1/
2 Psig or 15” W.C.)
DO NOT connect to an unregulated or high pressure
propane line or to a high pressure commercial natural
gas line.
GAS LINE SIZING
-It is strongly recommended that the Natural Gas pipe
be Black Iron pipe the entire distance from the outside
meter to the inlet of the 425 HN regulator. 1/2” Black Iron
pipe up to 10 feet, 3/4” Black Iron pipe up to 40 feet and
1” Black Iron pipe up to 150 feet distances. Flex line tubing
is NOT recommended, but if used then oversize it.
-It is strongly recommended that the LP Gas pipe be
semi-rigid copper or Black Iron pipe from the outside
regulator to the inlet of the 425 HN regulator. For semi-
rigid copper piping: 5/8” up to 20 feet and 3/4” up to 60
feet distances. For Black Iron piping: 1/2” up to 45 feet
and 3/4” up to 160 feet distances. Flex line tubing is NOT
recommended, but if used then oversize it.
THESE FIGURES ARE FOR 425 HN SUPPLY ONLY, ALL
OTHER APPLIANCES IN THE BUILDING WILL NEED TO
BE INCLUDED IN THE PIPE SIZING.
National Fuel Gas Code equi es that a sediment t ap (d ip
leg) be installed on gas appliances not so equipped. The
d ip leg must be accessible and not subject to f eezing
conditions. Install in acco dance with the ecommendations
of the se ving gas supplie .
WARNING: The heate must be disconnected f om
the gas supply piping system du ing any p essu e testing of
that system at test p essu es in excess of 0.5 psig.
The wate heate must be isolated f om the gas supply piping
system by closing the manual shutoff valve du ing any
p essu e testing of the gas supply piping system at test
p essu es equal to o mo e than 0.5 psig.
The wate heate , including the p essu e egulato p ovided
with it, must not be ope ated at gas supply p essu es in
excess of 0.5 psig. If ove p essu e has occu ed, such as
th ough imp ope testing of the gas lines o malfunction of
the supply system, the gas valve and egulato must be
checked fo safe ope ation. Make su e that the egulato
vent is p otected against blockage.
When you connections a e made, check fo gas leaks at
all joints (not just the ones you made). Apply some soapy
wate to all gas fittings and gas valve. Soap bubbles are a
sign of a leak.
NOTE: Do not apply soap solution to the pilot o pilot o ifice
a ea. If you have a leak, shut off the gas. Afte ve ifying that
equi ed gaskets a e in place, tighten app op iate fittings to
stop leak. Tu n the gas on and check again with a soapy
solution. Never test for gas leaks using a match or flame.
Fig. 5 - Installation of Gas Pressure Regulator
GAS INLET PIPE
PRESSURE TAP
PRESSUREREGULATOR
(with directional arrow on
reverse side pointing
upward)
The p essu e egulato p ovided with the heate is adjusted
to delive the p ope gas p essu e (as indicated on the ating
plate and in the manual fo altitude up to 2000 feet (660
mete s) above sea level. On appliances being installed above
2000 ft (660 mete s) elevation, the inlet gas p essu e should
be set at installation to the value shown below.
PLASTICVENT CAP
d aft dive te outlet must be connected to an unobst ucted
vent of the same size, o la ge .
In Canada, CAN/CGA-B149 Installation Code fo detailed
equi ements
In U.S.A., ANSI Z223.1 - NFPA 54, National Fuel Gas
Code fo detailed equi ements.
The vent connection fo the 425 HN is 5 inches. Howeve ,
fo installations at high altitude (ove 2000 feet above sea
level) a six inch vent is equi ed. A 5" by 6" vent adapto
must be affixed to the flue colla of the 425 HN with at
least two sc ews. See fig. 4.
WARNING: Failu e TO INCREASE THE VENT SIZE
TO SIX INCHES AND/o assu e that the manifold p essu e
is set to p ope value listed fo applications at high altitudes
will cause unsafe venting, asphyxiation, and will void CSA
Ce tification.
Altitude Natural Gas Liquid Propane
inches W.C: inches W.C:
0' - 2.000 ft 5.7" 10.5"
2.000 ft - 4.500 ft 4.6" 8.4"
Above 4.500 ft consult your local gas provider.
MAXIMUM INLET GAS FLOW PRESSURE SETTING
Fig. 4 - Vent Adaptor for High Altitude Installations
5” X 6” ADAPTOR
USED ON HIGH ALTITUDE
NOTE: The gas pressures specified below refer to
pressures taken at the pressure tap on the gas inlet pipe
just above the regulator (See Fig 5). These readings
should be taken while the heater is operating at full input
— i.e. maximum water flow with the temperature dial
selector turned all the way clockwise.

10 6 720 607 026
Nominal
Iron Pipe Black Iron Pipe
Size, Length of Pipe, Feet
Inches 10 20 30 40 50 60 70 80 90 100 125 150
1/2 275 189 152 129 114 103 96 89 83 78 69 63
3/4 567 693 315 267 237 217 196 185 173 162 146 132
1 107 732 590 504 448 409 378 346 322 307 275 252
1 1/4 220 149 121 103 913 834 771 724 677 630 567 511
1 1/2 330 229 185 155 141 127 118 108 102 976 866 787
2 622 433 346 299 264 239 220 204 192 1811 1606 1496
Copper
Outside Length of Tubing, Feet
Diameter,
Inch 10 20 30 40 50 60 70 80 90 100
3/8 39 26 21 19 _ _____
1/2 9262 5041 37 3531292726
5/8 199 131 107 90 79 72 67 62 59 55
3/4 329 216 181 145 131 121 112 104 95 90
7/8 501 346 277 233 198 187 164 155 146 138
GAS PIPING, CONNECTIONS and Gas Regulator
Befo e connecting the gas supply, check the ating plate on the ight side of the f ont cove to be su e that the heate is
ated fo the same gas to which it will be connected.
In the United States: The installation must confo m with local codes o , in the absence of local codes, the National Fuel
Gas Code ANSI Z223.1/NFPA 54.
In Canada: The Installation should confo m with CGA B149 INSTALLATION CODES and /o local installation codes.
FOR NATURAL GAS
Maximum Capacity of pipe in Cubic Feet of Gas pe Hou
fo Gas P essu e of 0.5 Psig o less and a P essu e d op of
0.3 in Wate Column.
(Based on a 0.60 Specific G avity Gas) Btu numbe s given
in thousands.
GAS LINE SIZING
The gas supply piping should be sized acco ding to the Applicable Plumbing Code fo a maximum d aw of 117,000 BTUH. Fi st dete mine
the effective length of the gas supply line by measu ing the actual length of piping, and then adding 5 ft. fo eve y elbow o “T” to the actual
length. Use the cha ts below to dete mine the pipe diamete necessa y to accommodate the BTU demand of the unit. If the e a e mo e
gas d awing appliances on the line, size acco ding to the maximum amount of BTU demand.
Follow boxed numbe s fo piping just one 425 HN
(example: 3/4” B.I. Natu al Gas pipe fo 30 ft. will handle
152,000 btu’s). Fo multiple appliances combine the total
btu input load and then efe to applicable cha t below.
FOR LP GAS
Maximum Capacity of Pipe in Thousands of
BTU pe Hou of Undiluted Pet oleum Gases
(at 11 inches Wate Column Inlet P essu e)
(Based on a P essu e D op of 0.5 Inch Wate
Column).
Maximum Capacity of Semi-Rigid
Tubing in Thousands of BTU pe Hou of
Undiluted Liquified Pet oleum Gases (at 11
inches Wate Column Inlet P essu e)
(Based on a P essu e D op of 0.5 Inch Wate
Column)
* Sou ce National Fuel Gas Code NFPA 54, ANSI Z223.1.
Nominal
Iron Length of Black Iron Pipe, Feet
Pipe Internal
Size, Diameter
inches inches 10 20 30 40 50 60 70 80 90 100 125 150 175 200
1/4 0.364 32 22 18 15 14 12 11 11 10 9 8 8 7 6
3/8 0.493 72 49 40 34 30 27 25 23 22 21 18 17 15 14
1/2 0.622 132 92 73 63 56 50 46 43 40 38 34 31 28 26
3/4 0.824 278 190 152 130 115 105 96 90 84 79 72 64 59 55
1 1.049 520 350 285 245 215 195 180 170 160 150 130 120 110 100
1 1/4 1.380 1050 730 590 500 440 400 370 350 320 305 275 250 225 210
1 1/2 1.610 1600 1100 890 760 670 610 560 530 490 460 410 380 350 320
2 2.067 3050 2100 1650 1450 1270 1150 1050 990 930 870 780 710 650 610
2 1/2 2.469 4800 3300 2700 2300 2000 1850 1700 1600 1500 1400 1250 1130 1050 980
3 3.068 8500 5900 4700 4100 3600 3250 3000 2800 2600 2500 2200 2000 1850 1700
4 4.026 17,500 12,000 9,700 8,300 7,400 6,80 6,200 5,800 5,400 5,100 4,500 4,100 3,800 3500

116 720 607 026
OPERATING INSTRUCTIONS
Befo e p oceeding with the ope ation of the heate make
su e that the system is filled with wate .
Open the cold wate inlet supply to the heate fully.
Open a hot wate faucet to pe mit the wate to fill the heate
and the piping and to eliminate the ai t apped in the system
Close the hot wate faucet afte the wate flows f eely and
all the ai has escaped f om the system. The wate heate is
now eady to ope ate.
FOR YOUR SAFETY READ BEFORE
OPERATING YOU R HOT WATER HEATER
Warning: If you do not follow these instructions exactly,
a fire or explosion may result causing property damage,
personal injury or loss of life.
A. This appliance is equipped with elect onic ignition fo
lighting the pilot and main bu ne s. When tu ning the heate
on, follow these inst uctions exactly.
B. Befo e ope ating the unit, set the On/Off switch to the
On ( ) position. The On/Off switch is located behind
the flip-down cove plate on the f ont panel st ip. Smell all
a ound the appliance a ea fo gas. Be su e to smell next to
the floo because some gas is heavie than ai and will settle
on the floo .
WHAT TO DO IF YOU SMELL GAS
- Do not t y to light any appliance.
- Do not touch any elect ic switch; do not use any phone
in you building
- Immediately call you gas supplie f om a neighbo ’s
phone. Follow the gas supplie ’s inst uctions.
- If you cannot each you gas supplie , call the fi e
depa tment.
C. Use only you hand to push in the on/off cont ol button.
Neve use tools. Follow these inst uctions exactly. If cont ol
button is jammed, close the gas supply and call a qualified
se vice technician. Attempted fo ceful epai may esult in a
fi e o explosion.
D. Do not use this appliance if any pa t has been unde
wate . Immediately call a qualified se vice technician to
inspect the appliance and to eplace any pa t of the cont ol
system and any gas cont ol which has been unde wate .
WATER CONNECTIONS
Install the heate cent ally in the building if possible and
make hot wate piping uns as sho t as possible When facing
the heate , the cold wate inlet will be on the ight and the
hot wate outlet on the left.
Although wate piping th oughout the building may be othe
than coppe , coppe o galvanized piping should be used
when connecting to the heate s ½” male NPT flex connecto s
(follow local codes if mo e st ingent). Plastics o othe PEX
type plumbing line mate ials a e not suitable fo connecting
di ectly to the wate heate . Keep wate inlet pipe to no less
than ½” (19.05mm) diamete to allow the full flow capacity.
If the cold and hot connections to the heate a e eve sed,
the heate will not function.
The 425 HN is p ovided with one flexible type connecto
that must be connected to the cold inlet fitting of the wate
valve as shown in Fig. 6. The union end of the flexible
connecto should be attached to the ea inlet po t of the
wate valve with the supplied washe gasket. No pipe dope
o th ead tape is to be used at this joint. The ½” flexible hot
wate outlet line is supplied attached to the heate .
Be ce tain the e a e no loose pa ticles o di t in the piping.
Blow out o flush the lines befo e connecting to the wate
heate . Full po t valves should be installed on both the cold
wate supply and hot wate outlet lines to facilitate se vicing
the heate (see Fig. 7). Fo installation on a p ivate well
system with the use of a p essu e tank, the lowest p essu e
ange setting ecommended is 30-50 psi (2.07-3.45 ba ).
Fig. 6 - Water valve - top view
Connecting the pressure relief valve (PRV)
A listed p essu e elief valve supplied with the heate must
be installed at the time of installation. No valve is to be
placed between the PRV and the heate . No educing
coupling o othe est iction may be installed in the discha ge
line.
The discha ge line must be installed such that it allows
complete d ainage of both the PRV and the line.
The location of the PRV must be eadily accessible fo
se vicing o eplacement, and be mounted as close to the
wate heate as possible. See Fig 7. To install the PRV, a
suitable fitting connected to an extension on a “T” fitting
can be sweated to the hot wate line.
Suppo t all piping.
Fig. 7 - Pressure Relief Valve
Pipe to
app op iate
discha ge
Hot Cold

12 6 720 607 026
The position you select on the tempe atu e adjustment knob
will depend on the tempe atu e of the incoming wate (50°F
is ave age in the U.S.). In wa m weathe egions whe e the
incoming cold wate is gene ally wa me , o du ing the hot
weathe months in some othe a eas a midway setting on
the tempe atu e adjustment knob would p oduce a
tempe atu e ise of about 70°F, giving an output of
app oximately 120°F. At this setting, if it is still necessa y,
one could mix a small amount of cold wate in a showe and
have a comfo table showe at about 3 gallons/minute. Du ing
the colde months, o in cold climate a eas, it might be
necessa y to set the cont ol to the maximum position, all
the way clockwise to the ight.
If you supply the 4 5 HN, with preheated water (i.e.) in a
solar installation the unit could overheat. It will then shut off
on safety when the outlet tempe atu e eaches 194°F. Fo
this type of application, pu chase a sola model 125B LPS
o 125B NGS.
If the inlet wate tempe atu e to the wate heate is ve y
wa m the heate can p oduce tempe atu es that may be
found to be too hot. A tempe atu e balance showe valve
can automatically mix in cold wate to educe such hot wate
tempe atu e, in the event of any tempe atu e instability with
the use of a tempe atu e balance showe valve, efe to
showe valve manufactu es inst uctions fo inte nal
adjustment setting. Adjustments should be made to the
hottest setting in the showe valve. Additionally the
tempe atu e cont ol of the heate can be adjusted to p oduce
a mo e comfo table hot wate tempe atu e.
Fig. 8 - Temperature Adjustment Knob
LIGHTING AND OPERATING INSTRUCTIONS
1. STOP! Read the safety info mation above on this label.
2. The gas valve must be shut off by putting the ON/OFF
switch to position “ ”. Wait five (5) minutes to clea
out any gas. If you smell gas, STOP! Follow “B” in the
safety info mation above. If you do not smell gas, go to
the next step.
3. This wate heate is equipped with a safety pilot bu ne
and an automatic ignition cont ol system.
4. Set the ON/OFF switch (flip down cove plate on f ont
panel st ip) to the “ ” position. In this position, the
wate heate is eady to use.
5. If the ed LED indicato light is flashing, this is a wa ning
that the wate p essu e is not enough to ignite the main
bu ne .
6. Tu n the hot wate faucet on to the minimum flow ate
equi ed to activate the heate . The automatic ignition
system fi st ignites the safety pilot bu ne which then
ignites the main bu ne in about 4 seconds.
7. The g een LED indicato is on when the main bu ne is
on.
8. The pilot flame will extinguish 10-30 seconds afte the
bu ne s come on. The bu ne s will emain on until the hot
wate tap is tu ned off.
NOTE: On a fi st time initial installation, existence of ai in
the gas supply line and in the wate line may cause some
ignition delay. In that case, epeatedly open and close the
hot wate tap in o de to esta t the ignition p ocess until all
the ai has been pu ged.
TO TURN OFF GAS TO APPLIANCE
Tu n off the manual leve on the gas supply line to the heate
and set the ON/OFF switch to the OFF ( ) position.
See Fig 9.
SETTING THE WATER TEMPERATURE
The 425 HN LP and 425 HN NG wate heate s have a gas
cont ol that modulates bu ne input in esponse to flow. Its
pu pose is to ensu e that the hot wate tempe atu e will
emain steady, although the wate flow demand might va y
(down to 1/2 gal/minute). To adjust the tempe atu e on you
425 HN, tu n on a hot wate faucet to its maximum flow. At
the wate heate , tu n the la ge knob located beneath the
main gas cont ol on the f ont of the heate all the way to the
ight (clockwise). See Fig 8. This will p oduce a tempe atu e
ise of app oximately 90°F at flow ates between 0.5 and
2.0 gallons pe minute. Given that ave age incoming wate
tempe atu es a e 50°F, this heate will p oduce
app oximately 140°F wate at these flow ates. Tu ning the
dial all the way to the left (counte clockwise) will p oduce a
tempe atu e ise of app oximately 45°F fo flow ates between
1.1 and 3.7 gallons pe minute. It is safest to select a
comfo table tempe atu e and not have to mix in cold wate . Temperature Adjustment Knob
To start and to shut down
the appliance press the
button “ ”.
The switch is located
behind the flip down cover
plate on the front panel
strip.
G661_004
Decrease temperature
and increase flow
Increases temperature
and decreases flow
Fig. 9 - Principles of Operation
- on
- off
Green light on = main
burner on.
Green light off = main
burner off.
If the red light is flashing,
that means the water flow
is not enough to ignite
the burner.

136 720 607 026
Fig. 10 - Characteristic Pilot Flame
(Pilot assembly shown without pilot hood)
MAINTENANCE AND SERVICE
REVIEW MAINTENANCE TABLE ON THE BACK OF THIS
MANUAL.
App oximately once a yea , the 425 HN should be checked
and cleaned. To emove the f ont cove , fi st emove the
incandescent pa ticle t ay, then pull off the tempe atu e
adjustment knob and unsc ew and emove the plastic colla .
Pull main cove out towa d you and lift up and out. THE
FOLLOWING OPERATIONS SHOULD BE PERFORMED
BY A QUALIFIED SERVICE PERSON:
Vent System: Should be checked annually. Clean and epai
as needed.
Water Valve (Pa t # 8707002649): The wate valve on this
heate should be se viced pe iodically. Lub icate component
36 on page 18 with a small amount of silicon, faucet o
lithium g ease eve y two yea s to keep its o ings f esh and
push od sliding smoothly. Eve y 3-5 yea s eplace component
36 on page 18. The f equency will depend on the mine al
content of the wate and conditions of use o wheneve
signs of co osion appea at the gas and wate valve joint.
Pe iodically check that the wate inlet filte (#27 on page
18) is clean as well. NOTE: If wate valve is emoved, be
su e to also inspect the o ing seal on the end of the ight
hand side wate pipe befo e e-installing. Lub icating the o
ing with lithium o faucet and valve g ease is ecommended.
Pilot Flame: The pilot flame should bu n with a clean, sha p,
blue flame and should esemble the diag am in Fig. 10. If
the flame is soft and yellow, the pilot bu ne o ifice may
need to be cleaned o eplaced. The pilot flame should be
app oximately 2 inches long, extending past the flame senso .
If the flame is too small, it will not each the flame senso
and the bu ne s will not come on.
Main Burner Flames: The main bu ne flames should be
blue, with a mo e intense blue cone in the cente co e. Yellow
flames could be a sign of w ong size gas o ifices o di ty
bu ne s, o a blockage on the heat exchange s fins. If some
bu ne s have yellow flames while othe s have good flames,
it is likely that dust, lint o spide webs have pa tially clogged
the bu ne ventu is. To clean the bu ne s, contact a gas
se vice pe son.
Mineral Scale Build-up: The 425 HN heate , when ope ated
at lowe tempe atu e settings, does not accumulate mine al
build-up. If howeve , the heate is used at the highe
tempe atu e settings and the wate has a high mine al
content, pe iodic descaling may be necessa y. The heating
coils should be flushed with a descaling solution. The
descaling is done only to the heat exchanger. The
chemicals will damage the hyd o-gene ato and othe pa ts.
Consult you se vice pe son.
TROUBLE SHOOTING
Introduction
The 4 5 HN burners are activated by a water flow valve.
Numerous water related problems can cause this water valve
to malfunction such as: Insufficient water flow volume to
activate the burners at its minimum flow requirement; Dirt
in the water flow valve causing it to malfunction; Sediment
build-up in faucet aerators, or shower heads; Uneven
pressures between cold and hot. (with single lever faucets)
Plumbing cross overs. These water flow related problems
can cause the heater to deliver less than its full output, or
to fail to ignite or to shut down completely.
To clean the pilot burner and/ or the pilot orifice : Tu n
off the gas to the unit. Set the On/Off switch to Off (position
). Remove the cove of the heate . To do so, pull off
the tempe atu e adjustment knob and unsc ew and emove
the plastic colla . Pull main cove out towa d you and lift up
and out. The pilot o ifice should be cleaned o eplaced. Do
not enlarge the orifice.
Do not use any wi e o sha p object to clean o ifices. Natu al
gas o ifices a e la ge enough that you can usually clean
them by blowing th ough them. LP o ifices, on the othe
hand, a e too small to clean and should be eplaced.
To access the pilot o ifice, emove 2 sc ews holding pilot
assembly in place. Then loosen comp ession fittings to expose
pilot o ifice.
3mm
Correct gap between pilot
burner tip and electrode tip
PiezoElectrode
Flame sensor
Piezo
Fig. 11 - Pilot burner with pilot orifice
(Pilot assembly shown without pilot hood)
PILOTORIFICE

14 6 720 607 026
P oblems a e stated in uppe case, bold face. Most common
causes fo the p oblems follow in o de of likelihood. The
suggested solutions equi e that the cove be taken off. To
do this, emove incandescent pa ticle t ay, pull off the
tempe atu e adjustment knob and unsc ew and emove the
plastic colla . Pull main cove out towa d you and lift up and
out.
NO SPARK AT THE PILOT
1. On/Off switch is not ”On” (Position ).
The On/Off switch is located behind the flipdown cove
plate on the f ont panel st ip. See fig. 9.
2. Cross over in water lines
To confi m the e is no c oss-ove in the plumbing, shut off
the cold wate supply to the 425 HN and open all hot wate
faucets. The e should not be any wate flowing at any faucets.
Wate unning is a sign of a plumbing c oss-ove . Consult
you se vice pe son.
3. Water flow is not sufficient to activate heater.
Wate flow ate at faucet is below minimum flow needed to
activate heate . When tempe atu e adjustment knob is tu ned
all the way clockwise, the model 425 HN equi es 1/2 gallon
pe minute flow to activate the heate . As a efe ence, this
is a flow which would fill a qua t ja in 30 seconds. If the
tempe atu e adjustment knob is tu ned fully
counte clockwise, a flow ate of 1.1 gallons/minute is equi ed
to activate the heate .
The ed LED light (located behind the cove plate on the
f ont panel) flashes when the wate flow is not enough.
4. Water inlet filter is clogged
Wate flow is est icted, p eventing needed flow to activate
heate . Clean wate inlet filte sc een.
5. Cold incoming water connection made to wrong side
of heater
Make su e cold wate inlet connection is on the ight side of
heate when you a e facing heate .
6. Water valve parts may be dirty or components damaged
Fi st check that ventu i is f ee of di t pa ticles. Wate valve
and component pa ts must be totally f ee of di t. In ha d
wate a eas, mine al deposits can eventually (3-5 yea s in
ha d wate a eas) co ode the wate valve pa ts to a point
whe e they will need eplacing. Any sign of moisture or
corrosion at the joint of the water valve and the gas valve is
a sign that the water valve components need to be serviced
immediately.
7. Poor circuit connections at the ECO (overheat sensor)
or the flue gas safety device
Ignition spa ke will not ope ate if the elect ical ci cuit is
inte upted. Check that the connections to the ECO and
Flue Gas Safety Device a e secu e and tighten if necessa y.
If the 4 5 HN is Powervented, check spill switch reset
SPARK APPEARS AT PILOT WHEN HOT WATER TAP
TURNED ON, BUT PILOT AND BURNERS WILL NOT IGNITE
1. Air in the Gas Line
No mally this is a p oblem at time of initial installation, afte
the pipes have been wo ked on, o afte a gas supply line
has been allowed to empty. Bleed the ai t apped in the gas
line by tu ning the hot wate faucet on and off until the ai
has been clea ed f om the line.
2. No gas to the 425 HN
A. Gas cock on gas line may not be open
B. Gas egulato may be shut o damaged. Replace o unlock
the egulato . Note: The egulato fu nished with the heate
is exclusively designed fo low gas p essu e. Excessive
p essu e will lock it up. Locking usually happens when using
p opane gas if the gas p essu e between the gas tank and
the wate heate ’s gas egulato has not been educed. See
Page 2 fo ecommended co ect gas p essu e. To unlock a
egulato , consult you se vice pe son.
3. Pilot orifice or gas valve is dirty
Clogging of the pilot bu ne can be caused by dust in the
ai o di t in the gas. The pilot o ifice o gas filte may need
to be cleaned o eplaced.
4. Pilot electrovalve switch may need to be replaced (See
Fig 13).
Measu e milivoltage at pilot elect ovalve (see Fig. 13).
5. Electronic Control Box may need to be replaced (See
Fig 13).
Check wi e connections and test ope ation of elect onic
cont ol box (see Fig. 13).

156 720 607 026
Fig. 13 - Electrical wiring diagram
5. Flue gas safety device tripped
Ventilate the oom and ope ate the appliance 10 minutes
late . P ovide mo e combustion ai to the oom and/o inspect
and co ect venting system. Afte wa ds, if it happens again,
contact you se vice pe son.
WATER IS TOO HOT
1.Temperature Selection too high
Tu n the tempe atu e adjustment knob counte -clockwise
(to the left) to lowe the maximum wate tempe atu e.
Note: This will increase the activation flow rate.
2. Inlet water temperature is too hot (as with solar
preheated water or high ground water temperatures)
Dec ease gas p essu e. Note: Only the model 125B LPS o
125B NGS should be used if the inlet wate is p eheated.
3. Ground water temperature is very warm
Reduce inlet gas supply to heate when inlet wate
tempe atu e is ve y wa m (60-70 F). See also HOT WATER
TEMPERATURE FLUCTUATES / UNIT DEACTIVATED.
WATER IS NOT HOT ENOUGH
1. Temperature selection too low.
Change the setting. Tu n the tempe atu e adjustment knob
clockwise (to the ight).
Note: This will decrease the activation flow rate.
2. Btu input is too low due to insufficient gas pressure
It is extremely important for a tankless instantaneous
water heater to have the right size gas line to obtain
the correct gas pressure
See specifications on page 2. Unlike sto age tank wate
heate s, the bu ne s of a tankless wate heate must be ve y
powe ful to heat wate instantaneously since they do this
only at the time hot wate is actually being used. It is the efo e
impe ative that the gas p essu e equi ement be met exactly.
Insufficient gas p essu e will di ectly affect the wate
tempe atu e at the time of usage. See page 2 fo co ect
gas p essu e settings and fig. 5 fo locations whe e gas
p essu es a e taken.
3. Btu input is too low due to insufficient gas supply
Make su e you main gas line is fully opened. If using LP
gas, be su e that the size of the p opane tank is adequate to
supply the equi ed gas p essu e.
4. Cold water is mixing with the hot water between the
425 HN and the outlet
Compa e wate tempe atu e at outlet of the 425 HN (hold
the 425 HN outlet pipe with you hand) and at the tap. If
these two a e ve y diffe ent, check fo mixing valve o
plumbing c ossove (see “NO SPARK AT THE PILOT”
pa ag aph #4).
5. Parts in water valve are dirty or damaged, which will
prevent the gas valve from being fully opened.
Wate valve maintenance needs to be pe fo med, see #6
unde NO SPARK AT THE PILOT
PILOT LIGHTS BUT BURNERS WILL NOT COME ON
1. Confirm water valve assembly is working properly.
See Wate Valve unde MAINTENANCE AND SERVICE.
2. Confirm the flame sensor is in its proper position and
clean, lightly wipe clean if dirty. See Fig. 10
3. Confirm the burner electrovalve switch is functioning
properly.
Measu e milivoltage at bu ne elect ovalve (see Fig. 13).
4. The Electronic Control Box (Fig. 13) may need to be
replaced.
Check wi e connections and test ope ation of elect onic
cont ol box (see Fig. 13).
8 elect onic cont ol box
9 LED failu e indicato
10 LED indicato main
bu ne ope ation
11 on/off switch
12 hyd o-gene ato
13 flue gas safety device
1 bu ne elect ovalve
2 gas cont ol valve
3 pilot elect ovalve
4 flame senso
5 pilot elect ode
6 ove heat senso
7 tempe atu e limite
MAIN BURNERS GO OUT DURING HOT WATER USE
1. Flow rate diminished below activation rate
Inc ease hot wate flow ate.
2. Unbalanced pressure in the water lines
The added est iction caused by the 425 HN in the hot
wate system can esult in uneven p essu es between the
cold and the hot. In such cases when mixing cold wate at
the tap, the lowe hot wate p essu e may be ove powe ed
by a much highe cold wate p essu e, which may cause the
425 HN bu ne s to shut down. Make su e faucet ae ato s
and showe heads a e f ee of mine als and di t. Do not add
any flow est icto to the showe head.
3. Minimum inlet pressure on well is inade uate
Check the inlet wate p essu e. On a p ivate well, aise the
minimum p essu e setting to 30 psi. Confi m that the
p essu e tank is not wate logged.
4. ECO (overheat sensor) tripped due to overheating
e-ignite the appliance 10 minutes late . If it happens again,
contact you se vice pe son.

16 6 720 607 026
HOT WATER TEMPERATURE FLUCTUATES / UNIT
DEACTIVATED
1. Unbalanced pressure in waterlines
The added est iction caused by the 425 HN in the hot
wate system can esult in uneven p essu es between the
cold and the hot. In such cases when mixing cold wate at
the tap, the lowe hot wate p essu e may be ove powe ed
by a much highe cold wate p essu e, which may cause the
425 HN bu ne s to shut down. Make su e faucet ae ato s
o showe heads a e f ee of mine als. Do not add any flow
restrictor to the shower head.
2. Temperature balance valves
If the inlet wate tempe atu e to the wate heate is ve y
wa m the heate can p oduce tempe atu es that may be
found to be too hot. A tempe atu e balance showe valve
can automatically mix in cold wate to educe such hot wate
tempe atu e, in the event of any tempe atu e instability with
the use of a tempe atu e balance showe valve, efe to
showe valve manufactu es inst uctions fo inte nal
adjustment setting. Adjustments should be made to the
hottest setting in the showe valve. Additionally the
tempe atu e cont ol of the heate can be adjusted to p oduce
a mo e comfo table hot wate tempe atu e. See SETTING
THE WATER TEMPERATURE section.
3. Cold water is mixing with the hot water between the
425 HN and the outlet
See #4 unde “NO SPARK AT THE PILOT”.
4. Inlet water pressure is erratic due to inade uate supply
water pressure or saturated pressure tank on well
system
Check the inlet wate p essu e. Fo installation on a p ivate
well system with the use of a p essu e tank, the lowest
p essu e ange setting ecommended is 30-50 psi (2.07 -
3.45 ba ). Confi m that the p essu e tank is not wate logged.
5. Gas pressure is too low
See page 2 fo co ect specifications.

176 720 607 026
Fig. 14 - Diagram of 425 HN
1. Heat exchange
2. Pilot assembly
3. Bu ne manifold gas
p essu e test nipple
4. Main gas bu ne
5. Pilot gas tubing
6. LED failu e indicato
7. on/off switch
8. LED ope ation indicato
9. Wate valve
10. Tempe atu e adjustment selecto
11. Gas inlet gas p essu e test nipple
12. Hose
3
1
4
10
11
2
9
8
6
7
5
12
6 720 606 804-10.1AL

18 6 720 607 026
Fig. 15 - INTERIOR COMPONENTS DIAGRAM AND PARTS LIST

196 720 607 026
Fig. 15
INTERIOR COMPONENTS DIAGRAM
AND PARTS LIST 425 HN
1 Cover 8 705 421 752
2 Temperature adjustment knob 8 702 000 111
3 Temperature adjustment collar 8 700 403 008
4 Draft diverter 8 705 505 354
5 Heat exchanger 8 705 406 235
7 Main burner 8 708 120 298 NG
7 Main burner 8 708 120 296 LP
8 Burner assembly washer 8 710 103 060
9 Throttle disc 8 700 100 174 LP
10 Pilot burner 8 708 105 491 LP
10 Pilot burner 8 718 105 051 NG
11 Pilot electrode 8 708 107 006
12 Pilot orifice 8 708 200 069 NG
12 Pilot orifice 8 708 200 312 LP
13 Pilot tube 8 700 707 370
14 Washer pilot tube 8 700 103 173
15 Gas valve 8 707 011 917 NG
15 Gas valve 8 707 011 945 LP
16 Diaphragm switch 8 708 504 021 NG
16 Diaphragm switch 8 708 504 049 LP
17 Pilot electrovalve 8 708 501 249
18 Burner electrovalve 8 708 501 250
19 O-ring 8 700 205 023
20 Hydro-generator 8 707 406 043
21 Cold water pipe 8 700 705 947
22 Outlet flex hose 8 700 703 139
23 Washer 8 700 103 658
24 Water valve 8 707 002 649
25 Water valve cover 8 705 500 101
26 Diaphragm 8 700 503 053
27 Water inlet filter 8 700 507 001
28 Water valve selector screw 8 708 500 289
29 Volumetric water governor 8 705 705 009
30 Venturi 8 708 205 279
31 Electronic control box 8 707 207 085
32 Flame sensor cable 8 704 404 040
33 Electrode cable 8 704 404 039
34 Flue Gas Sensor 8 707 206 345
35 Set of cables 8 704 404 038
36 Pushrod 8 700 306 114
37 O-ring 8 700 205 120
38 O-ring 8 700 205 119
39 Water connection gasket 8 710 103 043
40 Water elbow fitting 8 700 703 114
41 Overheat sensor (ECO) 8 707 206 040
42 Connecting pipe hot 8 700 715 178
43 Washer 8 710 103 045
44 Flame sensor 8 708 107 007
45 Water valve set screw 8 703 401 051

20 6 720 607 026
Fig. 16 - Flow chart of 425 HN