Bosch GWH-425-HN-N User manual

Water heaters for use with Natural and L.P.G.
GWH 425 HN
Flow Modulated with Hydro-generated Ignition
Suitable 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
6 720 608 036 US (2007.05) AL
Warning: If the information in this manual is not
followed exactly, a fire or explosion may result
causing property damage, personal injury or death.
Do not store or use gasoline or other flammable
vapor and liquids in the vicinity of this or any other
appliance.
Improper installation, adjustment, alteration,
service or maintenance can cause injury or
property damage. Refer to this manual. For
assistance or additional information consult a
qualified installer, service agency or the gas
supplier.
In the Commonwealth of Massachusetts this
product must be installed by a licensed plumber or
gas fitter.
Upon completion of the installation, these
instructions should be handed to the user of the
appliance for future reference.
What to do if you smell gas
• Close gas valve. Open windows.
• 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
qualified installer, service agency or the gas supplier.

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2
Index
Index
1Warning 2
2 Appliance details 3
2.1 GWH 425 HN specifications (Technical data) 3
2.2 Unpacking the GWH 425 HN heater 4
2.3 General rules to follow for safe operation 4
2.4 Dimensions and installation clearances 5
3 Installation instructions 6
3.1 Introduction 6
3.2 Proper location for installing your heater 6
3.3 Heater placement and clearances 6
3.4 Mounting Heater 7
3.5 Combustion air requirements 8
3.6 Venting 8
3.7 Gas piping & connections 11
3.8 Measuring gas pressure 13
3.9 Water connections 13
3.10 Recirculation application 14
4 Operation instructions 14
4.1 For your safety read before operating your water heater 14
4.2 Lighting instructions 15
4.3 To turn off appliance 15
4.4 Setting the water temperature 16
4.5 Power adjustment 16
4.6 Purge the appliance 16
5 Maintenance and service 17
5.1 Maintenance intervals 17
5.2 Water valve 17
5.3 Pilot 17
5.4 Main burners 18
5.5 Vent assembly 18
5.6 Mineral scale build-up 18
6 Troubleshooting 19
6.1 Introduction 19
6.2 No spark at pilot with water flow 19
6.3 Spark appears when hot water is turned on, but pilot and
burners will not ignite 19
6.4 Pilot light, but burners will not come on. 20
6.5 Main burners go out during hot water use 20
6.6 Hot water temperature fluctuates at tap 21
6.7 Water is too hot 21
6.8 Water is not hot enough 22
6.9 Burners ignite without water flow 22
6.10 Low hot water pressure 22
6.11 Noise when heater is running 22
6.12 Burners do not burn cleanly, yellow
flames when operating 22
7 Protecting the environment 23
8 Interior components and diagram parts list 24
8.1 Interior components 24
8.2 Components diagram 25
8.3 Parts list 26
9 Fifteen Year Limited Warranty 27
1 Warning
Fig. 1
Warning: The heater must be isolated
from the gas supply piping system
during any pressure testing of that
system at test pressures equal to or
more than 0.5 psig.
Caution: Any changes or
modifications not expressly approved
by the party responsible for compliance
could void the user’s authority to
operate the equipment.

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Appliance details
3
2 Appliance details
2.1 GWH 425 HN specifications
(Technical data)
Approved in US/Canada
Capacity
GWH 425 HN: 4.36 GPM
Maximum output
GWH 425 HN: 91 525 Btu/hr
Maximum input
GWH 425 HN: 117 000 Btu/hr
Efficiency in %
Recovery efficiency 80%
Min. Output
GWH 425 HN: 23 906 Btu/hr
Gas Requirement
Gas connection (inches) - ¾” NPT
Inlet gas pressure under maximum operation*
• Propane: 10.5” - 14” water column
• Natural Gas: 5.7” - 14” water column.
* To measure Gas Pressure, see Measuring Gas
Pressure, chapter 3.8.
Venting
• Natural Draft
• Vent diameter (inches) - 5”
• Minimum height (feet) - 6’ with no elbows
• Vertical termination
• Sidewall termination only with AQ4 power vent kit.
Water
• Hot water connection (inches) - ½” NPT
• Cold water connection (inches) - ½” NPT
• Water valve material: Fibreglass - reinforced
polyamida (PA)
• Minimum water flow: 0.6 gallon/minute (2.2 l/m)
• Minimum recommended water pressure: 30 PSI
(2.07 bar)
• Connections:
– Bottom of heater
Dimensions
• Depth (in): 10.5” (265 mm)
• Width (in): 18.25” (460 mm)
• Height (in): 36.75” (936 mm)
• Weight: 64 pounds (29 kg).
Gas types
Natural Gas.
LP Gas
Safety devices
• Flame failure device (ionization flame rod sensor)
• Pressure relief valve (supplied with heater)
• Over heat prevention (temperature limiter).
Water protection
IP X4.

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Appliance details
2.2 Unpacking the GWH 425 HN heater
This heater is packed securely.
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 cover.
Fig. 2 Rating plate
ASerial number
BType of gas
The box includes:
• Pressure relief valve
•Mountingscrews
• Product registration card
• Installation manual
• Incandescent particle tray.
Do not lose this manual, there is a charge for a
replacement.
Please complete and return the enclosed product
registration card.
The GWH 425 HN is not approved or designed for:
• Manufactured (mobile) homes, RV's or boats
• Heating or other recirculating/pumping applications*
• Solar/preheat backup or high temperature booster
use
• Installation in bathroom, bedroom or other occupied
rooms normally kept closed.
* This includes domestic hot water circulator pump loop
systems that may be installed in home hot water system
prior to installing this unit. An approved recirculation
design can be found in chapter 3.10.
2.3 General rules to follow for safe
operation
1. You should follow these instructions when you install
your heater. In the United States: The installation must
conform with local codes or, in the absence of local
codes, the National Fuel Gas Code ANSI Z223.1/
NFPA 54.
In Canada: The Installation should conform with CGA
B149.(1,2) INSTALLATION CODES and /or local
installation codes.
2. Carefully plan where you install the heater. Proper
clearances must be followed.
3. The appliance must be isolated from the gas supply
piping system by closing its individual manual gas
shutoff valve (not supplied with heater) during any
pressure testing at pressures in excess of ½ Psig (3.5
kPa).
The appliance and its gas connection must be leak
tested before placing the appliance in operation.
4. Keep water heater area clear and free from
combustibles and flammable liquids. Do not locate the
heater over any material which might burn.
5. Correct gas pressure is critical for the optimum
operation of this heater. Gas piping must be sized to
provide the required pressure at the maximum output of
the heater, while all the other gas appliances are in
operation. Check with your local gas supplier, and see
chapter 3.7 and 3.8 to verify proper gas line sizing.
6. Should overheating occur or the gas supply fail to
shut off, turn off the gas supply at the manual gas shut
off valve, on the gas line. Note: manual gas shutoff valve
is not supplied with the heater.
7. Do not use this appliance if any part has been
underwater. Immediately call a qualified service
technician to inspect the appliance and to replace any
part of the control system and any gas control which
has been underwater.
iBOSCH is constantly improving its
products, therefore specifications are
subject to change without prior notice.

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Appliance details
5
2.4 Dimensions and installation clearances
Fig. 3 Dimensions in Inches and (mm)
4Heat exchanger
5Burner
10 Temperature control
11 Water valve
20 Gas connection
23 Igniter unit
26 Output control
34 LED indicator for burner status
36 Front cover
37 Hole for fixing to wall
38 Exhaust pipe to wall
39 Draught diverter with flue gas monitor
40 Gas valve
49 Observation window
51 On/Off switch
110 LED failure indicator
Fig. 4 Minimum clearances
Model GWH 425
TOP (A) 12 inches (306 mm)
FRONT (B) 4 inches (100 mm)
BACK 0 inches
SIDES 4 inches (100 mm)
BOTTOM (C) 12 inches (306 mm)
Dimensions
inches (mm)
GWH 425 HN
A16.73” (425)
B25.75” (655)
C13.15” (334)
D5.11” (130)
E21.25” (540)
F2.55” (65)
G1.18” (30)
H3/4”
Table 1 Dimensions in inches (mm)

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Installation instructions
3 Installation instructions
3.1 Introduction
Please follow these instructions. Failure to follow
instructions may result in:
BDamage or injury.
BImproper operation.
BLoss of warranty.
If you are unable to perform the tasks required to install
this heater properly, please contact a locally licensed
plumber or gas technician.
Please contact Bosch Water Heating with any
questions.
3.2 Proper location for installing your
heater
Carefully select the location of the water heater. For
your safety and for proper heater operation, you must
provide combustion air to the heater and a proper
exhaust vent system.
Follow the guidelines below:
B1. Locate the heater where venting, gas and
plumbing connections are feasible and convenient.
B2. The hot water lines should be kept short to save
energy. Centrally locating the water heater is best. It
is always best to have hot water lines insulated.
3.3 Heater placement and clearances
The GWH 425 HN is design certified for installation on
a combustible wall (see 3.4 Mounting installation)
provided the floor covering below the heater is
noncombustible. For installations in an alcove or closet,
maintain the minimum clearances to combustible and
non-combustible materials listed below. See also Fig. 4.
A. Top 12 inches (306 mm)
B. Front 4 inches (100 mm)
C. Back 0 inches
D. Sides 4 inches (100 mm)
E. Bottom 12 inches (306 mm)
Warning: The water in this water
heater is cold and always remains cold
except for the times that hot water is
being used DO NOT INSTALL IN AN
AREA WHERE IT COULD FREEZE.
Drain the heater entirely if freezing
temperatures are anticipated in area
where heater is installed. See chapter
4.6 for draining instructions.
To prevent any freeze damage,
introduce short bursts of compressed
air (20-40psi) through these
connections to remove the residual
water in the horizontal pipes and water
valve.
Warning: Flammable materials,
gasoline, pressurized containers, or any
other items or articles that are potential
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 liquids.
Warning: The heater must be isolated
from the gas supply piping system
during any pressure testing of that
system at test pressures equal to or
more than 0.5 psig.
Warning: Place the heater in a location
where water leaks will do NO DAMAGE
to adjacent areas.

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Installation instructions
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3.4 Mounting Heater
Remove cover and inspect.
BRemove the temperature control.
BUnsrew the cover fixing screws, see Fig. 6.
BRemove the outer case by sliding it forwards and
then lifting upwards.
BEnsure that the flue terminal is clear.
BAfter inspection, replace front cover and tighten
screws.
Install incandescent particle tray.
BInstall incandenscent particle tray using screws
provided as shown in Fig. 5.
Fig. 5 Incandescent particle tray illustration
Mounting heater.
The GWH 425 HN is design certified for mounting on a
wall.
The heater must be mounted on a wall using
appropriate anchoring materials. If wall is a stud wall
sheathed with plasterboard, 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. 7.
BSecure the two included L shaped hooks to wall
studs or support board 13 ¼” apart (See Fig. 7).
BHang heater on two L shaped hooks (See Fig. 8).
Fig. 6 Remove front cover
Fig. 7 Support board
Fig. 8 Secure heater to wall
Warning: before starting installation:
Bcheck that there are no loose parts
inside the appliance
Bensure that gas pipe, gas valve, and
burner have no damage and are
properly fitted.
BRead chapter 2.2 to verify proper gas
type and to check all parts are included
in box.
iFront cover should be removed in order to
inspect components visually (see
instructions below).
Warning:
BDo not install this appliance on a
carpeted wall.
Incandescent particle tray
Screws

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Installation instructions
3.5 Combustion air requirements
The Protankless water heater holds cold water in its
copper heat exchanger and water valve when not in use.
Because of this, any cold air that comes down through
the unit's vent pipe is capable of freezing these
components. This Installation Manual specifies the
minimum vertical vent pipe and the amount of
combustion air required for this unit. When all
requirements are followed, the unit will operate properly
and safely. However, there may still be a risk of freezing
due to negative draft if all the combustion appliances in
the area are not being supplied with a sufficient amount
of make-up air. A wood stove or furnace can rob the
makeup air in the Protankless vent pipe, leaving the cold
infiltrating air capable of freezing the cold water in the
Protankless heat exchanger. Supplying more
combustion air for all combustion appliances is the
solution. Follow the instructions on venting and
checking adequacy of make up air. A HVAC specialist
should be used to design solutions for providing more
make-up air if necessary. Observe the following
instructions concerning combustion air.
Appliances located in unconfined spaces:
a) An unconfined space is one whose volume is greater
than 50 cubic feet per 1000 Btu per hour of the
combined rating of all appliances installed in the space.
That would be 5850 cubic feet for the Protankless
GWH 425 HN alone.
b) Installations in structures that have been tightly
constructed (air infiltration rate of 0.40 ACH or less)
must be provided for combustion air per the National
Fuel Gas Code. Consult a HVAC specialist if your air
infiltration rate is questionable.
Appliances located in confined spaces:
The confined space must be provided with two
permanent openings, one commencing within 12
inches of the top and one commencing within 12 inches
of the bottom of the enclosure. Each opening must have
a minimum free area of one square inch per:
• 1000 Btu/hr if all air is taken from inside the building.
• 2000 Btu/hr if all air is taken from the outside by
horizontal ducts.
• 4000 Btu/hr if all air is taken from the outside by
direct openings or vertical ducts.
Or the confined space must be provided with one
permanent opening or duct that is within 12 inches of
the ceiling of the enclosure. This opening must have a
minimum free area of one square inch per:
• 3000 Btu/hr if all air is taken from the outside by a
direct opening or vertical duct.
Louvers, grills and screens have a blocking effect. If the
effective free area is not known, increase the sizes of
your openings by 400% if your louvers are wood and by
135% if your louvers are metal. Refer to the National
Fuel Gas Code for complete information. In buildings of
tight construction all air should be taken from outside.
3.6 Venting
BMinimum vent pipe diameter: 5 inches
BMinimum vertical vent height: 6 feet, with no elbows
BEstablish 12 inch rise before any elbow
The heater must be vented to the outside following all
local ordinances and specifications for installing a gas
appliance vent or chimney. The heater must be located
as close as practicable to a vertically rising chimney or
vent that has a listed vent cap at its termination point.
The venting system must be designed and constructed
so as to develop a positive flow adequate to remove flue
gasses to the outdoors. Consult the National Fuel Gas
Code if the vent will have elbows or share venting with
another natural draft appliance.
3.6.1 Horizontal venting
A power vent, with a proof-of-draft safety interlock
device, is required and is available for sidewall venting.
Contact your dealer. In the Commonwealth of
Massachusetts power vented applications must utilize
proof-of-draft safety interlock device.
3.6.2 Vertical venting
BA 5 inch diameter gas vent constructed of double
wall Type B gas vent is recommended. A 6 inch vent
is required in elevations greater than 2000 feet, see
Fig. 4. Under no circumstances should the vent pipe
be reduced in size.
BAn approved gas vent connector must be attached to
the top of the water heater and rise vertically at least
12" before entering into an approved gas vent
connector elbow.
BThe minimum vertical gas vent height allowed is 6
feet; horizontal vent connectors and elbows are not
to be considered in the total gas vent height.
Danger: Do not reduce the vent pipe
size. Do not put an elbow directly on top
of heater. Failure to follow venting
requirements may result in dangerous
exhaust gases to enter living space.
Warning: Do not combination vent with
a mechanically vented appliance.
Warning: Horizontally venting to a
vertically constructed vent stack along
an outside wall of a building is not
permissible.
Warning: Horizontally venting to a
sidewall vent terminator (without power
vent) is not permissible.

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Installation instructions
9
BAll gas vent sections must be secured to each other
with sheet metal screws and be properly supported.
Horizontal runs:
Any gas vent section that is greater than 45 degrees
from the vertical is considered horizontal. Horizontal
sections must slope upwards at least ¼ inch for every
foot of its horizontal length and be properly supported.
Keep the horizontal section short and avoid too many
elbows. The maximum horizontal run allowed is half of
the total vertical vent height; horizontal vent connectors
and elbows are not to be considered in the total gas
vent height.
Vent termination:
The gas vent constructed of double wall Type B gas
vent must terminate above the roof surface with a listed
vent cap at a height that's in accordance with Fig. 11
and table 2, provided they are at least 8 feet (2.4 m)
from a vertical wall or similar obstruction. All other gas
vents that are not able to terminate within the minimum
specified height allowed must terminate not less than 2
feet (0.6 m) above the highest point where it's passed
through the roof and at least 2 feet (0.6 m) higher than
any vertical wall or similar obstruction within 10 feet (3.1
m).
Fig. 9 Flat roof
Fig. 10 Vent Adaptor for High Altitude Installations
LISTED VENT CAP
MINIMUM 6
FEET (1.8M)
LISTED GAS VENT
ESTABLISH A ONE
FOOT RISE BEFORE
ANY ELBOWS
5” X 6” ADAPTOR
USED ON HIGH
ALTITUDE

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Installation instructions
Masonry chimney
Masonry chimneys shall be built and installed in
accordance with NFPS 211 or local codes. A minimum
5" diameter gas vent pipe (metal double wall Type B),
or an approved clay flue liner or a listed chimney lining
system must be used when venting into a naturally
drafting, internal masonry chimney. 6 inch is required in
elevations greater than 2000 feet, see Fig. 10. Local
codes may require the use of both gas vent and an
approved lining system when venting into a masonry
chimney. The Commonwealth of Massachusetts
requires the use of a listed liner. Lining systems include
approved clay flue lining, a listed chimney lining system
or other approved material that will resist corrosion,
erosion, softening, or cracking from exhaust flue gases
at temperatures up to 1800 degrees F. The lining
system must be listed for use with naturally drafting,
draft hood equipped gas appliances. Follow local
codes and refer to NFGC 54 and NFPA 58.
Existing interior masonry chimney
The metal gas vent pipe should be permanently
mounted inside the masonry chimney. Double wall Type
B gas vent is recommended. The masonry chimney may
have to be tile or metal lined before the insertion of the
gas vent pipe; check local codes for clarification. The
lining material must be listed for use only with naturally
drafting, draft hood equipped gas appliances. Follow
manufactures instructions for installation of listed lining
material. You may not vent any other fuel burning
appliances into any free space remaining in the
chimney. The minimum vertical gas vent length within
the masonry chimney should be no less than 5 ft (1.5
m); the vent terminator should extend at least 3 feet (0.9
m) above where the chimney meets the roofline and at
least 2 feet (0.6 m) higher than any vertical wall or
similar obstruction within 10 feet (3.1 m). The top of the
gas vent should have an approved vent terminator. See
Fig. 12.
Fig. 11 Pitch roof
Fig. 12 Masonry chimney
GAS VENT TERMINATIONS FOR LISTED VENT CAPS
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
Table 2
LISTED VENT CAP
LOWEST DISCHARGE
OPENING
MINIMUM 6
FEET (1.8M)
X
12
ROOF
PITCH IS
X/12
LISTED GAS VENT
H (minimum) height
from roof to lowest
discharge opening
ESTABLISH A ONE
FOOT RISE BEFORE
ANY ELBOWS
GAS
VENT
LISTED
VENT CAP
VENT
CONNECTOR
Establish a
one foot
rise before
any elbows

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Installation instructions
11
3.7 Gas piping & connections
Before connecting the gas supply, check the rating
plate on the right side of the heater to be sure that the
heater is rated for the same gas to which it will be
connected.
In the United States: The installation must conform with
local codes or, in the absence of local codes, the
National Fuel Gas Code ANSI Z223.1/NFPA 54.
In Canada: The Installation should conform to CGA
B149 INSTALLATION CODES and/or local installation
codes.
Mount regulator to gas inlet pipe as shown in Fig. 13.
The arrow on the back of the regulator indicates the
direction of gas flow and should point toward the
appliance.
Fig. 13 Installation of Gas Pressure Regulator
Note: The GWH 425 HN comes with a gas pressure
regulator. Failure to install or altering the gas pressure
regulator 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 14" W.C.).
GAS CONNECTIONS
BInstall a manual gas shut off valve, on the gas supply
line.
BInstall a union when connecting gas supply.
BAttach the appliance regulator to the inlet gas pipe.
BThe minimum diameter required for any appliance
connector used is ¾” NPT.
BNational Fuel Gas Code requires that a sediment
trap (drip leg) be installed on gas appliances not so
equipped. The drip leg must be accessible and not
subject to freezing conditions. Install in accordance
with the recommendations of the serving gas
supplier.
When connections are made, check for gas leaks at all
joints. Apply some gas leak detection solution to all gas
fittings. Bubbles are a sign of a leak. A combustible gas
detector may also be used to detect for leaks.
HIGH ALTITUDE INSTALLATION
The pressure regulator provided with the heater is
adjusted to deliver the proper gas pressure (as
indicated on the rating plate and in the manual for
altitude up to 2000 feet (660 meters) above sea level.
On appliances being installed above 2000 ft (660
meters) elevation, the inlet gas pressure should be set
at installation to the value shown below.
Note: The gas pressures specified below refer to
pressures taken at the pressure tap on the gas inlet pipe
just above the regulator. See chapter 3.8 for measuring
gas pressure.
MAXIMUM INLET GAS FLOW PRESSURE SETTING
GAS LINE SIZING
The gas supply piping should be sized according to the
applicable code for a maximum draw of 117,000 BTUH.
Measure the length of gas supply line and use the tables
in Fig. 14 or the gas line manufacturer’s sizing tables to
determine the pipe diameter necessary to
accommodate the BTU demand of the unit. If there are
more gas appliances drawing on the same line, size the
gas line according to the total maximum amount of BTU
draw for all appliances.
Note: Under sizing the gas line may result in diminished
output and improper operation. See chapter 3.8 for the
procedure to confirm gas pressure.
Warning: DO NOT connect to an
unregulated or high pressure propane
line or to a high pressure commercial
natural gas line.
Warning: The heater must be isolated
from the gas supply piping system
during any pressure testing of that
system at test pressures equal to or
more than 0.5 psig. If overpressure has
occurred, such as through improper
testing of the gas lines or malfunction of
the supply system, the gas valve must
be checked for safe operation.
Danger: If you have a leak, shut off the
gas. Tighten appropriate fittings to stop
leak. Turn the gas on and check again
with a gas leak detection solution.
Never test for gas leaks using a match
or flame.
Altitude Nat. Gas
inches W.C.
Liquid Propane
inches W.C.
0’ - 2 000 ft 5.7” 10.5”
2 000 ft - 4 500 ft 4.6” 8.4”
Table 3 Above 4.500 ft consult your local gas provider

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Installation instructions
FOR NATURAL GAS
Maximum Capacity of pipe in Cubic Feet of Gas per Hour for Gas Pressure of 0.5 Psig or less and a Pressure drop
of 0.3 in Water Column (0.75mbar).(Based on a 0.60 Specific Gravity Gas) Btu numbers given in thousands.
Copper tubing is prohibited for use with Natural Gas in the Commonwealth of Massachusetts and not recommended
elsewhere.
Fig. 14
Follow boxed numbers for piping just one GWH 425 HN (example: ¾” B.I. Natural Gas pipe for 20 ft (6.1m). will
handle 190,000 btu’s (55.7 kWh). For multiple appliances combine the total btu input load and then refer to
applicable chart below.
* EHD = Equivalent Hydraulic Diameter. The greater the
value of EHD, the greater the gas capacity of the tubing.
Maximum Capacity of Semi-Rigid copper Tubing in
Thousands of BTU per Hour of Undiluted Liquefied
Petroleum Gases (at 11 inches Water Column Inlet
Pressure).
(Based on a Pressure Drop of 0.5 Inch Water Column)
* Source National Fuel Gas Code NFPA 54, ANSI Z223.1
- No Additional Allowance is necessary for an ordinary
number of fittings
FOR LP GAS
Maximum Capacity of Pipe in Thousands of BTU per Hour of Undiluted Petroleum Gases (at 11 inches Water Column Inlet
Pressure) (Based on a Pressure Drop of 0.5 Inch Water Column).
* EHD = Equivalent Hydraulic Diameter. The greater the
value of EHD, the greater the gas capacity of the tubing.
Tube
size,
inches EHD*
10 20 30 40 50 60
1/2 18 EHD 82 58 47 41 37 34
3/4 23 EHD 161 116 96 83 75 68
1 30 EHD 330 231 188 162 144 131
1 1/4 37 EHD 639 456 374 325 292 267
Length of Flexible Corrugated Stainless Steel Tubing (CSST), Feet
Tube
size
inches EHD*
10 20 30 40 50 60
1/2 18 EHD 129 91 74 64 58 53
3/4 23 EHD 254 183 151 131 118 107
1 30 EHD 521 365 297 256 227 207
1 1/4 37 EHD 971 661 528 449 397 359
Length of Flexible Corrugated Stainless Steel Tubing (CSST), Feet
Copper (LP gas only)
Outside Length of Tubing, Feet
diameter
Inch 10 20 30 40 50 60 70 80 90 100
3/8 392621 19 ______
1/2 926250 41 373531292726
5/8 199 131 107 90 79 72 67 62 59 55
3/4 329 216 181 145 131 121 112 104 95 90
Nominal
Iron pipe
size
inches 10 20 30 40 50 60 80 100 125 150 200
1/2 291 200 160 137 122 110 94 84 74 67 58
3/4 608 418 336 287 255 231 197 175 155 140 120
1 1145 787 632 541 480 434 372 330 292 265 227
Black Iron Pipe
Lenght of pipe, Feet
Nominal
Iron
Pipe Internal
Size Diameter
inches inches 10 20 30 40 50 60 70 80 90 100 125 150 175 200
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
Length of Black Iron Pipe, Feet

6 720 608 036
Installation instructions
13
3.8 Measuring gas pressure
Connecting manometer
BShut off gas.
BRemove front cover and locate inlet gas pressure
measuring point on the right side, see Fig. 15.
BLoosen screw from test point on the right side and
connect manometer tube on test point.
Fig. 15 Gas pressure measuring point
AInlet tap
Static pressure test
BTurn gas supply back on.
BRecord static gas pressure reading on back page of
manual.
Operating pressure test
BTurn on all hot water taps served by the water heater.
BSet flow control knob fully clockwise.
BTurn power adjustment knob fully clockwise.
BOperate all other gas appliances on same gas piping
system at maximum output.
BRecord operating gas pressure reading on back
page of manual.
Gas pressures lower than 5.7" W.C. for Natural Gas or
10.5" W.C. for LP Gas will result in insufficient degree
rise to the hot water being used, and must be corrected.
See Gas Line Sizing under chapter 3.7.
3.9 Water connections
When facing the heater, the ½” cold water inlet is on the
bottom right and the hot water outlet is on the bottom
left.
BInstall unions or the Webstone service valves
when connecting plumbing to the water
heater. This will facilitate any necessary
cleaning and servicing.
Although water piping throughout your structure may be
other than copper, we recommend that copper piping
or suitably rated stainless steel flex line piping be used
for at least three feet before and after the heater (follow
local codes if more stringent). Never sweat any rigid
piping directly to or beneath the water connections or
damage can occur to the internal water valve from
heating of the pipe. Plastics or other PEX type plumbing
line materials are not recommended for connecting
directly to the water heater. Keep water inlet and outlet
pipes to no less than ½" (12.7mm) diameter to allow the
full flow capacity.
It is recommended that all water piping below the heater
be properly insulated to avoid heat loss. The freeze
protection provided with the heater will not prevent any
freeze damage to the exterior water piping.
If the cold and hot connections to the heater are
reversed, the heater will not function. Be certain there
are no loose particles or dirt in the piping. Blow out or
flush the lines before connecting to the water heater.
Full port valves should be installed on both the cold
water supply and hot water outlet lines to facilitate
servicing the heater (see Fig. 16). For installation on a
private well system with the use of a pressure tank, the
lowest pressure range setting recommended is 30-50
psi (2.07 and 3.45bar).
Connecting the pressure relief valve (PRV)
The pressure relief valve supplied with the heater must
be installed at the time of installation. No valve is to be
placed between the PRV and the heater. No reducing
coupling or other restriction may be installed in the
discharge line. The discharge line must be a minimum of
4” above a drain and installed such that it allows
complete drainage of both the PRV and the line.
The location of the PRV must be readily accessible for
servicing or replacement, and be mounted as close to
the water heater as possible. See Fig. 16. To install the
PRV, a suitable fitting connected to an extension on a
“T” fitting can be sweated to the hot water line.
Support all piping.
Fig. 16 Plumbing Connections and Pressure Relief
Valve

6 720 608 036
14
Operation instructions
3.10 Recirculation application
Since recirculation through the heater is not
permissible, the following drawing is provided to outline
a proper recirculation application using the Protankless
water heater with an Ariston minitank. This schematic is
for illustration only and must not be used for actual
installation without appropriate engineering and
technical advice from a professional properly licensed in
locality where the installation is made.
Fig. 17 Recirculation application
1Full port isolation valve
2Circulator
3Check valve
4PRV
The use of a small electric mini-tank (4-6 gallon size)
should be used for this application and designed so the
pump will circulate the water through the mini-tank and
the building's hot water return loop only. A timed or
thermostatically controlled operation of the pump is
commonly done. The GWH 425 HN should be
plumbed in line before the mini-tank water heater.
Contact Bosch Water Heating if further instruction is
needed.
4 Operation instructions
4.1 For your safety read before
operating your water heater
A. This appliance is equipped with electronic ignition for
lighting the pilot and main burners. When turning the
heater on, follow these instructions exactly.
B. Before operating the unit, set the On/Off button to
the On position. The On/Off button is located on
the front panel. Smell all around the appliance area for
gas. Be sure to smell next to the floor because some
gas is heavier than air and will settle on the floor.
WHAT TO DO IF YOU SMELL GAS
BClose gas valve. Open windows.
BDo not try to light any appliance.
BDo not touch any electric switch; do not use any
phone in your building.
BImmediately call your gas supplier from a neighbors
phone. Follow the gas supplier’s instructions.
BIf you cannot reach your gas supplier, call the fire
department.
C. Use only your hand to push in the on/off control
button. Never use tools. Follow these instructions
exactly. If control button is jammed, close the gas
supply and call a qualified service technician.
Attempted forceful repair may result in a fire or
explosion.
D. Do not use this appliance if any part has been under
water. Immediately call a qualified service technician to
inspect the appliance and to replace any part of the
control system and any gas control which has been
under water.
Warning: If you do not follow these
instructions exactly, a fire or explosion
may result causing property damage,
personal injury or loss of life.

6 720 608 036
Operation instructions
15
4.2 Lighting instructions
B1. STOP! Read the previous safety information.
B2. The gas valve must be shut off by putting the ON/
OFF switch to position OFF . Wait five (5)
minutes to clear out any gas. If you smell gas, STOP!
Follow “B” in chapter 4.1. If you do not smell gas, go
to the next step.
B3. This water heater is equipped with a safety pilot
burner and an automatic ignition control system.
B4. Set the ON/OFF button to the position ON .
In this position, the water heater is ready to use.
Fig. 18
B5. Open hot water tap to a flow rate above the
minimum activation rates listed in table 4. The
automatic ignition system first ignites the safety pilot
burner which then ignites the main burner in about 4
seconds.
B6. The green LED indicator is on when the main
burner is on.
B7. The pilot flame will extinguish 10-30 seconds after
the burners come on. The burners will remain on until
the hot water tap is turned off.
Note: The GWH 425 HN operates in two modes. See
SETTING THE WATER TEMPERATURE.
4.3 To turn off appliance
BSet the ON/OFF switch to the position OFF .
Fig. 19
BShut installer supplied manual gas shutoff valve on
the supply line to the heater.
iNote: On first time initial
installation, existence of air in the
gas supply line and in the water
line may cause some ignition
delay. In that case, repeat the
ignition process until all the air
has been purged.

6 720 608 036
16
Operation instructions
4.4 Setting the water temperature
The GWH 425 HN has a gas control that modulates
burner input in response to flow. Its purpose is to
ensure that the hot water temperature will remain
steady, although the water flow demand might vary. The
output temperature can be adjusted with either the
temperature adjustment knob or the gas control knob.
Temperature adjustment knob
The temperature adjustment knob on the front bottom of
the heater (see Fig. 20) adjusts temperature by
adjusting flow capacity. See table 4 for details.
Fig. 20
As the temperature adjustment knob is turned counter-
clockwise, the output temperature will lower and the
activation rate will be raised. Turning the knob
clockwise will raise the temperature and lower the
activation rate.
4.5 Power adjustment
Less power.
Lower water temperature.
Fig. 21
More power.
Higher water temperature.
Fig. 22
4.6 Purge the appliance
If there is a risk of freezing, proceed as follows:
BRemove the retaining clip from threaded bushing
(pos. 1).
BRemove threaded bushing (pos. 2) from water valve.
BEmpty the appliance of all water.
Fig. 23 Purge
1Retaining clip
2Threaded bushing
Knob
position Clockwise Counter- clockwise
Degree rise 90°F 45°F
Activation rate 0.5 GPM 1.1 GPM
Max flow rate 2.0 GPM 3.7 GPM
Table 4 Clearances

6 720 608 036
Maintenance and service
17
5 Maintenance and service
5.1 Maintenance intervals
The GWH 425 HN requires periodic maintenance. The
below time maintenance intervals should keep the unit
operating for many years.
Every year
BInspect inlet water filter screen ( 5.2)
BInspect pilot assembly and flame ( 5.3)
BInspect burner assembly ( 5.4)
Every 2 years
BLubricate and clean water valve ( 5.2)
Every 3 to 5 years
BRebuild water valve ( 5.2)
BClean pilot assembly and clean or replace orifice
(5.3)
5.2 Water valve
The water valve is the main control that tells the heater
to fire. If the periodic maintenance is neglected, more
costly damage may occur over time.
Inspecting inlet water filter screen
Shut off the installer supplied cold water isolation valve
to the heater. If one is not installed, install before
proceeding. Open the nearest hot water tap to drain the
plumbing lines. Position a bucket under the heater's
water valve assembly to catch any water that may drain
from the heater. Remove the filter retaining clip located
at base of water valve (see Fig. 23). Remove the
threaded bushing to access the cylindrical filter inside.
Remove filter, clean with water and inspect for damage.
If the filter is at all damaged, it should be replaced.
Lubricating the water valve
A more detailed instruction is available on our website
at www.boschpro.com. Shut off the installer supplied
cold water isolation valve to the heater. If one is not
installed, install before proceeding. Open the nearest
hot water tap to drain the plumbing lines. Position a
bucket under the heater's water valve assembly to catch
any water that may drain from the heater. Disconnect
the inlet water pipe going to the back of the water valve.
Disconnect the water pipe leading out of water valve to
the heat exchanger. Do this by removing the retaining
clip on water valve assembly and pulling the pipe free
from the water valve. Loosen the two set screws at the
bottom of the water valve where it connects to the gas
valve. If the set screws are seized, STOP and call
Bosch tech support before proceeding. Forcing and
snapping seized set screws may cause irrevocable
damage to the water valve and gas valve assemblies.
Remove the water valve by pulling it to the right.
Separate the two halves of the water valve by removing
the five screws on the left side of the water valve. Apply
lithium or faucet & valve grease to the pushrod and o-
ring. (See Fig. 27). Inspect the venturi and clean if
necessary. (see Fig 27).
Rebuilding the water valve
Several parts within the water valve should be replaced
every 3 to 5 years depending on water quality and
usage. The parts list and instructions can be found on
our website at www.boschpro.com. Failure to rebuild
the water valve can result in decreased performance or
damage to the heater.
5.3 Pilot
Inspecting pilot
The pilot should burn with a clean sharp blue flame. The
flame should fully engulf the tip of the thermocouple. If
it does not, clean the pilot assembly per procedure
below.
Cleaning pilot assembly
Shut off gas supply to the heater using installer supplied
manual gas shutoff. Locate pilot gas tubing mounted in
the center of the burner assembly. At the base of the
pilot gas tubing there is a retaining clip. Release pilot
gas tubing from this clip. Push upwards on other end of
pilot gas tubing to release it from burner mount.
Remove brass hex headed pilot orifice from gas valve
(located near base of pilot gas tubing). Soak orifice and
pilot gas tubing in carburetor cleaner for 30 minutes.
Dry off and reinsert orifice in gas valve. If heater is an LP
unit, replace orifice. Reinstall the pilot gas tubing
following removal instructions in reverse. Open gas
supply and return heater to service.
Warning: Failure to perform
recommended maintenance may result
in complete failure of the unit over time.
The warranty does not cover failures
due to improper or insufficient
maintenance.
Caution: Do not ream or poke orifice as
it will enlarge orifice opening.

6 720 608 036
18
Maintenance and service
5.4 Main burners
The main burner flames should be blue, with a more
intense blue cone in the center core. Yellow flames
could be a sign of wrong size gas orifices or dirty
burners, a blockage on the heat exchangers fins. If
some burners have yellow flames while others have blue
flames, it is likely that dust, lint or spider webs have
partially clogged the burner venturis. To clean the
burners, contact a gas service person.
5.5 Vent assembly
Inspect the draft hood and heat exchanger fins for signs
of soot build-up or any other foreign material such as
spider webs. Clean out any debris found in the vent
hood. Signs of soot indicate insufficient combustion air
or exhaust draft. Check for vent assembly blockage or
combustion air blockage on the underside of the unit.
5.6 Mineral scale build-up
The GWH 425 HN, when operated at lower
temperatures settings, does not accumulate mineral
build-up. If however, the heater is used at the higher
temperature settings and the water has a high mineral
content, periodic descaling may be necessary. The
heating coils should be flushed with a descaling
solution.
5.6.1 Descaling heat exchanger
1. Disconnect electrical supply from the water heater.
2. Shut off the water supply to the water heater using
(installer supplied) isolation valve.
3. Open hot water taps to drain and relieve pressure
from the plumbing system.
4. Drain water from the unit's heat exchanger by
disconnecting inlet and outlet water connections.
5. Connect the line (A) from the outlet of the circulating
pump (installer supplied) to the inlet water fitting on the
water heater.
6. Using another line (B), connect to the water outlet
fitting on the water heater. Route the other end of this
line into a descaling reservoir.
7. Using a 3rd line (C) from the descaling reservoir,
connect to the inlet side of circulating pump. Verify
there is a filter on the end of the line in the descaling
reservoir.
8. Make sure all connections are "hand tight."
9. Fill tank with descaling solution so lines inside are
submersed. We recommend a straight white vinegar
solution. If using a commercial descalant, refer to
manufacturer's instructions on dilution with water.
10. Operate the circulating pump.
11. Make sure there are no leaks and the solution is
flowing from the descaling reservoir through the heat
exchanger and returning to the reservoir.
12. Run solution through the heat exchanger until the
solution returning to the descaling reservoir comes out
clear. (Changing to a fresh solution may be necessary
during this process).
13. Disconnect all lines and drain all solution from heat
exchanger. Properly discard of solution.
14. Position a container below the hot water outlet and
connect cold water supply. Open cold water supply
isolation valve and flush heat exchanger with clean
water.
15. Shut cold water isolation valve and reconnect hot
water supply to the water heater.
16. Reconnect electrical supply to unit, open water
isolation valves, and return the unit to service.
Fig. 24

6 720 608 036
Troubleshooting
19
6Troubleshooting
6.1 Introduction
Many of the questions customers ask regarding
operation of this unit can be answered by following the
troubleshooting steps as outlined below. Visit our web
site at www.boschpro.com for more detailed
troubleshooting. For best results, perform each step
before proceeding to the next. The suggested solutions
may require that the cover be taken off. (See chapter
3.4).
6.2 No spark at pilot with water flow
1. On/Off button is not ”On”.
The On/Off button is located in the front cover.
2. Verify the cold water supply is connected to the right
side rear of the water valve.
3. Close installer supplied cold water shut off valve (if
none installed, install before proceeding). Open all hot
water taps supplied by the heater. Wait 5 minutes and
check all taps. Any water running is a sign of a plumbing
crossover. Consult a local plumber or service person for
help in correcting a plumbing crossover.
4. Confirm that there is an adequate flow rate. With the
temperature adjustment knob on the unit positioned all
the way clockwise, a 1/2 gallon per minute (GPM) flow
rate is required for heater activation. Fully open a hot
water faucet and fill a quart container. If the container
fills in 30 seconds or less, the flow rate (1/2 GPM) is
sufficient to activate the water heater.
NOTE: When the dial is turned all the way
counterclockwise, a flow rate of 1.1 GPM is required for
heater activation.
5. Inspect the water path for obstructions. Make sure all
outlets (i.e. showerheads, faucet aerators and whole
house filters) are clear of debris. Inspect and clean the
water heater's inlet filter screen. See chapter 5.2.
6. Verify all wire connections are secure, paying close
attention to the wire connection between the piezzo
electrode and the electronic control box.
7. Check for voltage at the hydro-generator. Disconnect
plug connector on wires leading from hydro-generator
(approximately 3" from hydro-generator). Verify voltage
on hydro-generator side of plug connector with hot
water running. The voltage reading should be 1.6V AC
or greater. If voltage is lower, then remove and flush the
hydro-generator. Reinstall hydro-generator and take
another voltage reading. If voltage is at least 1.6V AC,
proceed to next step. If voltage is still low, hydro-
generator may be damaged/defective. Visit
www.boschpro.com to purchase replacement part, or
call Bosch Water Heating at 866-330-2730 for
replacement part if still under warranty.
8. If using a power vent, check the safety spillage switch
reset button (the safety spillage is wired through the
water heater's thermocouple circuit). The safety
spillage switch should be located at the top of the water
heater close to the draft diverter. Consult power vent
manual for more detailed instructions.
9. Verify the spade connectors on the two overheat
sensors (ECO) & the flue gas sensor are plugged in
securely and their terminals are free of corrosion. The
terminals can be cleaned with a pencil eraser. The
ECO's are located on the lower left hot water outlet
pipe and the upper right side of heat exchanger. The
flue gas sensor is located on the upper right side of
draft diverter.
10. Verify the microswitch located on bottom of gas
valve is firmly mounted and wire clip connector is
secure.
11. Electronic control box may be damaged or
defective. Visit www.boschpro.com to purchase
replacement part, or call Bosch Water Heating at 866-
330-2730 for replacement part if still under warranty.
6.3 Spark appears when hot water is
turned on, but pilot and burners
will not ignite
1. Verify gas type indicated in the rating sticker located
on the cover's right hand side, coincides with the gas
type you are using. NG is a natural gas unit and LP is
for liquid propane (see chapter 2.2).
2. If the unit was just installed or the gas lines have been
worked on, there may be air in the gas line. Bleed out
the air trapped in the gas line by turning the hot water
faucet on and off until the pilot flame/burners ignite.
(Please note: depending on how much air is in the line
this could take numerous cycles of turning the faucet on
and off).
3. Verify gas supply is on at Natural Gas meter or
Propane tank. Make sure all manual gas shut off valves
are open.
4. Have a licensed gas technician confirm adequate gas
pressure at the inlet tap (See gas requirements in
chapter 2.1). If gas is not present, verify manufacturer
supplied Maxitrol regulator is in the upright position.
The arrow on the back of the regulator should point in
the same direction as gas flow.
5. Confirm wire connection to the pilot electro valve is
secure. See Components Diagram for location. Inspect
terminals for corrosion. To clean terminals, remove the
spade connectors and clean with a pencil eraser.
6. Open a hot water faucet. While sparking, hold a
flame from a lit match or lighter in the pilot area. If the
pilot successfully lights, the pilot orifice may be dirty or
clogged. See chapter 5.3 on how to clean the pilot.
7. Verify adequate voltage at the spade connection of
the pilot electro valve. When sparking, voltage should

6 720 608 036
20
Troubleshooting
read 1.6V AC. If voltage is proper, pilot electro valve is
defective. If voltage is not proper, electronic control box
is defective.
Visit www.boschpro.com to purchase replacement part,
or call Bosch Water Heating at 866-330-2730 for
replacement part if still under warranty.
6.4 Pilot light, but burners will not
come on.
1. Verify gas type indicated in the rating sticker located
on the cover's right hand side, coincides with the gas
type you are using. NG is a natural gas unit and LP is for
liquid propane (See chapter 2.2).
2. Pilot flame should be blue in color and completely
engulfing the flame sensor. If not, the pilot orifice may be
dirty or clogged. See chapter 5.3 on how to clean the
pilot.
3. Flame sensor not recognizing pilot flame. Verify wire
connection clip on wire from the bottom of the flame
sensor is secure. If flame sensor is fully engulfed by the
pilot flame, it may be dirty. Clean entire surface area of
flame sensor with a pencil eraser.
4. Inspect the water path for obstructions. Make sure all
outlets (i.e. showerheads, faucet aerators and whole
house filters) are clear of debris. Inspect and clean the
water heater's inlet filter screen. See chapter 5.2.
5. Confirm wire connection to the burner electro valve is
secure (see components diagram for location of burner
electro valve). Inspect terminals for corrosion. To clean
terminals, remove the spade connectors and clean with
a pencil eraser.
6. The water valve assembly, which allows gas flow to
the burners, requires periodic maintenance (every 2-5
years depending on water quality and use). Visit
www.boschpro.com to purchase water valve rebuild kit
to service the water valve.
7.Electric control box, burner electro valve or flame
sensor maybe damaged/defective. Verify adequate
voltage at the spade connection of the burner electro
valve (see components diagram for location of burner
electro valve). After the pilot lights, voltage should read
1.6V AC on the burner electro valve. If voltage is
proper, the burner electro valve is defective. If voltage is
not proper, electronic control box or the flame sensor is
damaged/defective.
Visit www.boschpro.com to purchase replacement part,
or call Bosch Water Heating at 866-330-2730 for
replacement part if still under warranty.
Fig. 25 Electrical wiring diagram
2Pilot electrode
3Flame sensor
18 Microswitch
23 Electronic control box
25 Burner electro valve
28 Pilot electro valve
31 Temperature limiter
32 Flue gas safety device
34 LED indicator main burner operation
35 On/Off button
50 Hydro-generator
111 Overheat limiter
6.5 Main burners go out during hot
water use
1. Hot water flow rate is dropping below required
activation flow rate. Turn temperature adjustment knob
all the way clockwise. Fully open a hot water faucet and
fill a quart container. If the container fills in 30 seconds
or less, the flow rate (1/2 gallon per minute) is sufficient
to activate the water heater. If flow rate is not sufficient,
the water heater's inlet filter should be inspected and
cleaned. See chapter 5.2.
2. Close installer supplied cold water shut off valve (if
none installed, install before proceeding). Open all hot
water taps supplied by the heater. Wait 5 minutes and
check all taps. Any water running is a sign of a plumbing
crossover. Consult local plumber or service person for
help correcting a plumbing crossover.
3. The minimum water pressure for the home should be
30psi or greater. For installation on a private well
system with use of a pressure tank, the lowest pressure
range setting recommended is 30-50 psi (2.07-3.45
bar).
4. Hot water is very hot out of the tap, requiring a lot of
cold water to be added with it in order to attain a
useable hot water temperature. The addition of too
much cold will overpower and slow the flow within the
tankless heater, decreasing it below activation point,
which shuts off the burners. The end result is nothing
but cold water coming out of the outlet. Reduce the