U.S. Boiler Company SteamMax STMX075 Service manual

111043-03 - 9/21
SteamMax
Models:
•STMX075
•STMX100
•STMX125
•STMX150
•STMX175
WARNING
This boiler must only be installed, serviced, and repaired by a qualified installer, service agency or gas
supplier. Improper installation, adjustment, alteration, service or maintenance can cause severe personal
injury, death, or substantial property damage. For assistance or additional information, consult a qualified
installer, service agency or gas supplier. Read these instruction carefully before installing.
!
Installation, Operating and Service Instructions for
Manual Contents Page
1. Qualified Installer, Service Agency or Gas Supplier
Read Before Proceeding ......................3
2. User/Homeowner Read Before Proceeding .......4
3. Specifications ...............................5
4. Locating Boiler .............................10
5. Preparing Boiler.............................14
6. Venting ....................................15
7. Steam Piping ...............................18
8. Gas Piping .................................22
9. Electrical ..................................24
10. System Start-Up and Checkout ................31
11. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
12. Before Leaving Jobsite .......................40
13. Service and Maintenance .....................41
14. How it Works ...............................45
15. Troubleshooting ............................49
16. Service Parts ...............................56
Appendix A Steam Boiler Skimming ...............71
Appendix B Indirect Water Heater .................72
Appendix C McDonnell & Miller 150S-MD Auto Reset
LWCO/Pump Control ............................74
•STMX200
•STMX250
•STMX299
•STMX350
•STMX399
•STMX450
•STMX500
•STMX550
WARNING
Attention Installer - Affix these instructions
adjacent to boiler. Provide model number and
serial number when seeking information and
support.
Attention Building Owner - Retain these
instructions for future reference. Contact a
qualified installer, service agency or gas
supplier for all issues and support.
!
• Steam Boiler
• Cast Iron
• Chimney Vent
• Gas Fired
NOTICE: Knock-Down Boilers: These instructions
address installation and controls. Assembly is
addressed in separate instructions shipped with
knock-down boilers.

2
SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
The following terms are used throughout this manual to bring attention to the presence of hazards of various
risk levels, or to important information concerning product life.
Massachusetts Board of Plumbers and Gas Fitters has listed SteamMax Boilers. See Massachusetts Board of
Plumbers and Gas Fitters website for latest Approval Code or ask your local Sales Representative.
Commonwealth of Massachusetts requires this product to be installed by a licensed Plumber or Gas fitter.
NOTICE: Indicates special instructions on
installation, operation, or service which are
important but not related to personal injury
hazards.
DANGER
Indicates a hazardous situation that, if not
avoided, will result in death or serious injury.
CAUTION
Indicates a hazardous situation that, if not
avoided, could result in minor or moderate
injury.
WARNING
Indicates a hazardous situation that, if not
avoided, could result in death or serious injury.
! !
!
Important Definitions of Warnings
DANGER
Explosion Hazard. DO NOT store or use gasoline or other flammable vapors or liquids in the vicinity of
this or any other appliance.
If you smell gas vapors, DO NOT try to operate any appliance - DO NOT touch any electrical switch or
use any phone in building. Immediately, call gas supplier from a remotely located phone.
Follow gas supplier’s instructions or if the supplier is unavailable, contact fire department.
WARNING
This boiler must only be serviced and repaired by a qualified installer, service agency or gas supplier.
• If any controls are replaced, they must be replaced with identical models.
• Read, understand and follow all instructions and warnings contained in all sections of this
manual.
• If any electrical wires are disconnected during service, clearly label wires and assure that wires
are reconnected properly.
• Never jump out or bypass any or operating control or component of this boiler.
• Assure that all and operating controls and components are operating properly before placing
boiler back in service.
• Inspect boiler annually.
!
!

3111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
WARNING
Asphyxiation Hazard, Burn Hazard, Electrical
Shock Hazard.
• Carefully read all instructions in literature
packet and posted on boiler before attempting
installation, startup, or service of this boiler.
Perform steps in order given. Failure to follow
all instructions in proper order could result in
severe personal injury, death or substantial
property damage.
• Use proper personal protective equipment
when installing, servicing, or working near this
boiler. Materials of construction, flue products,
and fuel contain alumina, silica, heavy metals,
carbon monoxide, nitrogen oxides, and/or
other toxic or harmful substances that can be
hazardous to health and life and that are known
to the State of California to cause cancer, birth
defects, and other reproductive harm.
• Do not disconnect pipe fittings on boiler or
in heating system without first verifying system
is cool and free of pressure and that your
clothing will protect you from a release of hot
water or steam. Do not rely solely on boiler
temperature and pressure gauge when making
this judgement.
• Install all guards, cover plates, and
enclosures before leaving boiler in operation.
• Disconnect electrical supply before installing
or performing maintenance.
• Do not install above 2,000 ft.
• Not for use with LP gas.
!NOTICE:
1. Size boiler properly. A grossly oversized boiler
will cycle excessively leading to premature
failure of boiler and its components. Warranty
does not cover damage from excessive
cycling.
2. Systems employing baseboard and/or
radiators, use an industry accepted
sizing method such as the I=B=R "Residential
Hydronic Heating Installation and Design"
guide published by the Air Conditioning,
Heating and Refrigeration Institute (AHRI).
3. Inspect shipment carefully for signs of
damage. See Figure 5-1 for parts shipped
loose with packaged boilers. Any claim
for damage or shortage in shipment
must be filed immediately against carrier
by consignee. No claims for variances
or shortages will be allowed by boiler
manufacturer, unless presented within 60
days after receipt of equipment.
1Qualified Installer, Service Agency or Gas Supplier Read Before Proceeding

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SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
2User/Homeowner Read Before Proceeding
CAUTION
Burn Hazard.
Keep children and pets away from hot surfaces
of the boiler including boiler piping, vent piping,
and vent terminals.
!
NOTICE: Protect your home in freezing weather.
A power outage, operating control, or component
failure will prevent your boiler from lighting. In
winter your pipes may freeze and cause extensive
property damage. Do not leave heating system
unattended during cold weather unless alarms
or other safeguards are in place to prevent such
property damage.
NOTICE: Boiler may leak water or steam at the
end of its useful life. Be sure to protect walls,
carpets, and valuables from water or steam that
could leak from boiler.
WARNING
Asphyxiation Hazard. Fire Hazard.
• A qualified installer, service agency or gas
supplier should annually inspect boiler.
• If you are not qualified to install or service
boilers, do not install or service this one.
• Carbon monoxide is an odorless, deadly gas
that may be introduced into your home by any
malfunctioning fuel burning product or vent
system failure. Consider installing CO alarms
near bedrooms in all levels of building to
warn you and your family of potential CO
exposure.
• Do not block air flow into or around boiler
or opening for fresh air into boiler room.
Insufficient air may cause the boiler to produce
carbon monoxide or start a fire.
!

5111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
Table 3-1: Ratings
SteamMax Gas Series
Model
Number
Input1
(MBH)
Output
(MBH)
Net AHRI Rating2
AFUE
(%)
Thermal
Efficiency
(%)
Combustion
Efficiency (%)
Steam
(square foot)
Steam
(MBH)
STMX075 75 62 196 47 82.0
STMX100 100 82 258 62 82.0
STMX125 125 103 321 77 82.0
STMX150 150 124 388 93 82.0
STMX175 175 144 450 108 82.0
STMX200 200 165 517 124 82.0
STMX250 250 206 646 155 82.0
STMX299 299 246 771 185 82.0
STMX350 350 284 890 213 81.0 82.5
STMX399 399 323 1012 242 81.0 82.5
STMX450 450 365 1143 274 81.0 82.5
STMX500 500 405 1269 304 81.0 82.5
STMX550 550 446 1397 335 81.0 82.5
1Input ratings can be used for elevations up to 2,000 ft.
2Net AHRI Ratings shown are based on a piping and pickup allowance of 1.333. The manufacturer
should be consulted before selecting a boiler for installations having unusual piping and pickup
requirements, such as intermittent operation, extensive piping systems, etc.
3Specifications

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SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
3Specifications (continued)
Boiler
Model
Number
Shipping
Weight
Shipping
Crate Depth
Shipping
Crate
Width
Shipping
Crate
Height
STMX100 480 37 32 47
STMX125 595 37 32 47
STMX150 595 37 32 47
STMX175 725 37 47 47
STMX200 725 37 47 47
STMX250 840 37 47 47
Table 3-3: Shipping Weights and Dimensions
Boiler
Model
Number
No. of
Sections Height
(in)
Width (in) Depth1
(in)
Supply2
(NPT)
Return
(NPT)
Vent3
(in)
Gas4
(NPT)
Relief
Valve
(NPT)
Min.
Relief
Valve
Capacity
(lb./hr.)
STMX075 3 41 16 5/16 30 (1) 2 2 5 1/2 3/4 62
STMX100 3 41 16 5/16 30 (1) 2 2 5 1/2 3/4 82
STMX125 4 41 21 3/8 30 (1) 2 2 6 1/2 3/4 103
STMX150 4 41 21 3/8 30 (1) 2 2 6 1/2 3/4 124
STMX175 5 41 26 7/16 30 (2) 2 2 7 3/4 3/4 144
STMX200 5 41 26 7/16 30 (2) 2 2 7 3/4 3/4 165
STMX250 6 41 31 1/2 30 (2) 2 2 8 3/4 3/4 206
STMX299 7 41 36 9/16 30 (2) 2 2 8 3/4 3/4 246
STMX350 8 41 41 5/8 30 (3) 2 2 6 & 7 1/2 & 3/4 3/4 284
STMX399 9 41 46 11/16 30 (3) 2 2 6 & 8 1/2 & 3/4 1 323
STMX450 10 41 51 3/4 30 (4) 2 2 7 & 8 3/4 & 3/4 1 365
STMX500 11 41 56 13/16 30 (4) 2 2 8 & 8 3/4 & 3/4 1 405
STMX550 12 41 61 7/8 30 (4) 2 2 8 & 8 3/4 & 3/4 1 446
1Depth less draft hood. Depth with draft hood installed is 40 in.
2(3) 2 indicates three 2 inch risers.
36 & 7 indicates both 6 and 7 inch vent connections.
41/2 & 3/4 indicates both 1/2 and 3/4 inch gas connections.
Drain connection = 3/4 inch NPT
Maximum allowable working pressure (MAWP) = 15 psi
Table 3-2: Dimensions and Connections

7111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
3Specifications (continued)
Figure 3-4: Dimensional Drawing STMX075 through STMX299
Boiler A B C D
STMX075 16 5/16 5 6 8 1/2 STMX100 16 5/16 5 6 8 1/2 STMX125 21 3/8 6 6 1/2 11 STMX150 21 3/8 6 6 1/2 11 STMX175 26 7/16 7 7 13 1/2 STMX200 26 7/16 7 7 13 1/2 STMX250 31 1/2 8 7 1/2 16 STMX299 36 9/16 8 7 1/2 18 1/2
Left Side Right SideFront
6
1 2
"
'A'
8
3
16
"
'D'
8
3
16
"
5
3 8
"
2" Riser (Required STMX175 - STMX299)
2" Riser
(All sizes)
3
1 2
"
26"
16
1 2
"
20"
26 5
8
"
NWL
Auto Reset
Pressure Limit
BVS
Auto Reset
LWCO
Drain
Valve
Gas Line Opening
Gauge
Glass
Bottom
of Boiler
11
1
16
"
40"
'C'
28
1 2
"
'B'
40
1 2
"
40
3 4
"
Relief Valve
Skimming Connection
Alternate Gas Line Opening

8
SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
3Specifications (continued)
Figure 3-5: Dimensional Drawing STMX350 through STMX550
Boiler A B C D E F G
STMX350 41 5/8 23 3/16 NA 11 17 1/2 6 7 STMX399 46 11/16 23 3/16 NA 11 20 6 8 STMX450 51 3/4 18 3/16 28 3/16 13 1/2 22 1/2 7 8
STMX500 56 13/16 23 3/16 33 3/16 16 25 8 8
STMX550 61 7/8 23 3/16 38 3/16 16 27 1/2 8 8
Left Side Right SideFront
6
1 2
"
'A"
8
3
16
"
8
3
16
"
'B'
'C'
5
3 8
"
2" Riser
(All sizes)
3
1 2
"
26"
20"
16
1 2
"
26 5
8
"
NWL
BVS
Gauge
Glass
Auto Reset
Pressure Limit
Gas Line Opening
Auto Reset
LWCO
Drain
Valve
Bottom
of Boiler
11
1
16
"
40"
28
1 2
"
40
1 2
"
40
3 4
"
Relief Valve
BVS
Skimming Connection
Alternate Gas Line Opening
'D'
'E'
'F'
'G'
6
1 2
"
'A"
8
3
16
"
8
3
16
"
'B'
'C'
5
3 8
"
2" Riser
(All sizes)
3
1 2
"
26"
20"
16
1 2
"
26 5
8
"
NWL
BVS
Gauge
Glass
Auto Reset
Pressure Limit
Gas Line Opening
Auto Reset
LWCO
Drain
Valve
Bottom
of Boiler
11
1
16
"
40"
28
1 2
"
40
1 2
"
40
3 4
"
Relief Valve
BVS
Skimming Connection
Alternate Gas Line Opening
'D'
'E'
'F'
'G'

9111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
Table 3-7: Trim and Control Installation in Section Tappings
Tapping Size
(NPT) Function
A 2 Supply
B 2 Return and/or Bushed Down for Drain
C 2
Skimming Connection or Bushed Down for Auto
Reset Pressure Limit, Manual Reset Pressure Limit
and Pressure Gauge
D 1/2 Gauge Glass
E 3/4 Auto Reset LWCO
F 1 Relief Valve
G 3/4 Manual Reset LWCO
H 1-1/4 IWH Connection
Figure 3-6: Section Tappings
3Specifications (continued)
Intermediate Section
Left End Section Right End Section
F D C
A
B
E
D
G H H
C
D F
E
D
B
G

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SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
4Locating Boiler
Code Requirements
2. All wiring must comply with National Electrical
Code ANSI/NFPA 70.
3. Where required by authority having
jurisdiction, installation must conform to
Standard for Controls and Safety Devices for
Automatically Fired Boilers, ANSI/ASME CSD-1.
Location Considerations
1. Provide combustion and ventilation air in
accordance with section "Air for Combustion and
Ventilation," of National Fuel Gas Code, ANSI
Z223.1/NFPA 54, or applicable provisions of local
building codes.
2. If replacing an existing boiler, check for and
correct common system problems including:
A. System leaks resulting in premature heat
exchanger failure from oxygen corrosion or
hardness deposits
B. Inadequate freeze protection resulting in
system freezing and leaking
C. Dirt or debris left in existing piping if it has not
been properly flushed or cleaned
3. Appliance is listed for installation on combustible
flooring and must not be installed on carpeting.
4. Install on level floor. For basement installation
provide concrete base if floor is not level or if
water may be encountered on floor around boiler.
5. Protect gas ignition system components from
water (dripping, spraying, etc.) during appliance
operation and service.
6. Locate boiler to avoid water damage in case
there is a leak. If boiler must be located in an
area sensitive to water damage, install drain
pan underneath boiler and pipe to a suitable
drain location. Manufacturer will not be held
responsible for water damage resulting from
this appliance or any of its components.
7. Check for and remove any combustible
materials, gasoline, or other flammable liquids
from area around boiler.
8. Check for and remove any potential combustion
air contaminants from area around boiler. See
Table 4-1.
Clearances
1. Provide clearances between boiler jacket and
combustible material in accordance with
authority having jurisdiction. Minimum
clearances to combustible material shown in
Figure 4-2 and 4-3.
2. Recommend 24 in. service clearance from left
side, right side and front.
NOTICE: If recommended service clearances
are not provided, it may be necessary to remove
boiler for service.
3. Boiler must not be installed on carpeting.
1. Installations must conform to requirements of
authority having jurisdiction or, in the
absence of such requirements, to National Fuel
Gas Code, ANSI Z223.1/NFPA 54.
Table 4-1: Corrosive Combustion Contaminants
and Sources
Contaminants to avoid:
Spray cans containing chloro/fluorocarbons (CFC’s)
Permanent wave solutions
Chlorinated waxes/cleaners
Chlorine-based swimming pool chemicals
Calcium chloride used for thawing
Sodium chloride used for water softening
Refrigerant leaks
Paint or varnish removers
Hydrochloric acid/muriatic acid
Cements and glues
Antistatic fabric softeners used in clothes dryers
Chlorine-type bleaches, detergents, and cleaning
solvents found in household laundry rooms.
Adhesives used to fasten building products and
other similar products
Excessive dust and dirt
Areas likely to have contaminants:
Dry cleaning/laundry areas and establishments
Swimming pools
Metal fabrication plants
Beauty shops
Refrigeration repair shops
Photo processing plants
Auto body shops
Plastic manufacturing plants
Furniture refinishing areas and establishments
New building construction
Remodeling areas
Garages with workshops
NOTICE: Flue side corrosion caused by
contaminants is not covered by warranty.

11111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
NOTICE: 24 in. service clearance recommended
from left side, right side, and front.
4Locating Boiler (continued)
Figure 4-2: Minimum Alcove Clearances STMX075 through STMX299
Boiler A
STMX075 16 5/16
STMX100 16 5/16
STMX125 21 3/8
STMX150 21 3/8
STMX175 26 7/16
STMX200 26 7/16
STMX250 31 1/2
STMX299 36 9/16
'A'
40
3
4
"
16
1
4
"
6" 18"
57"
6"
6"
Bottom of
Boiler

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SteamMax Installation, Operating & Service Instructions
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4Locating Boiler (continued)
Figure 4-3: Minimum Alcove Clearances STMX350 through STMX550
Boiler A
STMX350 41 5/8
STMX399 46 11/16
STMX450 51 3/4
STMX500 56 13/16
STMX550 61 7/8
NOTICE: 24 in. service clearance recommended
from left side, right side, and front.
'A' 18" 18"
72"
40
3
4
"
31
1
4
"
6.00
6.00
Bottom
of Boiler

13111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
4Locating Boiler (continued)
After Removing Existing Boiler
When an existing boiler is removed from a
common venting system, the common venting
system is likely to be too large for proper venting
of appliances remaining connected to it.
At time of removal of an existing boiler, the
following steps shall be followed with each
appliance remaining connected to the common
venting system placed in operation, while the
other appliances remaining connected to the
common venting system are not in operation:
1. Seal any unused openings in the common venting
system.
2. Visually inspect venting system for proper size
and horizontal pitch and determine there is no
blockage or restriction, leakage, corrosion, and
other deficiencies which could cause an unsafe
condition.
3. Insofar as is practical, close all building doors
and windows and all doors between the space
in which the appliances remaining connected
to the common venting system are located and
other spaces of building. Turn on clothes dryers
and any appliance not connected to the common
venting system. Turn on any exhaust fans, such
as range hoods and bathroom exhausts, so they
will operate at maximum speed. Do not operate a
summer exhaust fan. Close fireplace dampers.
Before Removing Existing Boiler
Take pictures and measure and/or mark existing
steam and condensate return piping to ensure
proper re-piping of new boiler. Support existing
piping before disconnecting to prevent change in
pitch.
4. Place in operation appliance being inspected.
Follow Operating Instructions. See Figure 10-
1. Adjust thermostat so appliance will operate
continuously.
5. Test for spillage at draft hood relief opening after 5
minutes of main burner operation. Use flame of a
match or candle, or smoke from a cigarette, cigar
or pipe.
6. After it has been determined that each appliance
remaining connected to the common venting
system properly vents when tested as outlined
above, return doors, windows, exhaust fans,
fireplace dampers and any other gas-burning
appliance to their previous condition of use.
7. Any improper operation of the common venting
system should be corrected so the installation
conforms with the National Fuel Gas Code, ANSI
Z223.1/NFPA 54. When resizing any portion of the
common venting system, the common venting
system should be resized to approach minimum
size as determined using appropriate tables in
Chapter 13 of the National Fuel Gas Code, ANSI
Z223.1/NFPA 54.

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SteamMax Installation, Operating & Service Instructions
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5Preparing Boiler
Remove boiler and miscellaneous parts carton from
outer shipping carton.
Unpack Boiler
Figure 5-1: Packaged STMX100 through STMX250
DANGER
Use precautions and appropriate rigging
apparatus when moving heavy objects.
!
Relief Valve
Drain Valve
BVS
Misc. Parts Carton
Vent Damper
Rear Air Dam
Draft Hood Carton
Pressure Gauge
Boiler

15111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
6Venting
General
1. Locate boiler as close to chimney as possible.
See Figure 4-2 and 4-3 for minimum clearances.
2. Install vent system in accordance with local
codes, authority having jurisdiction or "National
Fuel Gas Code, ANSI Z223.1/NFPA 54. SteamMax
boilers are Category I, draft hood equipped
appliances. A vent damper is provided with
STMX100 through STMX299 boilers. STMX350
through STMX550 boilers do not have vent
dampers.
A. Acceptable Chimneys
i. Listed Type B or Type L gas
vent. Install in accordance with
manufacturer's installation
instructions, local codes, authority
having jurisdiction or National Fuel
Gas Code, ANSI Z223.1/NFPA 54.
ii. Masonry chimney. Build and install
in accordance with local codes,
authority having jurisdiction or
Standard for Chimney's Fireplaces,
Vents and Solid Fuel Burning
Appliances, NFPA 211.
B. Acceptable Vent Connections
i. Listed Type B or Type L gas vent.
ii. Single wall galvanized vent. Use
0.018" (26 gauge or heavier). The
size and location of the chimney
may not permit the use of a single
wall connector in some cases. See
National Fuel Gas Code, ANSI
Z223.1/NFPA 54.
Do not use single wall pipe for vent
connections in attics.
C. Size chimney and vent connector in
accordance with National Fuel Gas Code,
ANSI Z223.1/NFPA 54.
3. Do not connect vent of this appliance into any
portion of a mechanical vent system operating
under positive pressure.
4. Do not connect boiler into a chimney flue
servicing an open fireplace or other solid fuel
appliance.
5. Prior to boiler installation, inspect chimney for
obstructions or other defects and correct as
required. Clean chimney as necessary.
6. The vertical section of vent pipe coming off
boiler should be as tall as possible, while still
maintaining the proper clearance from horizontal
vent connector to combustibles.
7. Vent pipe should slope upward from draft hood
not less than one inch in four feet. No portion of
vent pipe should run downward or have sags.
Vent pipe must be securely supported.
8. Vent pipe should be installed above bottom of
chimney to prevent blockage.
9. Vent pipe must be inserted flush with inside face
of chimney liner and the space between vent pipe
and chimney sealed tight.
10. Do not install vent damper in any portion of vent
system which is used by appliances other than
the boiler being installed.
Figure 6-1: Vent Damper, Rear Air Dam and BVS
Installation Diagram (STMX075 through STMX299)
WARNING
Asphyxiation Hazard.
• Failure to vent this boiler in accordance with
these instructions could cause products of
combustion and/or carbon monoxide to enter
living space, resulting in severe personal injury,
death or substantial proper damage.
• Vent installation shall be in accordance with local
codes, authority having jurisdiction or National
Fuel Code, ANSI Z223.1/NFPA 54 or applicable
requirements of the local building codes.
• Follow instructions in Section 4 Locating Boiler
when removing a boiler from an existing vent
system.
• Maintain clearances to vent piping and draft
hood per Figures 4-2 and 4-3.
• Do not use CPVC, PVC, Polypropylene or any
other non-metallic vent pipe.
!
Vent Damper
Rear Air Dam
Draft Hood
Weld Stud
#6 SMS
BVS
Wires are
interchangeable

16
SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
6Venting (continued)
A. Vent damper must be same size as outlet
of vent supplied with boiler (see Table 3-2).
Unpack damper carefully - DO NOT FORCE IT
CLOSED! Forcing damper may damage motor
and is not covered by warranty.
Insert pilot vent plug into gate and fold over
tabs.
B. Mount the vent damper assembly onto diverter.
See damper manufacturer instructions for more
information. Do not modify either diverter or
vent damper. See Figure 6-1.
NOTICE: Provide adequate access for
servicing.
C. Locate vent damper position indicator to be
visible following installation. See Figure 6-2.
D. Plug vent damper connector into damper
motor polarized receptacle.
3. Install Blocked Vent Switch (BVS). See Figure
6-1.
A. Attach BVS to diverter using supplied #6
sheet metal screws.
B. Attach Black/White (BVS) and Black (BVS/
FRS) quick connects to BVS.
Wires are interchangeable.
4. Install vent piping.
A. See Figure 3-4 for vent sizes and locations.
See Figure 4-2 for clearances.
B. Install vent piping from vent damper outlet
to chimney. See Figure 6-3 for typical vent
installation.
WARNING
This installation is not complete until rear air
dam is installed. Product performance will be
negatively affected if rear air dam is not installed
which can cause severe personal injury or
death.
!
Figure 6-2: Vent Damper Position Indicator
(shown in open position)
CAUTION
Do not use one vent damper to control more
than one heating appliance.
!
Single Draft Hood System
(STMX075-STMX299)
1. Install vent damper.
OPEN VENT DAMPER CARTON and remove
Installation Instructions. READ INSTALLATION
INSTRUCTIONS THOROUGHLY before
proceeding.
2. Install rear air dam.
Hang air dam on screws located at back of
boiler. See Figure 6-1 for location.
Figure 6-3: Typical Single Draft Hood Installation
STMX075 through STMX299
Bottom
of Boiler
Air Gap
Front
of Boiler
Cleanout
Thimble
Chimney
Slope Up

17111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
6Venting (continued)
Dual Draft Hood System
(STMX350-STMX550)
1. Install rear air dams. Hang air dams on screws
located at back of boiler. See Figure 6-5 for
reference.
2. Install Blocked Vent Switches (BVS).
A. Attached one BVS to each diverter using
supplied #6 sheet metal screws. See Figure
6-5.
B. Attached Black/White (BVS) and Black (BVS/
FRS) quick connects to right BVS. Wires are
interchangeable.
C. Attach Black (BVS/FRS) and White (BVS/
Relay) quick connects to left BVS. Wire are
interchangeable.
3. Install vent piping.
A. See Figure 3-5 and Table 6-4 for vent
sizes and locations. See Figure 4-3 for
clearances.
B. Install vent piping from vent damper outlets
to chimney. See Figure 6-6 for typical vent
installation.
WARNING
This installation is not complete until rear air
dams are installed. Product performance will
be negatively affected if rear air dams are not
installed which can cause severe personal injury
or death.
!
Model
Minimum Common
Vent Connector Inside
Diameter (in)
STMX350 9
STMX399 9
STMX450 10
STMX500 10
STMX550 12
Figure 6-5: Typical Vent Installation
STMX350 through STMX550
Figure 6-6: Typical Dual Draft Hood Installation
STMX350 through STMX550
Bottom
of Boiler
End Cap
Air Gap
Front
of Boiler
Cleanout
Thimble
Chimney
Common Vent
Connector
Slope Up
Table 6-4: Common Vent Connector Sizes
Draft Hood
Rear Air Dam
Weld Stud
BVS
#6 SMS
Wires are
interchangeable
BVS

18
SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
7 Steam Piping
NOTICE:
Boiler manufacturer does not recommend use of copper pipe for the following reasons:
1. High coefficient of thermal expansion can induce mechanical stresses and cause expansion/ contraction
noises if not accounted for in piping system design and installation,
2. High heat transfer rate (heat loss) of uninsulated copper piping must be included in normal piping and
pickup factors used to size the boiler,
3. Soldering or brazing pastes and fluxes that end up in the system can cause poor heat transfer, surging, an
unsteady water line and wet steam if not thoroughly removed during the skimming procedure and,
4. Galvanic corrosion of adjoining metal may occur due to dissimilar metals in certain water
chemistry if dielectric unions are not used.
General Piping Notes
Primary purpose of this near boiler piping is to
separate tiny water droplets from steam exiting
the boiler so that “dry” steam is sent to distribution
system. If near boiler piping is not correct, wet
steam will enter distribution system and the
following problems may occur:
• Short cycling on low water in boiler
• Boiler or system flooding
• Water hammer
• Failure to heat one or more radiators
Avoid three common piping mistakes shown in
Figure 7-1. This applies even if existing boiler has
one of the piping mistakes shown in Figure 7-1
and appears to be working. If two or more steam
mains must be connected to boiler, connect a
separate take-off for each main into the header
between riser(s) and equalizer. Also note the
following points:
1. A size reduction must be made to connect header
and equalizer. This reduction must be made in the
equalizer line. Do not make this size reduction in
the horizontal header.
2. One pipe steam systems require air vents on
each radiator, as well as at the end of each main.
For system to work properly, these vents must
be properly installed, sized, and be in good
condition. Inspect and replace any defective
vents. If there are no vents at ends of the mains,
install them.
3. For installations with condensate or boiler feed
pumps, follow pump manufacturer’s piping
instructions. Such systems generally do not
require Hartford loops.
4. Do not use a check valve in place of, or in
addition to, a Hartford loop on a gravity return
system.
5. Pipe fill connection from a clean source of water.
When water supply is from a well, make sure that
a strainer is installed in the well system.
WARNING
• Do not use softened water in steam boilers. Accelerated boiler corrosion will result. Tie in fresh water supply
to boiler upstream of a water softener.
• Oxygen contamination of boiler water will cause corrosion of iron and steel boiler components, and can
lead to boiler failure. Check for and correct system leaks.
!

19111043-03 - 9/21
SteamMax Installation, Operating & Service Instructions
7 Steam Piping (continued)
Figure 7-1: Common Near Boiler Piping Mistakes
WRONG
Take-off
between risers.
WRONG
Header bullheaded
into take-off and
equalizer.
WRONG
Take-off bullheaded
into two mains.

20
SteamMax Installation, Operating & Service Instructions
111043-03 - 9/21
Near Boiler Piping
1. Evaluate existing steam system.
Single most important factor in determining
expected life cycle of a steam boiler is the amount
of fresh water added to boiler during operation.
Fresh water brings minerals and oxygen into the
boiler. These contaminants greatly accelerate
corrosion of cast iron boiler sections.
Assure all system radiators, piping and vents are
absolutely leak tight.
A. When a steam boiler is installed in an
existing system, ALL air vents should be
replaced at the same time. This assures
the new boiler will not be compromised by
existing system leaks.
B. If system contains hidden supply or return
piping (hidden behind walls, buried in
concrete, etc.) pressure test this piping to
assure there are no leaks.
2. Repair any leaks in the system.
3. Install accurate water meter with counter on fresh
water supply to boiler.
4. Remove parts carton from boiler crate.
5. Install relief valve (spindle must be in vertical
position) into tapping on boiler right side using
nipple and elbow supplied. See Figures 3-4 and
3-5, 3-6, and Table 3-7.
DANGER
• Pipe relief valve discharge to a safe location.
• Do not install any valves between boiler and
relief valve.
• Do not install any valves between relief valve
and discharge.
• Do not move relief valve from factory location.
• Do not plug relief valve discharge.
• Do not install a relief valve with a setting greater
than 15 psi.
!
6. Install drain valve into tapping on boiler left side
using 2 x 3/4 bushing provided. See Figures 3-4,
3-5, 3-6 and Table 3-7.
7. See Figure 7-2 for near boiler piping of Hartford
Loop.
7 Steam Piping (continued)
Indirect Water Heater
All SteamMax boilers are equipped with tappings to
permit connection of an indirect water heater (IWH).
See Appendix B for installation.
WARNING
• Pipe discharge of relief valve to a location
where water or steam will not create a hazard or
cause property damage if the relief valve opens.
• End of discharge pipe must terminate in an
unthreaded pipe.
• If relief valve discharge is not piped to a drain it
must terminate at least 6 inches above floor.
• Termination of the relief valve discharge piping
must be in an area where it is not likely to
become plugged by debris or subjected to
freezing.
!