Rinnai 37AHB SERIES User manual

2100-536 Rev A
37AHB SERIES
HYDRONIC FURNACE
SIZES 045 THRU 090
Installation,Operation,andMaintenanceManual
Consumer Safety Information ................................2
Overview of the Rinnai Hydronic Furnace ............3
Model Number Nomenclature................................3
Physical Data .........................................................4
Receiving & Checking Equipment ......................... 5
Installation
Clearances.....................................................5
Locating and Mounting ...............................6-9
Plumbing .................................................10-13
Electrical Connections ............................14-16
Dip Switch Options....................................... 17
Thermostat Installation............................17-18
Start-Up Procedure ...................................... 19
Troubleshooting ......................................20-23
Sequence of Operation ...................................24-26
Maintenance...................................................26, 27
Selection Guide..............................................27, 28
Air Distribution Guide .....................................28, 29
Quick Reference Duct Sizing Chart ....................30
Hydronic Furnace’s Specifications.......................31
Air Delivery and Performance Data ................32-51
Accessories.......................................................... 59
Wiring Diagrams .............................................60-62
Parts List .........................................................63-64
Limited Warranty.............................................65-66
To register your hydronic furnace or tankless water heater,
please visit www.rinnairegistration.com.
Quality Assurance
This product is manufactured in a
facility registered by UL to ISO 9001.

2 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Consumer Safety Information
SAFETY DEFINITIONS
Indicates safety alerts. When this symbol is seen on the Hydronic Furnace and in all instructions and/or
manuals, be alert to the potential for personal injury. Recognize signal words DANGER, WARNING,
and CAUTION. These words are used with the safety alert symbol.
Indicates an imminently hazardous situation which, if not avoided, will result in death or
serious injury.
Indicates a potentially hazardous situation which, if not avoided, could result in death or
serious injury.
Indicates a potentially hazardous situation which, if not avoided, could result in minor or
moderate injury. It may also be used to alert against unsafe practices.
SAFETY CONSIDERATIONS
Before any work is undertaken, it is imperative to
observe all precautions as stated in this manual, on
tags, and/or labels, together with any other safety
measures that may apply.
• Wear safety glasses and work gloves.
• When practical, objects to be brazed shall be
moved to a designated safe location or, if the
objects to be brazed cannot be readily moved, all
movable fire hazards in the vicinity shall be taken
to a safe place, or otherwise protected.
• Use quenching cloth for all brazing and un-brazing
operations.
• Suitable fire extinguishing equipment shall be
immediately available in the work area and shall
be maintained in a state of readiness for instant
use.
Read these installation instructions carefully and
adhere to all WARNINGS and CAUTIONS. Consult
local building codes, Occupational Safety & Health
Administration (OSHA) and National Electrical Code
(NEC) for special requirements.
Improper installation, modification, service,
maintenance, or use of Hydronic systems can cause
electrical shock, burns or other conditions which may
cause personal injury or property damage. Consult a
qualified installer, service agency, or your distributor
for information or support. The qualified installer or
agent must use factory authorized kits and/or
accessories when installing this product. Refer to the
appropriate Rinnai® literature for listing.
Read the entire instruction manual before starting the installation.
CAUTION
WARNING
NOTICE This is used to highlight important information which will aid in installation, improve
reliability or enhance operation.
Application of this Hydronic Furnace should be
indoors. Special attention should be given to unit
sizing and piping, filling, and purging.
NOTICE
Failure to follow this caution may result in personal
injury. Sheet metal parts may have sharp edges or
burrs. Use care and wear appropriate protective
clothing.
CAUTION
DANGER
Before installing or servicing the Hydronic Furnace,
always turn off all power to unit. There may be more
than 1 disconnect switch. Electrical shock can cause
personal injury or death.
WARNING

Rinnai Corporation Hydronic Furnace (37AHB) Manual 3
INTRODUCTION:
The optimum in hydronic technology: the updated
Rinnai® multi-position Hydronic Furnaces offer a
unique solution for a wide variety of small and medium
sized residential and light commercial applications.
They are compact and ready to fit in tight spaces
which may include, but not limited to, attics,
basements, closets, crawlspaces, and utility rooms.
The 37AHB units are equipped with an intelligent
microprocessor control that allows for domestic hot
water priority and adapts to available hot water flow for
space heating by automatically regulating the pump
and blower sequence to maximize comfort.
These unique Hydronic Furnaces are designed to
work in combination with our line of Rinnai® tankless
products to deliver a wide variety of heating capacities
that cover the entire residential and light commercial
heating spectrum.
Because our units are designed specifically to the
Rinnai® tankless products, our stated capacities are
fine tuned and are based on the “Hydronic Furnace /
Tankless Water Heater” match set and NOT a given
water flow rate.
CODES AND STANDARDS:
It is the responsibility of the installer to follow all
national codes, standards and local ordinances, in
addition to instructions laid out in this manual. The
installation must comply with regulations of the local
building, heating, plumbing, and other codes. Where
local codes are not applicable, the installation must
comply with the national codes and any and all
authorities having jurisdiction.
Overview of the Rinnai Hydronic Furnace (RHF)
The following is a suggested list of codes and
standards for the United States and Canada:
General Installation
Installation of Air Conditioning and Ventilating Systems
NFPA 91 (latest edition)
Duct Systems
Sheet Metal and Air Conditioning Contractors National
Association (SMACNA)
American Society of Heating, Refrigeration, and Air
Conditioning Engineers (ASHRAE)
2001 Fundamentals Handbook Chapter 34 or 2000
HVAC Systems and Equipment Handbook Chapters 9
and 16
US and CANADA: Air Conditioning Contractors
Association (ACCA) Manual D
Acoustical Lining and Fibrous Glass Duct
US and CANADA: current edition of SMACNA; NFPA
90B as tested by UL Standard 181 for Class I Rigid Air
Ducts
Electrical Connections
US: National Electrical Code (NEC) ANSI/NFPA 70
(latest edition)
CANADA: Canadian Electrical Code CSA C22.1
(latest edition)
Plumbing Systems:
US and CANADA: ICC International Plumbing Code
(IPC); Uniform Mechanical Code (UMC); Uniform
Plumbing Code (UPC)
Model Number Nomenclature
37AH B 045 08 K A 5 Multi-position
5 = Yes
2 = No
Engineering Digit
Denotes minor change
(not present in sales or service
literature)
Voltage Code (V-Ph-HZ)
K = 115 - 1 - 60
L = 240 - 1 - 50 (export models)
Model
37AH - Multi-Speed Hydronic Furnace
Series
A - Unit with PSC Motor
B - Unit with ECM Technology
Nominal Heating
Capacity (BTU/h)
045 = 45,000
060 = 60,000
075 = 75,000
090 = 90,000
Cooling / Heating Air Flow Range (CFM)
08 = 800 (650-800)
12 = 1200 (650-1200)
14 = 1400 (1000-1600)
16 = 1600 (1200-1750)

4 Rinnai Corporation Hydronic Furnace (37AHB) Manual
6"
17 13/16"
1"
18 5/16"
1"
C
B
34"
A22"
E
13/8"
D
17 13/16"
21/8"
Table 1 - Physical Data
UNIT SIZE A B C D E
in. mm in. mm in. mm in. mm in. mm
37AHB04508KA5 14 355.6 18 457.2 12 304.8 10 - 1/2 266.7 19 482.6
37AHB06012KA5 17 -1/2 444.5 18 457.2 16 406.4 16 - 11/16 423.9 19 - 1/4 489.0
37AHB07514KA5 21 533.4 18 457.2 20 508 18 - 11/16 474.7 19 - 1/2 495.3
37AHB09016KA5 24 - 1/2 622.3 18 457.2 24 609.6 21 - 1/8 536.6 19 482.6
DIMENSIONS
Figure 1
Water In
Water Out
7/8” Dia. KO Supply
Power Wire Entry
(Typ. for both sides)
7/8” Dia. KO Thermostat Wire
Entry (Typ. for both sides)

Rinnai Corporation Hydronic Furnace (37AHB) Manual 5
Receiving and Checking Equipment
IDENTIFY UNIT
The unit model number and serial number are stamped
on the unit identification / name plate (located on the
top right side of unit). Check this information against
shipping papers and job requirements.
INSPECT SHIPMENT
Upon receipt of a 37 Series Hydronic Furnace the
packaging should be checked for peripheral signs of
transportation damage while unit is still in the shipping
package. If unit appears to be damaged or is torn
loose from its anchorage, the unit shall be immediately
examined by the receiving party before removal. If
damage is found, the receiving party must sign the
driver’s delivery receipt noting all damage (i.e. carton
damage and/or product damage) as well as contact the
last carrier immediately, preferably in writing,
requesting inspection by the carrier’s agent. All claim
papers MUST be forwarded to Rinnai® America
Corporation for processing. In general, upon receipt of
product, be sure to check all items against shipping list;
if items are found to be missing, it should be noted as
such on the driver’s delivery receipt; and the receiving
party shall also immediately notify the area
distributor. To prevent loss or damage, leave all parts
in original packages until installation.
Installation
Figure 2: Minimum Clearance to Combustible
Construction
The 37AHB Series Hydronic Furnace needs to be
installed and commissioned by a knowledgeable
qualified professional.
NOTES:
1. This Furnace is approved for up-flow, down-flow,
and horizontal configurations.
2. Clearance arrows do not change with Hydronic
Furnace orientation.
3. This Hydronic Furnace is for indoor
installation only.
4. Unit(s) shall be installed in such a way as to
ensure that the electrical components are
protected from any contact with water.
5. Unit(s) shall not be installed directly on any
combustible material other than wood flooring.
6. This unit is designed to be used with an air
distribution system (ductwork). Refer to section
the Air Distribution.
7. The installer shall provide ample space for
servicing and cleaning. Always comply with
minimum clearances as shown in Figure 2.
8. The 37AHB units are designed to be installed
vertically or horizontally on the floor; units may
also be hung from the ceiling or wall. Be sure to
allow appropriate clearances for wiring, piping,
and servicing.
Do not install this unit if the unit is damaged.
Do not install this unit if any part or all of unit has been
under water. Refer to the Receiving and Checking
Equipment section.
WARNING
TOP/PLENUM
zero
SIDE
zero
BACK
zero
SIDE
zero
FRONT
zero
BOTTOM
zero
SERVICING
24” (610 mm)

6 Rinnai Corporation Hydronic Furnace (37AHB) Manual
LOCATING AND MOUNTING THE HYDRONIC
FURNACE
General
The multi-position 37AHB Series Hydronic Furnaces
are shipped in packaged configuration. This means
that the units may be installed without assembly and/
or modifications when configured for bottom return air
inlet application; however, some modifications and
assembly are necessary if units are to be installed in
an application that requires side return air inlet
arrangement. For instructions on required
modifications and assembly refer to Figures 3 and 4.
Installation
Figure 3: Modification of Unit to Accommodate
Side Filter Rack Installation
USE EXISTING SCREW HOLE
TO LOCATE FILTER RACK
17 3/4"
23/16"
1/8"
21 11/16"
3/8"
17 15/16"
Figure 4: Side Filter Rack Installation
NOTE: For side return application, obtain Side Filter
Rack” and “Bottom Fill Plate from your area
authorized Rinnai® distributor.

Rinnai Corporation Hydronic Furnace (37AHB) Manual 7
Installation
Upflow Installations
The 37AHB Hydronic Furnace is ready to install in the
up-flow position without modifications.
The unit MUST be supported on the bottom ONLY and
set on a field supplied supporting frame or plenum.
Supporting frame or plenum must be anchored to the
unit and to the floor or wall.
The 37AHB Furnaces are shipped without a bottom fill
plate. If side return installation is desired, the return
opening (Bottom) must be blanked off. If a bottom fill
plate is required, install only a factory authorized
bottom fill plate. Refer to the Rinnai Accessory list for
details. For side filter rack installation instructions
refer to Figures 3 and 4.
Downflow Installation
The 37AHB Hydronic Furnace is ready to install in the
down-flow position without assembly or modifications
when configured for bottom return air inlet installation.
If side return air inlet installation is desired refer to
Figures 3 and 4.
Horizontal Left and Right Installations
Without Cased Coil:
If a cased coil is NOT being installed, the cabinet can
be placed on either side for horizontal airflow as
shipped, when configured for bottom return air inlet
installation. If side return air inlet installation is desired
refer to Figures 3 and 4.
With Cased Coil (Field Supplied):
Refer to the manufacturer’s Cased Coil installation
instructions for details.
Blower located to the right of
coil section. Conditioned air is
discharged to the left.
Blower located below coil
section. Conditioned air is
discharged upward.
Blower located to the left of coil
section. Conditioned air is
discharged to the right.
Blower located above coil
section. Conditioned air is
discharged downward.
Figure 5: Multi-Position Orientation

8 Rinnai Corporation Hydronic Furnace (37AHB) Manual
IMPORTANT: When a 37AHB unit is matched with an
evaporative type (cased coil/condensing unit) split
system for cooling application and the system is
installed above a finished ceiling and/or an occupied
space, building codes may call for a secondary
insulated condensate pan (by others) to be installed
under the entire unit. In other instances, some local
codes may allow the running of a separate, secondary
condensate line in lieu of the required drain pan. It is
the responsibility of the installer to consult local codes
for compliance.
It is the installer’s responsibility to use an appropriate
hanging method capable of supporting the unit’s
weight. Refer to the specification section of this
document for the respective unit’s installed weights.
WARNING
Installation
CLOSET INSTALLATION (RETURN AIR THRU
OPENING OR GRILL)
The 37AHB Hydronic Furnace can be installed in a
closet on a supporting stand or be mounted from the
closet wall using the closet as the return air plenum.
Unit should be high enough from the floor to provide
unimpeded return air flow into the bottom of the
cabinet.
Closet return air opening can be on the front (in closet
door), side (thru the wall) or a combination of both,
providing there is clearance on the sides between
unit’s cabinet and closet. Refer to ACCA Manual D or
SMACNA for sizing and free area recommendations.
NOTE: Local codes may limit application of systems
without a ducted return to single story dwellings.
Figure 6: Horizontal Unit Suspension
SUSPENDED CABINET INSTALLATION
If the cabinet cannot be supported on a frame or
supported from the wall, it may be suspended.
Use metal strapping or threaded rod with angle iron
under cabinet for support. These supports MUST run
parallel with the length of the cabinet (see Figures 6
and 7).
Ensure that there is adequate room to remove service
and access panels after installing supporting brackets.
If an auxiliary drain pan is required, the support is to
be placed under the drain pan. In such installations
the unit will need to be supported on vibration isolators
(rubber or Styrofoam blocks).
For seismic hanging requirements, refer to local
codes.
NOTICE
DOOR
ASSEMBLY
8” MIN FOR
DOOR REMOVAL
(2) HEX NUTS, (2) WASHERS & (2)
LOCK WASHERS REQ. PER ROD
USE 1” SQUARE, 1-1/4 x 1-1/4 x 1/4
ANGLE IRON OR EQUIVALENT
SECURE ANGLE
IRON TO BOTTOM
OF CABINET WITH 3
#8 x 3/4” SCREWS
TYPCIAL FOR 2
SUPPORTS
1/4” THREADED ROD
(4 REQUIRED)

Rinnai Corporation Hydronic Furnace (37AHB) Manual 9
Installation
DUCT CONNECTIONS
Supply Duct
The supply ductwork must be attached to the outside
of the flange on the air discharge end of unit. Flexible
connectors may be used if desired.
Return Duct
The return ductwork should be attached to the air
return side (bottom or side) of unit using sheet metal
screws or other fasteners.
For side return air inlet installation see the Figures 3
and 4.
FILTER INSTALLATION
Internal filter rack and a 1 inch disposable filter are
standard on all models. Refer to the Specifications
section for dimensions.
AIR SYSTEM
Existing Ductwork
It is the responsibility of the installer to inspect all
previously installed air distribution systems to
determine its suitability for the new heating and/or
cooling system. Existing ductwork may have to be
modified and/or insulated to provide satisfactory air
distribution.
Ductwork Installation
Connect the supply-air duct over the outside of 3/4-in.
flange on the unit’s discharge side. Secure the duct to
the flange with proper fasteners for the type of duct
used. Support the duct independently.
Use flexible connectors (if desired between the
ductwork and the unit to prevent transmission of
vibration.
Use insulation with vapor barrier for ductwork passing
Attachment Methods Using Straps
Method 1
Use (4) #8 x 3/4 sheet metal screws for each strap.
Straps to be vertical against the unit’s sides and not
allowed to be pulled away from the sides.
Method 2
Fold all straps under the furnace and secure with (4)
#8 x 3/4 sheet metal screws (2 screws at the side and
2 screws at the bottom. (Care must be taken not to
drive the screw through the coil.)
Figure 7: Horizontal Unit Suspension with Straps
The air inlet is not allowed to be at
the front or back of the furnace
Front
Back
Do not position the furnace on its
back or with it face down.
COIL
INTERFACE
AREA
PROHIBITED INSTALLATIONS
Figure 8:
Figure 9:
NOTE:
Multiple Furnace configured for installation with a
single Rinnai Tankless Water Heater is prohibited.
DOOR
ASSEMBLY
1 INCH x 22 GAUGE
GALVANIZED STRAPS
TYPICAL FOR 4 STRAPS
RETURN AIR
OPENING
SUPPLY AIR OPENING
BACK OF UNIT

10 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Installation
PLUMBING
Codes:
Observe all local sanitary codes when installing water
lines. The water supply mating connection to the
37AHB Hydronic Air-Handling Units are made via the
two (3/4 in. Dia. X 2-1/2 in. Long) copper stubs to the
front-left of the unit labeled “WATER IN” and “WATER
OUT” (see Figure 1). Mating connectors to be two
field supplied 3/4 in. FNPT-sweat ends or two field-
supplied 3/4 in. SharkBite type FNPT-push fitting ends
or equivalent.
All associated hydronic piping MUST comply with ICC,
UPC and any other local codes or ordinances having
jurisdiction. USE POTABLE GRADE COPPER OR
OTHER PIPING MATERIALS. MATERIALS TO BE
LEAD FREE APPURTENANCES ONLY.
NOTE: Recommended piping, fittings, valves and
other appurtenances (exclusive of those indicted as
accessories that are available through Rinnai
distribution) called for in piping schematics to be field-
supplied.
Flow Sensor Installation:
(Required for Open Loop Systems)
Care must be taken to ensure that the flow sensor is
not damaged due to excessive tightening. The torque
must not exceed the maximum limit stated below. The
installation should be checked to ensure that no
leaking is evident.
Mating connectors to be (2) 3/4” FNPT fittings (field
supplied).
Pipe-work/connector alignment is imperative (avoid
bending stress).
Polytetrafluoroethylene (PTFE) thread seal tape (teflon
tape), or equivalent, is recommended.
Tighten fittings to maximum torque of 15lb/ft (20Nm).
Soldering Copper Tubing:
The common method of joining copper tubing in
hydronic heating systems is soft soldering. Plumbing
codes do not allow solders containing lead to be used
for domestic water service. USE ONLY 95/5 tin/
antimony solder for all piping systems that incorporate
a domestic water supply.
Note: Precautions must be taken during soldering to
avoid debris or solder from lodging in piping
system.
Mechanical Joining of Tubing:
Where used, refer to the respective mechanical
system manufacturer’s installation instructions.
Tubing Insulation:
Any tube conveying fluid at a temperature greater than
that of the surrounding air releases heat.
Insulate all accessible hot water lines and associated
valves with material, such as expanded neoprene or
polyurethane 3/8-in. to 1⁄2-in. thick.
Match the pipe sleeve's inside diameter to the pipe’s
outside diameter for a snug fit. Place the pipe sleeve
so the seam will be face down on the pipe. Tape,
wire, or clamp insulation every foot or two to secure it
to the pipe. If taping is desired, use acrylic tape
instead of duct tape.
Copper Tubing Support:
Copper tubing must be properly supported to prevent
sagging or buckling. On horizontal runs with hard
temper tubing, the following maximum support spacing
is suggested:
• 1/2 in. to 3/4 in. tube: 5 feet maximum spacing
• 1 in. to 1-1/4 in. tube: 6 feet maximum spacing
• 1-1/2 in. to 2 in. tube: 8 feet maximum spacing
The above suggested spacing does not account for
extra weight of piping components such as an
expansion tank, etc. When such components are
present the piping should be supported immediately
adjacent to the component.
On vertical runs, copper tubing should be supported at
each floor level or at a maximum of every 10 feet.
Thermal Expansion of Piping:
In all hydronic systems, piping undergoes temperature
swings as the system operates. This causes changes
in the length of the piping due to thermal expansion.
If the piping is rigidly mounted, this expansion can
cause annoying popping or squeaking sounds and in
extreme cases, the piping can even buckle.
To counter expansion movement, design piping
circuits with sufficient elbows, tees or expansion loops
(only used in large systems) or piping supports that
allow the tubing to expand and contract freely.
Another alternative is to install an expansion
compensator fitting capable of absorbing the
movement.

Rinnai Corporation Hydronic Furnace (37AHB) Manual 11
Installation
Hydraulic Resistance of Fittings, Valves, and Other
Devices:
Before the total hydraulic resistance of a piping circuit
can be found, the individual hydraulic resistances of all
fittings, valves, or other such components must be
determined. One approach is to consider each fitting,
valve, or other device as an equivalent length of
copper tube of the same pipe size (see Table 2).
By using the equivalent length of piping for all
components in the circuit, the circuit can be treated as
if it were a single piece of pipe having a length equal
to the sum of the actual pipe length, the total
equivalent lengths of all fittings, valves, or other
devices. Refer to Figure 10 and the associated
computation of equivalent lengths.
Pipe Sizing Considerations:
When selecting a pipe size for a given flow rate, the
resulting average flow velocity should be between 2
and 4 feet per second.
At water flow velocities of approximately 2 feet per
second, flowing water will carry air bubbles along a
vertical pipe. Average flow velocities of 2 feet per
second or higher can draw along air bubbles in a
downward flow. At the above stated velocities air
bubbles shall be routed to an air separator where they
can be collected and discharged from the system.
Use Taco 4900 series air separator, Model 49-075, or
equivalent (field supplied).
Average flow velocities higher than 4 feet per second
could cause flow noise and should be avoided.
Expansion Tanks:
All liquids used in hydronic heating systems expand
when heated. For all practical purposes, liquids are
incompressible. Any container completely filled with a
liquid and sealed from the atmosphere will experience
a rapid increase in pressure as the liquid is heated.
To prevent this from occurring, all modern hydronic
systems MUST be equipped with an expansion tank.
Refer to expansion tank manufacture’s instructions for
proper sizing and installation.

12 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Fitting or Valve 3/8" 1/2" 3/4" 1" 1 1/4" 1 1/2" 2" 2 1/2" 3"
90 deg. Elbow 0.5 1 2 2.5 3 4 5.5 7 9
45 deg. elbow 0.35 0.5 0.75 1 1.2 1.5 2 2.5 3.5
Straight thru tee 0.2 0.3 0.4 0.45 0.6 0.8 1 0.5 1
Side port tee 2.5 2 3 4.5 5.5 7 9 12 15
Reducer coupling 0.2 0.4 0.5 0.6 0.8 1 1.3 1 1.5
Gate valve 0.35 0.2 0.25 0.3 0.4 0.5 0.7 1 1.5
Globe valve 8.5 15 20 25 36 46 56 104 130
Angle valve 1.8 3.1 4.7 5.3 7.8 9.4 12.5 23 29
Ball valve 1.8 1.9 2.2 4.3 7 6.6 14 0.5 1
Swing check valve 0.95 2 3 4.5 5.5 6.5 9 11 13
Flow check valve NA NA 83 54 74 57 177 85 98
Butterfly valve NA 1.1 2 2.7 2 2.7 4.5 10 15.5
Rinnai Flow Sensor NA NA 3.2 NA NA NA NA NA NA
Taco 49-075 Air NA NA 0.3 NA NA NA NA NA NA
Table 2: Equivalent Length of Straight Pipe for Valves and Fittings (ft)
Rinnai Flow Sensor
ball valves
TACO Model 49-075
Air Separator
gauge
cap
3/4 in. type M copper tubing
4 ft 9 ft 15 ft
5 ft
10 ft
3 ft
3 ft
3 ft3 ft3 ft3 ft
12 ft
10 ft
FS
PROCEDURE FOR CALCULATING THE TOTAL
EQUIVALENT LENGTH OF PIPE
Given piping assembly as shown in Figure 10 below,
what is the total equivalent length of the system?
First determine the total straight pipe lengths; next
refer to table 2 to determine the equivalent straight
pipe length for each fitting shown. Add together the
equivalent lengths of piping and fittings.
Installation
Where possible the length of pipe
should not exceed 150 feet total
equivalent length.
Any piping running through
unconditioned space MUST be
insulated to prevent heat loss,
and possible freezing of the line.
Stickers indicating direction of
flow, (WATER IN, and WATER
OUT) are labeled on the outside
of the cabinet. DO NOT reverse
these lines, as this will cause the
unit to malfunction.
NOTICE
¾” Tubing (total straight pipe length)....68 ft.
(6) ¾” 90 deg. Elbows….……..6(2) = 12 ft.
(2) ¾” Side port tee……….…..2(3) = 6.0 ft.
(1) ¾” Taco air separator….1(0.3) = 0.3 ft.
(1) ¾” Rinnai flow sensor.....1(3.2) = 3.2 ft.
(2) ¾” Ball valves….….….....2(2.2) = 4.4 ft.
Total Equivalent length……..……93.9 ft.
Figure 10 Equivalent Length Calculation

Rinnai Corporation Hydronic Furnace (37AHB) Manual 13
Installation
Figure 11 - Typical Piping Arrangement For Direct Space Heating and Domestic Water Supply with Tankless
Water Heater. Open Loop
Piping Configuration
When employing a Tankless Water Heater in a
combination hydronic hot water heating system, the
system is considered an Open Loop System; i.e. the
system must be configured to simultaneously deliver
both domestic hot water and space heating. By
definition, if the circuit is sealed off from the
atmosphere at all locations (as is true for most modern
hydronic systems) it is called a closed loop system.
Conversely If the circuit is open to the atmosphere at
any point, it is called an open loop system. Current
Rinnai tankless products are not certified for closed
loop applications. Furnaces may be used in closed
loop application only with the new Rinnai Condensing
Boilers. Refer to Boiler Manuals for more details.
Open Loop System
If piping is done in accordance with the recommended
schematic diagram shown in Figure 11, the following
purge and priming procedure applies.
PURGING AND PRIMING THE SYSTEM:
The following procedure describes how the Rinnai®
system may be piped to eliminate the need for a
“purge cart” to fill the system and remove entrapped
air bubbles.
STEP 1: CLOSE the air separator venting valve.
STEP 2: CLOSE ball valve 3 (BV3);
STEP 3: OPEN drain valve 3 (DV3) to which a hose
MUST be connected and draining to a sink, drain or
outdoors.
STEP 4: CLOSE drain valves 1 & 2 (DV1 and DV 2)
and OPEN ball valve 2 (BV2).
STEP 5: OPEN cold water supply main valve (ball
valve 1 - BV1). The system will begin the prime/purge
process using the street pressure. Entrapped air
bubbles being pushed out of the system will be evident
by a slight vibration of the discharge hose connected
to drain valve 3 (DV3). The hose will stop vibrating
when laminar flow is achieved.
STEP 6: CLOSE drain valve 3 (DV3);
STEP 7: OPEN ball valve 3 (BV3). The system is now
purged, primed and ready to go.
STEP 8: OPEN the air separator venting valve.
Note: For an open loop system, use expansion tank
approved for potable water use only.
All piping to be 3/4 inch.
(BV)
(DV)
Field Supplied Ducting
Field Supplied
Evaporator Coil

14 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Before installing or servicing system, always turn off
all power to system. There may be more than 1
disconnect switch. Electrical shock can cause
personal injury or death.
ELECTRICAL CONNECTIONS
Line-Voltage Connections:
U.S. INSTALLATIONS: Make all electrical connections
in accordance with National Electrical Code (NEC)
ANSI/NFPA 70 and all local codes or ordinances
having jurisdiction.
CANADIAN INSTALLATIONS: Make all electrical
connections in accordance with Canadian Electrical
Code CSA C22.1 and all authorities having
jurisdiction.
Check all factory wiring per unit wiring diagram and
inspect factory wiring connections to be sure none
were loosened in transit.
NOTE: Prior to making any electrical connections,
ensure that supply voltage, frequency, and phase are
as specified on unit rating plate.
Check to ensure that the existing electrical service is
adequate to handle the additional load imposed by the
Hydronic Furnace. Refer to unit wiring diagram for
proper electrical connections.
All electrical connections MUST comply with NEC and
any other local codes or ordinances having
jurisdiction. USE COPPER WIRE ONLY. Provide
separate branch electric circuit with field supplied
disconnect switch.
Location of disconnect switch to be in clear site,
accessible and in close proximity to the unit.
Correct polarity MUST be maintained for 115 V wiring.
If polarity is incorrect unit will NOT operate.
Control Box Relocation:
The Control Box is factory installed in the blower
compartment upper left corner (see Figure 12); if
factory location of Control Box is suitable, proceed to
next section. To relocate the Control Box to an
alternate location (blower compartment upper right
side) follow steps 1 thru 7 below:
1. Remove and keep one screw and cover from the
Control Box.
2. Remove and keep two screws holding Control Box
to casing of 37AHB unit (See Fig. 12.).
3. Remove wire tie from looped wires attached to
Control Box.
4. Before Control Box is reinstalled, remove the
scored piece of insulation from the desired side.
Remove two knockouts in the casing where the
Control Box is to be installed.
5. Secure Control Box to casing with the two screws
removed and kept from Step 2.
6. Reinstall two plastic plugs (from spare parts bag)
where indicated in openings on adjacent side of
Control Box.
7. Route Control Box wiring within unit away from hot
surfaces, sharp edges and rotating parts.
CAUTION
If a disconnect switch is to be mounted on the unit,
select a location where a drill or fastener will not be in
contact with electrical or hydronic components.
Electrical shock can cause personal injury or death.
Installation
Do NOT remove ground screw inside control box.
WARNING
Figure 12: Control Box Relocation
WARNING

Rinnai Corporation Hydronic Furnace (37AHB) Manual 15
Electrical Connection to Control Box
1. Route the furnace power wires through aligned
holes in casing and Control Box and make field
wire connections in Control Box. Use best
practices for wire bushings, strain relief, etc.
Field wiring to the unit must be grounded and
conform to the National Electrical Code C22.1 Part
1 - latest edition. Use only UL listed conduit and
conduit connectors to connect supply wires to the
unit and provide appropriate grounding.
Grounding may also be accomplished by
grounding the control box per appropriate local
codes. Electric wires that are field installed shall
conform to the temperature limitation for 63° F
(35° C) rise when installed in accordance with
instructions. Refer to Table 4 for specific furnace
electrical data.
2. Route and secure field ground wire to ground
screw on Control Box.
3. Connect line voltage leads as shown in Figure 13.
4. Reinstall cover to Control Box. Ensure that wires
are not pinched between cover and edge of
Control Box.
24V Control System Connections to Unit’s Printed-
Circuit Board (PCB):
Refer to Figures 25 through 27 for factory wiring
details. For low voltage connections between the unit
and the thermostat, use No. 18 AWG color-coded,
insulated (63° F / 35°C minimum) wires. (Refer to
Figures 16 through 19.)
Installation
Failure to follow this warning could result in a fire. Do
not use aluminum wire between the Hydronic Furnace
and the disconnect switch. USE COPPER WIRE
ONLY.
WARNING
Figure 13: Field Wiring Diagram
NOTES:
1. Connect Y1 terminal as
shown for proper operation.
2. Rinnai thermostats require a
“C” terminal connection as
shown.
3. If any of the original wire, as
supplied, must be replaced,
use the same type or
equivalent wire.
Low Voltage Connections:
These units use a grounded 24 volt AC low voltage
circuit and require at least a Single stage heating and
a Single stage cooling thermostat.
The “R” terminal is the hot terminal and the “C”
terminal is grounded.
“G” terminal is the fan input.
“Y1” terminal is the compressor Stage 1 input.
“Y2” terminal is the compressor Stage 2 input.
“O” terminal is the reversing valve input. The
reversing valve must be de-energized for heating
mode.
“R” terminal is 24 VAC hot.
“C” terminal is 24 VAC grounded.
“W” terminal is the heat input. This terminal also
energizes the emergency heat if configured for heat
pump.
WGR C Y1
Y1
Y2
W
G
O
R
C
P3
FS
BLK BLK
WHT WHT
GND
GND
HYDRONIC FURNACE
115 Volt Fuse Disconnect
(Field Supplied)
Flow Sensor
(Packaged with Unit)
5 Wire
3 Wire Heating Only
Single Stage
Thermostat
(Available Accessory)
24 Volt FS / WH Connector
24 Volt Terminal Block Disconnect
(Field Supplied)
208 / 230 Volt
Single Phase
Control Box
PCB
Condensing Unit
(Field Supplied)
Field 115, 208 / 230 Volt Wiring
Field 24 Volt Wiring
Factory 24 Volt Wiring
Factory 115 Volt Wiring
Junction Box
L1
L2

16 Rinnai Corporation Hydronic Furnace (37AHB) Manual
FLOW SENSOR
CONNECTIONS
SW1 SET-UP SWITCH
AND HEATING BLOW
OFF DELAY
24 V THERMOSTAT
TERMINALS
TRANSFORMER 24 V
CONNECTIONS
P3
P4
P1
3AMPFUSE
OPERATING MODE
JUMPER (SHUNT)
STATUS LED LIGHT
115 VAC (L2)
NEUTRAL
CONNECTIONS
PUMP
CONNECTIONS
115 VAC (L1) LINE
VOLTAGE
CONNECTION
FAN
PARK
FAN
CONNECTION
HEATING
FAN
CONNECTION
COOLING
HUMIDIFIER CONNECTIONS
(DRY CONTACT) 24 VAC OR
115 VAC
FLASH UPGRADE
CONNECTION
(FACTORY ONLY)
u1
AN1
Installation
Figure 14: Hydronic Furnace Control Board
P7
Notes:
1. For proper operation of an open loop system with the flow sensor refer to Figures 11 and 14; Note that the
jumper (shunt) position on the PCB “point P7” MUST be in the FS position.
2. When changing the shunt position ensure that the unit’s power is turned off.
Wire Gauge Maximum Distance (feet)
20 gauge 45
18 gauge 60
16 gauge 100
14 gauge 160
12 gauge 250
System Low Voltage Wiring Diagrams
NOTE: Local codes may require thermostat wiring to
be routed through conduit or raceways. In such
instances splices can be made inside the Hydronic
Furnace. All wiring must be NEC Class l and must be
separated from incoming power leads.
Provide field supplied disconnect. Refer to Table 4
(Specifications) for maximum fuse or circuit breaker
sizes.
Transformer is factory wired for 115v operation. (See
Figures 27 through 29.)
The secondary circuit of the transformer is protected
by a 3-amp fuse mounted on the printed-circuit board.
IMPORTANT: Where possible, use a Rinnai factory
authorized thermostat with the 37AHB series Hydronic
Furnaces. If a thermostat other than specified is used,
refer to the manufacturer’s installation instructions for
further details.

Rinnai Corporation Hydronic Furnace (37AHB) Manual 17
Dip Switch Options (Smart Operating System):
The Rinnai® exclusive Smart Operating System is a
feature of your 37AHB series Hydronic Furnace’s
control system that is designed to allow the installer
(via DIP Switch – SW1) to configure the unit for single
or two stage, A/C or Heat pump systems with
selectable heat bower off delay.
Refer to Figures 14 and 15 for the proper dip switch
setting to be used with the desired configuration.
When viewed with the Furnace in the upflow position,
the dip switch will be as shown below (upside down.
Installation
Figure 15: Dip Switch Positions
THERMOSTAT INSTALLATION:
Safety Considerations:
All wiring must conform to local and national electrical
codes. Improper wiring or installation may damage
thermostat.
INSTALLATION CONSIDERATIONS:
Air Conditioner Model:
The Standard Model A/C thermostat may be wired
with or without connecting a common wire between
the indoor equipment and the thermostat. However, it
is recommended to use a common wire whenever
possible. Without a common wire this thermostat
becomes "power stealing." This means it will need to
steal a small amount of power from the equipment to
which it is connected. When "power stealing"
connection is used, the supplied 270 ohm resistor
must be connected at the indoor unit.
Heat Pump Model:
The Standard Model HP thermostat is not "power
stealing" and MUST have both ‘R’ and ‘C’ wires
connected to operate properly. This thermostat uses a
green LED to indicate auxiliary/emergency heat
operation.
Installation:
Thermostat should be mounted
• approximately 5 ft. (1.5 m) from floor
• close to or in a frequently used room, preferably on
an inside partitioning wall
• on a section of wall without pipes or duct work.
Thermostat should NOT be mounted
• close to a window, on an outside wall, or next to a
door leading to the outside.
• exposed to direct light and heat from a lamp, sun,
fireplace, or other heat-radiating object which may
cause a false reading.
• close to or in direct airflow from supply registers
and return-air grilles
• In areas with poor air circulation, such as behind a
door or in an alcove
Refer to Figures 16 through 19 for thermostat wiring
diagram and thermostat installation instructions for
further details.
SINGLE-STAGE A/
C
CONFIGURATION
(DEFAULT)
TWO-STAGE A/C
CONFIGURATION
1
O
N
23 41
O
N
23 4
SINGLE-STAGE HP
CONFIGURATION
TWO-STAGE HP
CONFIGURATION
1
O
N
23 41
O
N
23 4
30 SECONDS OFF
DELAY (DEFAULT)
60 SECONDS OFF
DELAY
1
O
N
23 41
O
N
23 4
90 SECONDS OFF
DELAY
120 SECONDS OFF
DELAY
1
O
N
23 41
O
N
23 4
WARNING
Before installing thermostat, turn off all power to unit.
There may be more than one power disconnect.
Electrical shock can cause personal injury or death.
1
O
N
Key:
Switch is in the ON position.
Switch does not affect this setting.
1
O
N
Switch is in the OFF position.

18 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Shunt Jumper Options:
An additional feature of the 37AHB series is its
selectable operating sequence option; the 3-pin shunt
header (P7) allows the control to operate the proper
heating logic based on the following system
requirements:
The 37AHB unit in Open Loop configuration:
“FS” Shunt selection: The “FS” logic sequence will
configure the unit for operation with all Rinnai®
Tankless Water Heaters; this logic monitors the ratio
of available flow for space heating (via Flow Sensor);
this status is then communicated to the PCB whose
operating characteristics is primarily determined by the
status of the Flow Sensor input (sequence allows
domestic priority).
Failure to follow this warning could result in an
electrical shock, fire, or death.
To minimize personal injury if an electrical fault should
occur, cabinet grounding MUST be an uninterrupted
ground and MUST comply with NEC, ANSI/NFPA 70
and all local codes having jurisdiction. The ground
may consist of electrical wire or metal conduit when
installed in accordance with existing electrical codes.
WARNING
THERMOSTAT WIRING DIAGRAMS
Installation
FIRST STAGE HEAT/COOL
AUX HEATING
FAN
24VAC HOT
24 VAC COMMON
SINGLE STAGE
HYDRONIC FURNACE
SINGLE SPEED
CONDENSING
UNIT
HEAT/COOL & COOL ONLY
THERMOSTATS
Y1
W
G
R
C
Y1
Y2
W
G
O
R
C
Y1
C
Single Stage Hydronic Furnace w/ Single Stage
A
Figure 16
FIRST STAGE HEAT/COOL
AUX HEATING
FAN
RVS COOLING
24VAC HOT
24 VAC COMMON
HEAT PUMP THERMOSTATS SINGLE STAGE SINGLE STAGE
HEAT PUMP
Y1
W
G
O
R
C
Y1
W
G
O
R
C
Y1
O
R
C
Y2
Figure 17 Single Stage Hydronic Furnace
w/ Single Stage Heat Pump
FIRST STAGE HEAT/COOL
SECOND STAGE HEAT/COOL
AUX HEATING
FAN
24VAC HOT
24 VAC COMMON
HEAT/COOL & COOL ONLY
THERMOSTATS SINGLE STAGE TWO STAGE
CONDENSING
UNIT
Y1
Y2
W
G
R
C
Y1
Y2
W
G
O
R
C
Y1
Y2
C
Figure 18 Single Stage Hydronic Furnace w/ Two Stage A/
C
FIRST STAGE HEAT/COOL
SECOND STAGE HEAT/COOL
AUX HEATING
FAN
RVS COOLING
24VAC HOT
24 VAC COMMON
HEAT PUMP THERMOSTATS SINGLE STAGE TWO STAGE
HEAT PUMP
Y1
Y2
W
G
O
R
C
Y1
Y2
W
G
O
R
C
Y1
Y2
O
R
C
Figure 19 Single Stage Hydronic Furnace
w/ Two Stage Heat Pump
HYDRONIC FURNACE
HYDRONIC FURNACE
HYDRONIC FURNACE

Rinnai Corporation Hydronic Furnace (37AHB) Manual 19
START-UP PROCEDURE (HEATING ONLY):
The following conditions must be met prior to unit start
-up.
Debris from soldering and/or other installation
activities can cause equipment failure. Ensure that all
associated lines and appurtenances are free of debris.
Check to ensure that unit is secure.
Check that blower wheel rotates freely within the scroll
housing.
Check all wiring to ensure that connections are tight.
Check all ductwork and pipe connections to ensure
proper seal.
Check to ensure that all packaging wraps are removed
from equipment.
Ensure that front access doors are properly installed.
Check to ensure proper connections to the appropriate
blower speed tap (Heat /Cool – High and Low). Refer
to Air Delivery and Capacity Charts and/or the
appropriate wiring diagram in this manual.
Perform all safety and start-up checks for Tankless
Water Heater as per manufacturer’s instructions.
Having verified all preceding checks, the Furnace’s
Start-Up Procedure is as follows:
STEP 1: Purge and fill system; follow appropriate
purging procedure as laid out in this manual in section
titled “Purging and Priming the System”.
STEP 2: Turn on power supply to Furnace. Caution:
blower and/or circulator may start to operate if
thermostat is on and a call is present.
STEP 3: Turn thermostat on and switch system to the
heating mode. The thermostat shall be set higher than
the actual room temperature; this will cause the
circulator to energize and initiate the heating cycle. (If
the pump does not start, or the Furnace is not
producing heat, refer to the Troubleshooting Section in
this manual).
STEP 4: Program room thermostat as desired by
homeowner.
START-UP PROCEDURE (COOLING SYSTEM)
Refer to field supplied evaporator coil and outdoor unit
manufacturer’s Installation Instructions for system
hook-up, start-up instructions and refrigerant charging
method details.
TROUBLESHOOTING BLOWER AND/OR PUMP
MOTOR AND CONTROLS
If blower and/or pump motor does not run:
Turn off power and check the following:
1. Check that door switch is in the CLOSED position.
2. Check 3 amp fuse on Printed Circuit Board (PCB).
3. Check for 24 VAC between COM and 24 VAC on
PCB. If no voltage is present, check transformer.
4. Check all connections for kinks which could cause
loose connections. Ensure connections are secure.
5. Verify that approximately 120 VAC is present
across L1 and L2 (refer to wiring diagrams).
If system still fails to start, refer to Figures 20 through
23 for additional help.
Installation
High voltage is at all times present at motor.
Disconnect power to the Hydronic Furnace before
removing or replacing or servicing motor. Wait at
least 5 min after disconnecting power before opening
motor. Failure to follow this CAUTION could result in
minor personal injury or product and property
damage.
CAUTION
TABLE 2.1: BLOWER MOTOR TROUBLESHOOTING
SYMPTOM POSSIBLE
CAUSES CORRECTIVE ACTION
Blown fuse Turn off motor. Replace
fuse
Incorrect
voltage
Verify motor voltage
matches system voltage
Improper
connections
Turn off motor. Verify
connections
Blower wheel
obstruction
Verify blower wheel is not in
contact with the blower
housing. Readjust blower
wheel position on motor
shaft.
Motor does
not come up
to full speed
Not applied
properly
Check speed taps as per
wiring diagram.
Motor stalls
during
operation
Overload
motor
Check for duct blockage
and/or verify that ducting
system is not restrictive.
Motor vibrates
or is
excessively
noisy
Loose or
defective fan
Turn off motor. Tighten fan
set screw or replace fan.
Motor fails to
start

20 Rinnai Corporation Hydronic Furnace (37AHB) Manual
Installation
Figure 20: Hydronic Furnace Start-Up and ‘LED’ Troubleshooting - Flow Sensor (FS Configuration)
NOTES:
1 Pipe system between Tankless Water Heater and Hyronic Furnace ‡
2 Leak check piping system
3 Purge and prime plumbing system
4 Perform required electrical work
5 Check system operation with power to condensing unit off (if installed)
6 Control board is sometimes refered to as 'PCB'
7 Hyronic Furnace is sometimes refered to as 'RHF'
8 Typical for all check, ensure system is in the following operating mode:
AC / SINGLE STAGE
9 For Amp loads refer to specification sheet
10 Thermostat is sometimes refered to as 'T'STAT'
‡ Refer to plumbing section in this manual
NO
YES
IS 'LED' ON
WITH DOOR SWITCH CLOSED
(SET SYSTEM IN HEATING
MODE - CALL FOR HEAT)
YES
NO
START
SYSTEM
PURGED
CHECK VOLTAGE ACROSS'L1'
AND 'L2'
RETURN TO PURGE & PRIME
STEPS ‡
IS 'LED'
RAPIDLY
FLASHING
STEADY 'LED' FLASH - NORMAL
OPERATION OR STAND-BY
MODE AWAITING
THERMOSTAT CAL
L
YES
NO
DOMESTIC HOT WATER
DEMAND PRESENT –SYSTEM
ON HOLD- SEARCH MODE
IS 'LED' RAPID
FLASH
CONSISTENT
YES
INCONSISTENT RAPID FLASH (PULSATING
PUMP OPERATION)– WRONG SHUNT
POSITION- REFER TO FIG.14 HYDRONIC
FURNACE CONTROL BOARD, JUMPER
(SHUNT) POSITION'P7'TO BE IN 'FS'
MODE
NO
CHECK BREAKER AND POWER
SUPPLY
IS POWER
SUPPLY OK
YES
NO
RECTIFYSUPPLY VOLTAGE
PROBLEM(S) ANDRETURN TO
START
VAC = 115V +/-
NO
CHECK DOOR SWITCH, IF
DEFECTIVE, REPLACESAME
AND RETURN TO START
YES
GO TO24 VOLTS
TROUBLESHOOTING CHART
CHECK FOR LOOSE OR
BROKEN WIRE
CHECK SYSTEM WIRING
AGAINST WIRING DIAGRAM -
RECTIFY PROBLEM(S)
L1/L2 = 115V +/-
NO
YES
IS 'LED' ON
NO
YES