Aqua-Hot 200 Series User manual

SERVICE MANUAL
200SERIES
250P and 250D

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Table of Contents
Introductions and Cautions ............................................ 1
Safety Instructions and Precautions ......................... 2
250D Cross Section ................................................... 5
250P Cross Section ................................................... 6
Operational Flow Chart .............................................. 7
Relay Control Board ................................................... 8
Interior Switch Panel .................................................. 10
Exhaust System Requirements ................................. 11
Aqua-Hot Components
Interlock Switch .......................................................... 12
Expansion Bottle Fluid Sensor................................... 13
Control Thermostat .................................................... 14
AC High Limit Thermostat .......................................... 16
Burner High Limit Thermostats ................................. 18
Low Temperature Cutoff Thermostat ........................ 19
3-way Valve................................................................. 21
Tempering Valve......................................................... 23
Stir Pump .................................................................... 24
AC Electric System...................................................... 26
Aqua-Hot 250P
System Features......................................................... 28
Propane Burner Overview .......................................... 29
Propane Burner Operational Flow Chart................... 30
Propane Switch Panel ................................................ 31
Propane Fenwal Control Card.................................... 32
250P System Schematic ........................................... 33
Detaching the Propane Burner.................................. 34
Re-attaching the Propane Burner.............................. 36
Propane Burner Mounting Gasket............................. 38
Hot Surface Igniter ..................................................... 39
Refractory Insulation.................................................. 41
Dual Stage Propane Gas Valve.................................. 42
Propane Combustion Blower Fan.............................. 44
Adjustment of the Propane Air Shutter ..................... 45
Exhaust High Back Pressure Switch.......................... 46
Fan Validation Switch................................................. 47
Combustion Blower Fan Buck-boost Module............ 48
Hot Surface Igniter Buck-boost Module .................... 49
Flame Sensor.............................................................. 51
Aqua-Hot 250D
System Features .........................................................53
Diesel Burner Overview ..............................................54
Diesel Burner Operational Flow Chart .......................55
Diesel Switch Panel ....................................................56
250D System Schematic............................................57
Diesel Burner Controller .............................................58
Disassembly of Burner Componentry ........................61
Re-assembly of Burner Componentry ........................62
Detaching the Diesel Burner ......................................63
Re-attaching the Diesel Burner ..................................65
Diesel Igniter Module..................................................66
Ignition Coil..................................................................67
Diesel Fuel Delivery System .......................................68
Diesel Combustion Blower Fan ..................................69
Adjustment of the Diesel Air Shutter .........................70
Combustion Blower Fan Buck-boost Module ............71
Photo-Eye.....................................................................73
250 Series Maintenance
Filling the Aqua-Hot.....................................................77
Purging the Aqua-Hot..................................................77
Diesel Burner Annual Service.....................................75
Winterizing and De-Winterizing the Aqua-Hot ...........78
Troubleshooting
General Troubleshooting ............................................79
Uneven, or Lack of Hot Water ....................................79
Unit Smoke ..................................................................80
Lack of Interior Heat ...................................................82
Burner Troubleshooting ..............................................83
Propane Burner Troubleshooting ...............................84
Anti-Freeze Heating Solution...........................................85
Measuring Propylene Glycol Concentration...............87

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
About the Aqua-Hot 250D and 250P
The Aqua-Hot 250D, and 250P are hydronic (water
based) heating systems that provide interior heat and tank-
less, continuous hot water in one small, easy to install pack-
age.
The heating system provides moist, quiet, comforta-
ble interior heat. It is equipped with one thermostatically
controlled temperature zone.
The tankless hot water system produces 90 gallons
per hour (1.5gal/minute) of continuous hot water
The system uses one or a combination of heat
sources to heat FDA–approved “Generally Recognized As
Safe” (GRAS) propylene glycol based anti-freeze solution
contained within the Aqua-Hot boiler tank.
The 250D and 250P use a 12-volt DC powered
burner (Propane or Diesel respectively) as the primary heat-
ing source. The propane and diesel burners should be used
as the primary heating source for hot water, and interior
heating needs. The 250 also has one 120-volt AC, 1000
watt electric element for use when shore power is available
as a supplemental heating source. Once the tank has been
brought to operating temperature by the propane or diesel
burner, the electric element can be used to maintain this
temperature, as well as provide hot water and interior heat
for light-duty applications. The burner, and the electric ele-
ment can be used together or separately.
For continuous hot water or heat in colder condi-
tions, the propane or diesel burner must be active.
Should additional assistance be required, please
contact the product application department at +1 (800) 685
-4298, Monday through Friday, between 7am and 4pm
Mountain Standard Time.
Caution Notes
As you read this information, take particular note of
the NOTICE, CAUTION, WARNING and DANGER symbols
when they appear. This information is important for safe and
efficient use of the Aqua-Hot system.
NOTICE signals a situation where potential damage to the
Aqua-Hot could occur.
CAUTION signals a situation where potential harm or risk of
minor or moderate injury could occur if you do not follow
instructions.
WARNING signals a hazardous situation where potential
harm, risk of serious injury, or death could result if instruc-
tions are not followed.
DANGER signals a situation where immediate risk of serious
injury or death will result if instructions are not followed.
Introduction and Cautions
NOTE: This manual will also use notes sections similar to
this one to draw attention to features and practices which
should be observed.

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Safety Instructions and
Precautions
Read all instructions before installing the Aqua-Hot unit. Aqua-
Hot Heating Systems is not liable for damage resulting from
failing to follow instructions contained in this, and any other
Aqua-Hot documentation relevant to this unit.
• Read this service manual before installing or using the
Aqua-Hot System to reduce the risk of injury to persons or
damage to the equipment
• The product identity label contains specifications of the
unit, to what standard it has been tested, and important
safety notices.
• The Aqua-Hot must be installed in a compartment that is
closed off from living quarters and accessible only from
the outdoors.
• Propylene glycol based anti-freeze “Generally Recognized
As Safe” by the FDA must be utilized for the anti-freeze
and water heating solution.
• An interlock switch prevents the Aqua-Hot heater from
operating when the cover is not installed, or is installed
incorrectly
• Disconnect electric wiring to the Aqua-Hot System before
welding or plasma cutting the coach to avoid damage to
equipment
• Air pressure to the tank must not exceed 18 PSI. Exceed-
ing this rating will cause internal damage to the Aqua-Hot.
• The Aqua-Hot’s exhaust is extremely hot, and must be
kept away from heat sensitive materials
• Use caution when working on or near the propane gas
system
• DO NOT connect the 12-volt DC power to the Aqua-Hot if
the vehicle requires welding
• At maximum operating temperature, the coolant will be
very hot and scalding hot vapor or coolant may result in
serious burns or injury.
• DO NOT activate the burner until the anti-freeze and water
heating solution has been added to the boiler tank to
avoid serious damage to the heater
Do not store or use gasoline or other flammable vapors and
liquids in the vicinity of this, or any other appliance.
WHAT TO DO IF YOU SMELL GAS
• Evacuate all persons from the vehicle
• Shut off the gas supply at the gas container or source
• Do not touch any electrical switch or use any phone or
radio in the vehicle
• Do not start the vehicle’s engine or electric generator
• Contact the nearest gas supplier or qualified service tech-
nician for repairs
• If you cannot reach a gas supplier or qualified service
technician, contact the nearest fire department
• Do not turn on the gas supply until the gas leak(s) have
been repaired.
• Installation and service must be performed by a qualified
installer, service agency, or gas supplier
If the information in these instructions is not followed exact-
ly, a fire or explosion may result causing property damage,
personal injury, or death.

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Safety Instructions and
Precautions

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
250D Cross Section
1. Domestic Cold Water Inlet
2. Domestic Hot Water Outlet
3. Diesel Fuel Connections
4. 12V DC Harness Junction
5. Expansion Tank Connection
6. Heating Zone Outlet Port
7. Heating Zone Return Port
8. 120V AC Connection
9. Interlock Switch
10.3-Way Valve
11.Diesel Burner Assembly
12.Boiler Tank
13.Tempering Valve
14.Diesel Burner Controller
15.Drain Valve
16.Zone Circulation Pump
17.Access Cover Screw
1 2 3 4 5 6 7 8
17
9
10
11
13
14
16
15
12
Figure 1

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
250P Components
1. Domestic Cold Water Inlet
2. Hot Water Outlet
3. Propane Inlet Ball Valve
4. 12V DC Harness Junction
5. Expansion Tank Connection
6. Heating Zone Outlet Port
7. Heating Zone Return Port
8. 120V AC Electrical Relay Box
9. Propane Burner Assembly
10. Tempering Valve
11. Boiler Tank
12. 3-Way Valve
13. Propane Burner Controller
14. Interlock Switch
15. Drain Valve
16. Zone Circulation Pump
17. Access Cover Screw
51 2 3 4 86 7
16
15
14
13
12
11
10
9
17
Figure 2

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Operational Flowchart
“Electric Element”
Switch Turned ON
“Burner” Switch
Turned ON
Burner fires into the Combus-
tion Chamber, which heats the
antifreeze and water heating
solution in the Boiler Tank.
Electric Heating Element
activates, which heats the
antifreeze and water heating
solution in the Boiler Tank.
Boiler Tank heats to
185°F
Heat Source is selected from
the Interior Switch Panel
Interior Zone Thermo-
stat calls for heat
A hot water faucet (e.g., kitchen
sink, shower, etc.) calls for hot
water.
Heat is transferred to the Do-
mestic Water System
Hot Water Flows from
the water faucet
Heat Exchanger fans acti-
vate
Electric Heating Element heat source
selected by Electric Element Switch
Burner heat source select-
ed by Burner Switch
The Pump activates for
the Heating Zone. The
heated antifreeze and
water heating solution is
transported throughout
the plumbing lines in that
zone
Antifreeze and water
heating solution moves
through the Heat Ex-
changers in the Zone
calling for heat
Heat is transferred into
the Zone calling for heat
The cooled antifreeze
and water heating solu-
tion is returned to the
Boiler Tank to be reheat-
ed
NOTE: The Burner and/
or Electric Heating Ele-
ment (as switched ON)
will automatically begin
reheating the antifreeze
and water heating solu-
tion when the Boiler
Tank drops below 155°F
NOTE: The circulation
pumps and heat exchang-
ers will not operate until
the Boiler Tank reaches a
minimum of 155°F

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Relay Control Board
Relay Control Board
The relay control board at use in your Aqua-Hot 200
series unit is designed to function in conjunction with the Fen-
wal burner control card, or diesel burner controller to allow the
heater to operate. This section will introduce you to the basic
functions of the board.
Indicator lights line the bottom of the relay control
board and are designated D1 through D6. Each of these lights
indicates a function or event that is taking place. The detailed
function of these lights will be explained on the next page.
• D1: Heating Status Light (orange)
• D2: Electric Element Power (green)
• D3: Pump Power (Blue)
• D4: Low-Temperature Cutoff Status (yellow)
• D5: Burner Status (green)
• D6: Interior Zone Heating (orange)
Fuse Functions, Locations, and Ratings
There are three fast-blow fuses included with this re-
lay board, and may need to be replaced if they cease to func-
tion. The cradles for these fuses are labelled FH1, FH2, and
FH3 and will contain either a 5A or 10A fuse. Only the 250P
will utilize the 5A fuse in slot FH3, the 250D uses a 20A fuse
integrated into the wiring harness to fill this need.
FH1: 10A, AC Activation Circuit and Burner
FH2: 5A, Switch Power
FH3: 5A, Burner Controller Power (250P only)
A note on diagnosing the Relay Control Board
If after diagnosing the Aqua-Hot you believe that the
relay control board is at fault, it is very strongly advised that
you contact Aqua-Hot Heating Systems Technical Support to
ensure that the issue has been accurately diagnosed. These
components have a very low rate of failure, and for that reason
we recommend contacting us before replacing it in an attempt
to reduce down time and unit repair costs.
Figure 3
AC Circuit and burner Power
Switch Power
Burner Controller Power (250P only)
D1 D2 D3 D4 D5 D6

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Heating Status (D1)
This indicator light will illuminate orange when the
burner and/or electric heating element switch on the interior
switch panel is on, and the control thermostat is calling for
heat.
If the heater is functioning and the light is not illumi-
nated, or the heater is not functioning and the light is illumi-
nated, there is an issue with the wiring or one of the compo-
nents.
Electric Heating Element Status (D2)
This indicator light shows the status of the electric
heating element by illuminating green when the electric ele-
ment switch is on. If the green LED does not illuminate, the
electric element switch is off or the unit is low on fluid.
The D2 light functions with the interior electric ele-
ment switch to provide functionality when requested. When
the electric element switch is on, the D2 light should also be
on.
If the D2 indicator light is not illuminated, but the
electric switch on the interior switch panel is in the “ON” posi-
tion, there is a short in the 12V DC side of the electric element
circuit. This could be due to a bad connection, a bad switch, a
bad fluid level sensor, or the fuse present in FH2 is not func-
tional.
Pump (D3)
This indicator light shows the power status of the cir-
culation pump within the Aqua-Hot. The circulation pump is
responsible for circulating fluid through the heating zone, and
stirring the fluid within the tank while it is heating.
The system functions correctly if the coach thermostat
is calling for heat, the pump is operating, and the light is illumi-
nated blue.
If the light is off while the coach thermostat is calling
for heat or hot water, there may exist an issue with coach wir-
ing, or with the relay control board.
Low-Temperature Cutoff Status (D4)
The low-temp cutoff status thermostat deactivates
interior heat when domestic hot water is being used, and acti-
vates the stir pump in conjunction with the burner switch.
When this light is on, it indicates that there is either a
demand for hot water, or the Aqua-Hot is not yet up to operat-
ing temperature.
Burner Status (D5)
This indicator light shows the status of the burner by
illuminating green when the burner switch is on. On the 250D
the indicator light on the switch panel will not illuminate if the
burner is faulting.
If the burner switch is in the “ON” position, and the
control thermostat is calling for heat, the D5 indicator light
should illuminate in addition to the D1 heating status light.
If the indicator light remains off while the burner
switch is in the “ON” position, it could indicate that there is a
short present, the fuse in FH2 may be burned out, or the unit
may be low on fluid.
Heating Zone Status (D6)
This light indicates the status of the coach thermostat
and the heat exchanger fans. The indicator light will illuminate
orange when the coach thermostat is requesting interior heat,
or hot water is being requested.
If a fault condition occurs, it could be due to a wiring
issue, a faulty thermostat, or issues with the heat exchanger
fans.
NOTE: It is possible for the Electric Heating Element and
Heating Status lights to be illuminated, and the electric ele-
ment not function due to an issue with the 110V AC power
supply.
Relay Control Board

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Interior Switch Panel
Interior Switch Panel
Overview:
The interior switch panel is used to control the two
potential heating sources for the Aqua-Hot’s boiler tank. When
a switch is activated, the indicator light on the switch will illu-
minate.
Burner Switch:
When the burner switch is in the ON position, the con-
trol thermostat will communicate with the relay control board,
and burner controller that heat is needed for the tank. The
burner will activate, and the unit will begin to warm up.
A cold boiler tank can be expected to reach tempera-
ture in approximately 10-20 minutes, depending on the ambi-
ent temperature. In colder temperatures, it may take longer to
heat the unit to its operating temperature.
In order to reach and maintain temperature under all
demands, the Burner switch must be in the “ON” position.
Keep in mind that the electric element is a supplemental only
heat source. This means that the element can provide hot
water for simple tasks such as hand washing, and maintaining
tank temperature at times when there is no load. The burner
is intended as the primary heat source.
Additionally, the burner switch can be used to reset a
low-voltage condition. This is accomplished by turning the
switch “OFF” for 30 seconds, and then turning it back on. This
is sometimes known as “power cycling.”
Electric Element Switch
When the coach is plugged into an AC power source
(e.g. shore power, or a generator) and the electric element
switch is “ON,” the electric heating element will be used to
provide heat to the boiler tank if the need arises. A cold boiler
tank can expect to be brought to operating temperature by the
electric heating element in 1-2 hours depending on the ambi-
ent temperature. The electric element is intended as a sec-
ondary, or supplementary heating source. If used alone, the
electric element will NOT be able to provide enough heat for
continuous hot water, or interior heat.
Figure 4

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Exhaust System Requirements
Exhaust Requirements
The Aqua-Hot’s exhaust is hot and must be kept away from
any heat-sensitive material. Therefore, the exhaust system
should be checked to ensure that it continues to meet the
following requirements.
• The exhaust must not be directed downward as a fire
could result when parked in dry, grassy areas.
• The exhaust must not terminate underneath the vehicle,
underneath an openable window or vent, in the awning
area of the coach (if applicable,) or near the slide-out are-
as
• The exhaust must be able to freely exit away from the ve-
hicle without any obstructions
• Use standard two-inch automotive type exhaust piping
and avoid bends if possible.
• The 3-inch and 4-inch black-pipe nipple and the exhaust
elbow, originally supplied with the Aqua-Hot must be used
(kit sold separately.)
• The exhaust tip must be used to prevent the burner from
extinguishing in winds of up to 40 mph.
• Mounting must be placed every three feet to adequately
support the exhaust system
• Total exhaust system length must not exceed 30ft in total
length, and shall contain no more than two 90° bends.
#8, 3/8”
#8, 3/8”
Exhaust Tip
Aqua Hot’s
Exhaust
Pipe
Figure 5
Figure 6

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Aqua-Hot Components
Components
This section of the Service Manual details various
components of the Aqua-Hot that may require troubleshooting
and/or replacement in the event of malfunction. Components
detailed here are interchangeable between the propane and
diesel units. Components specific to the diesel, or propane
units are detailed in their respective sections.
Replacement parts can be ordered online through
Aqua-Hot’s webstore.
If additional assistance is needed, the technical sup-
port team can be contacted at [phone], Monday
through Friday, 7am to 4pm Mountain Standard Time.
Sensors and Switches
Interlock Switch
The interlock switch is a safety device designed to
ensure that the Aqua-Hot’s access cover is securely installed
before the burner will operate.
Troubleshoot the interlock switch if the following conditions
occur;
• The burner fails to operate
• The burner indicator light fails to illuminate
Troubleshooting
 Ensure that the boiler tank has sufficiently cooled in order
to require heat from the burner
 Activate the burner switch located on the interior switch
panel
 Locate wires #37 and #24 on the wiring harness as they
lead into the interlock switch
 Disconnect the wires from the switch, noting that wire
#24 is connected to the terminal labelled “NO” and #37 is
connected to the terminal labelled “COM.”
 Using an ohmmeter, check the interlock switch for conti-
nuity while the switch button is depressed.
 If continuity is not present with the button pressed in, fol-
low the instructions in this section to replace the interlock
switch.
Replacement Procedure
 Disconnect the interlock switch wires by pulling the quick
connectors from the switch spade terminals
 Release the interlock switch from the Aqua-Hot cabinet by
pushing in on the locking tabs and pulling the interlock
switch
 Remove the defective interlock switch from the Aqua-Hot
 Install the replacement interlock switch onto the Aqua-Hot
ensuring that the locking tabs snap into place
 Connect the Aqua-Hot’s replacement wires to the replace-
ment interlock switch with wire #24 connected to the ter-
minal labelled “NO” and wire #37 connected to the termi-
nal labelled “COM.”
NOTE: Before attempting to troubleshoot any Aqua-Hot
component, please check all wiring to ensure that there is
no corrosion, loose and/or faulty wiring connection present
which may be causing failure.
Failure to disconnect all power supplies and/or allow the unit
to cool before servicing could cause serious damage or per-
sonal injury.
NOTE: If the interlock switch’s wires are connected incorrect-
ly, the interlock switch can fail to allow the Propane-burner
to operate when the access cover is correctly installed.
Locking Tab
Interlock Switch Button
“COM”
“NO” Figure 7

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Expansion Bottle Fluid Sensor
Scope
The fluid sensor monitors the level of anti-freeze and
water heating solution within the Aqua-Hot’s expansion bottle
to ensure that an adequate volume exists there, and within the
boiler tank.
Troubleshoot the overflow fluid sensor if one of the following
has occurred:
• The burner fails to operate
• The burner switch indicator light fails to illuminate
• The electric element fails to operate
• The indicator light on the electric switch does not illumi-
nate
Troubleshooting
 Verify that the Aqua-Hot’s expansion bottle is full of anti-
freeze and water heating solution
 Verify that the wiring is secure, and in good working condi-
tion
 Locate the wiring harness for the unit. Find wires #10,
#15, and #16. Wires #10 and #15 will co-terminate in a
yellow connector.
 Disconnect these wires from the expansion bottle, and
test for continuity across the expansion bottle sensor.
 If the bottle contains fluid, and continuity does not exist
across the fluid-level sensor, the sensor will need to be
replaced.
Replacement Procedure
 A new expansion bottle kit can be ordered online at our
webstore, or by phone at [phone].
 Using a hose clamp, clamp off the connection between
the expansion bottle and the fluid return port as depicted
below.
 Disconnect the fluid expansion hose from the expansion
bottle, and allow the bottle and the hose to drain into a
receptacle.
 Disconnect the wires leading from the expansion bottle to
the wire harness, paying particular attention to which
wires belong to each receptacle
 Remove any and all mounting hardware used to affix the
expansion bottle into place.
 Replace the faulty expansion bottle with a replacement,
using the previously removed mounting hardware to affix
the new bottle in position.
 Reconnect the expansion tubing, and fill the bottle until it
reaches the cold fill line.
 Reconnect the expansion bottle wires as they were re-
moved. Note that connecting these wires backwards will
not allow the expansion bottle sensor to function as in-
tended.
Failure to disconnect all power supplies and/or allow the
unit to cool before servicing could cause serious damage
and/or personal injury.
Before continuing this replacement procedure, ensure that
the unit has cooled to ambient temperature, all power
sources have been disconnected, and the unit is completely
shut off.
Fluid Return Port
Figure 8
Fluid-Level Sensor
Fluid Return Hose
Fluid Overflow Hose
Figure 9
Aqua-Hot Components

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Aqua-Hot Components
Thermostats
Control Thermostat
The control thermostat is installed into the Aqua-Hot’s
boiler tank and monitors the temperature of the anti-freeze
and water heating solution to determine when it is at operat-
ing temperature and when it requires heat. The Aqua-Hot is
considered to be at operating temperature between 155°F
and 188°F. Please reference page 8 for relay control board
LED indicator information.
Troubleshooting Condition
Troubleshoot the control thermostat if one of the fol-
lowing conditions has occurred:
• There is a lack of hot domestic water and interior heat
• The orange heating status light (D1) does not illuminate
while the unit is below 155°F.
Troubleshooting
1. Verify the following before troubleshooting the control
thermostat.
A. The interlock switch is depressed
B. The overflow bottle is adequately filled
C. The high-limit thermostats are not tripped
D. Fuses in the relay control board are functional
 Turn the burner and/or electric element switches to the
“ON” position.
 Check the relay control board to ensure that “Electric
Heating Element Status (D2)” and/or “Burner Status (D5)”
indicator lights are illuminated.
 Verify that the “Heating Status (D1)” indicator light on the
electronic relay control board is illuminated as it should be
whenever the Aqua-Hot is below 155°F.
If the “Heating Status (D1)” indicator light is not illuminated,
check the following;
• Verify that the temperature of the boiler tank has fallen
below the 155°F minimum operating temperature by
checking with an infrared thermometer. Take the reading
within 12 inches from the painted tank surface with the
thermometer set to “High Emissivity” or similar. Do not
attempt to take readings on a shiny surface.
• If below 155°F, disconnect the control thermostat’s wires
from their connection. Using an ohmmeter, check for con-
tinuity across the control thermostat.
If continuity does not exist across the thermostat under these
conditions, follow the directions in this section for replacing
the control thermostat.
If continuity exists across this thermostat, inspect it, and the
wiring harness for any damage.
NOTE: The Propane-burner and/or electric element switch
must be in the ON position for the “Heating Status” indicator
light on the relay control board to illuminate.
Figure 10

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Aqua-Hot Components
Control Thermostat Replacement Procedure
1. Ensure that the Aqua-Hot has been completely shut down,
all power sources disconnected, and the unit has been
allowed to cool
2. Clamp the hoses indicated below with pinch off pliers to
prevent drainage of the heating zone loop
3. Drain 1 gallon of antifreeze and water heating solution
into an external container to be reused later.
4. Disconnect the defective control thermostat wires by sep-
arating the quick disconnect terminals
5. Using a 7/8” deep wall socket, unscrew the control ther-
mostat from the Aqua-Hot’s boiler tank.
6. Wrap the threads of the replacement control thermostat
with 6 wraps of thread seal tape
7. Reconnect wire #14A and wire #23 to the new control
thermostat
8. Refill the Aqua-Hot’s boiler tank with anti-freeze and water
heating solution.
9. Refill the Aqua-Hot using an external fill pump, filling
through the zone fill port.
10. Test the Aqua-Hot for normal operation.
11. Test the interior heating loop to ensure that there are no
air pockets trapped within the interior heating loop.
12. If necessary, purge the interior heating loop according to
the guide on page 77.
Failure to disconnect all power supplies and/or allow the
unit to cool before servicing could cause serious damage
and/or personal injury.
Hose Pinch Off Pliers
CLAMP
CLAMP
ABOVE
TEE
Control Thermostat
Drain Valve
Figure 11
Zone Fill Port

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Aqua-Hot Components
AC High-Limit Thermostat
Scope:
The AC high-limit thermostat serves as a safety meas-
ure in the event that the electric heating element continues to
operate after the maximum operating temperature has been
reached. The high-limit thermostat allows the current for the
heating element to pass through until the boiler tank reaches
a temperature of 215°F. Should this temperature be reached,
the high-limit thermostat blocks the current to the element,
preventing the element to continue to operate.
Troubleshooting Condition
Troubleshoot the AC high-limit thermostat if the elec-
tric element fails to operate.
Troubleshooting
1. Verify the following before troubleshooting the AC high-
limit thermostat.
A. The control thermostat is calling for heat
B. The interlock switch is depressed
C. The overflow bottle is adequately filled
D. The high-limit thermostats are not tripped
E. The fuses in the relay control board are functioning
2. Ensure that the unit has been shut down, and cooled
completely before continuing.
3. Shut off the coach-side power breaker to the Aqua-Hot
4. Disconnect the 3-pin AC Molex plug from the AC relay en-
closure. Leave the DC power supply connected.
5. Activate the “ELECTRIC” switch on the switch panel inside
the coach.
6. Using a multi-meter, test for continuity across the AC high-
limit thermostat by placing one lead on the unit-side Mo-
lex connection as shown in figure 12.
7. If no continuity exists, locate the AC high limit thermostat
and depress the center button to reset it. Retest for conti-
nuity.
A. If continuity exists continue to step 8.
C. If continuity still does not exist, follow the instruc-
tions in this section for replacing the AC high-limit
thermostat.
8. Move the electric switch to the “OFF” position. Retest for
continuity. If continuity still exists, replace the AC relay
NOTE: If the high-limit thermostat is tripped, it is recom-
mended to test the control thermostat and AC relay for prop-
er operation.
Failure to disconnect all power supplies and/or allow the
unit to cool before servicing could cause serious damage
and/or personal injury.
Failure to disconnect all power supplies and/or allow the
unit to cool before servicing could cause serious damage
and/or personal injury.
AC High Limit Thermostat Figure 13
Figure 12

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Replacement Procedure
1. Shut off gas, electric and water supply to the Aqua-Hot.
2. Disconnect the domestic water inlet and outlet line fittings
from the Aqua-Hot
3. Unscrew the nut that affixes the mixing valve to the left
side of the Aqua-Hot cabinet wall
4. Push down on the mixing valve to expose access to the AC
high-limit thermostat.
5. Cut the wire tie which wraps the wires on the left side of
the unit
6. Cut the black wires on the AC high-limit thermostat as
close as possible to the body of the thermostat and strip
the other end.
7. Remove the faulty high-limit thermostat using a 5/8”
socket or crow’s foot
8. Butt-splice a new AC high-limit thermostat to the stripped
wire
9. Re-install the AC high-limit thermostat using a 5/8” sock-
et or crow’s foot
10. Re-secure the wires using the extra wire tie provided and
re-install the mixing valve assembly.
11. Make sure the replacement AC high-limit thermostat is
functioning properly by testing for continuity across this
thermostat.
12. Reposition the tempering valve assembly onto its mount-
ing stud on the left cabinet wall.
13. Re-secure the tempering valve assembly with the previ-
ously removed nut.
14. Re-connect the domestic water connections to the Pex
fittings of the tempering valve assembly.
15. Re-establish the gas, electric, and water supply to the Aq-
ua-Hot.
16. Test the unit for normal operation.
Figure 14
Figure 15
Aqua-Hot Components

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©Aqua-Hot® 200 Series Propane and Diesel Service Manual
Aqua-Hot Components
Burner High-Limit Thermostats
Scope:
The burner high-limit thermostats serve as a safety
measure in the event that the burner continues to operate
after the maximum operating temperature (215°F) is reached.
While the functional goal of the high limit thermostats
remains the same between the Propane and Diesel units, the
precise functionality herein differs between these units. These
operational differences are outlined below.
If the system reaches an over-temperature condition,
the thermostats on the 250D will both interrupt the function of
the fuel solenoid, preventing diesel fuel from entering the
combustion chamber.
The 250P will respond to an over-temperature condi-
tion by interrupting the power signal (left thermostat,) and the
fan validation signal (right thermostat.)
Troubleshooting Condition
Troubleshoot the high-limit thermostats if the follow-
ing conditions have occurred;
• The gas valve or fuel solenoid fails to operate
• The blower fan runs, but the burner does not ignite
Troubleshooting
 Locate the high-limit thermostats.
 Disconnect the burner thermostats using the information
below pertaining to the unit under service
250D: Wires #2 and #34 (left) or wires #34 and #8
(right)
250P: Wires #37 and #39 (left) or wires #38 and #36
(right)
 Place a jumper wire between the two wires on the harness
to bypass the high-limit thermostat. Check the Aqua-Hot
for normal operation.
 Disconnect the DC high-limit thermostat wires, then, using
an ohmmeter, check the thermostat for continuity
 If continuity does not exist, depress the button in the cen-
ter of the thermostat to reset it. Retest for continuity
 If the thermostat at this point still does not have continui-
ty, follow the instructions in this section to replace the
thermostat
Replacement Procedure
 Ensure that the Aqua-Hot has been completely shut down,
all power sources disconnected, and the unit is allowed to
cool completely before continuing
 Remove the two wires from the defective high-limit ther-
mostat
 Using a 5/8” socket or crow’s foot, remove the defective
high-limit thermostat from the face of the tank
 Install the replacement high-limit thermostat into the port
on the face of the boiler tank, and finger tighten the new
thermostat in place
 Reconnect wire #2 and wire #34 (left) and/or wire #34
and wire #8 (right) to the high limit thermostat which has
just been replaced
 Test for unit operation
NOTE: Bypassing the high-limit thermostats is for testing only
and must not be used as a substitute for normal operation.
Failure to disconnect all power supplies and/or allow the
unit to cool before servicing could cause serious damage
and/or personal injury.
NOTE: If the high-limit thermostat continues to trip, trouble-
shoot the control thermostat and verify that the boiler tank is
full of antifreeze and water heating solution
Right High-Limit Thermostat
Left High-Limit Thermostat
Figure 16