Found this old guide from Boots49 (Its long, but has links also)
I am not sure if this will help or not, sorry about the length:
RV FURNACE TROUBLESHOOTING
You’ve had a fun day but now it’s getting chilly, so you decide to fire up the RV furnace. You flip the
thermostat on, set the temperature and wait expectantly, but no heat comes from the outlets. It’s
Saturday night and you are camped far from civilization in that secluded spot by the lake. You tell the
kids to put their sweaters on, but it’s going to be really cold later tonight. What are you going to do to
get the furnace running?
This brief will attempt to provide you with some basic furnace troubleshooting advice. We will go over
symptoms and their causes, but first you need to understand a little about how an RV furnace operates
and know of few basic terms.
FURNACE OPERATION OVERVIEW
This is generic description of RV furnace operation and applies to most DSI type furnaces.
1. The Thermostat calls for heat, sending DC current to the furnace.
2. A Time Delay Relay is energized and passes battery DC power to the blower motor.
3. The blower motor and fans spin, pulling air from the Air Return and generating a stream
of air through the Heat Exchanger and out the Ducts. At the same time another fan pulls
air from the Combustion Air Intake into the Combustion Chamber and out the Exhaust.
4. The fan air stream causes the Sail switch to close as the blower gets near full speed.
5. Power flows to the High Limit Switch and onward to the Control Circuit Board.
6. The Control Circuit Board opens the gas valve and generates a spark at the DSI Igniter.
7. The Burner ignites, building heat in the Combustion Chamber.
8. Heat passes through the Heat Exchanger, warming the air circulating through the Ducts.
9. The Thermostat reaches the Set Temperature and opens.
10. Power is removed from the Control Board and the Burner shuts down.
11. The fan continues to run to cool the furnace, until the Time Delay Relay opens again.
For a Pilot Light model of furnace, the operation is similar but ignition does not depend on a
control board to trigger an igniter. Once the pilot is lit (manually), it stays lit and is the ignition
source. The furnace cycles on/off by control of the gas flow alone, with the gas valve opening or
closing in response to the thermostat demand.
An RV furnace runs strictly on 12 volt power (12 VDC) and does not depend on shore power or
generator to operate. As long as the RV’s batteries have sufficient voltage and amps, and there is
a supply of propane to produce heat, the furnace will function.
TERMINOLOGY
Air Return – an opening in the RV interior that allows inside air to enter the Blower Motor and
be recirculated out the Ducts. This air is not used to burn the propane in the combustion chamber
– there is a separate air source for that (see Combustion Air Intake).
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RV FURNACE TROUBLESHOOTING
Blower Motor – A 12VDC motor driving a pair of “squirrel cage” fans that circulate air through
the furnace. One fan brings RV interior air in via the Air Return and through the Heat Exchanger
and out the Ducts while the other brings outside air into the combustion chamber and out the
exhaust.
Circuit Breaker & On/Off Switch
Newer furnaces have an On/Off switch and a Circuit Breaker [CB] in the power feed to the
Blower Motor. The switch is for manual control, especially while serving the motor or burner.
The Circuit Breaker limits the power to the motor. If the CB has opened, it can be reset but the
conditions that caused it to open must be repaired for a lasting fix.
Combustion Air Intake – A fan with an air inlet on the exterior of the RV that provides air to
the Combustion Chamber. This air is used when the propane burns.
Combustion Chamber – The metal box where propane and air are burned.
DSI – is an acronym for Direct Spark Ignition. It refers to the use of an electronic Igniter to fire
the propane instead of a pilot light.
Ducts – round or rectangular tubes that carry heated air from the furnace to the RV interior.
Exhaust – An air outlet on the exterior of the RV that vents exhaust gases, including carbon
monoxide and carbon dioxide, from the Combustion Chamber. Exhaust gas contains carbon
monoxide and carbon dioxide and must never be allowed to enter the interior of the RV. Injury
or death can result if it does!
Fan Speed Controller – On models with two or more fan speeds, this small circuit board sets
the motor speed and thus governs the speed of both fans. This also affects the BTU (heat) output
delivered by the furnace – lower fan speed means lower BTUs and so the Gas Valve opening is
also reduced.
Heat Exchanger – A metal box that uses heat conducted from the Combustion Chamber to
warm the circulating air.
Gas Valve – an electrically operated valve which opens to allow propane to flow into the burner
chamber. It is controlled by the Control Circuit Board.
High Limit Switch – a normally closed switch that opens if it gets too hot. Its purpose is to
monitor temperatures in the Heat Exchanger and to prevent overheating that could burn a hole in
the metal. Opening this switch turns off power to the gas valve and the Controller Board,
shutting down the furnace. A hole in the Heat Exchanger would allow deadly poisonous gases to
enter the RV interior, so this is a crucial safety device.
Igniter – An electrical device that generates a spark to ignite the propane. In DSI furnaces, the
igniter serves in place of a Pilot Light. In newer furnaces, the igniter also serves as a flame
sensor, providing feedback to the Ignition Control Board to verify that ignition was successful.
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RV FURNACE TROUBLESHOOTING
Ignition Control Circuit Board – A printed circuit board that initiates gas valve opening and
triggers the Igniter. In some furnaces the Igniter sequence may last several seconds and make
multiple attempts to light the flame. You may hear a tick-tick-tick sound as the Igniter is cycled
repeatedly. Other boards will make three attempts
at approximately 60 second intervals. In older
models there is only one attempt to light and the
thermostat must be cycled off for several seconds
and back on again to initiate another ignition
cycle. In newer furnaces there is a feedback signal
from the igniter to confirm that ignition was
successful and a flame is present. If there is no
positive ignition feedback within 6-10 seconds,
the gas supply will be shut off and the furnace is
“locked out”. In some furnaces there are two
circuit boards – one for ignition and one for speed
control of the blower motor
Lock-out
The Control (Ignition) Circuit Board is shut down because an unsafe operating condition has
been detected.
Pilot Light – A tiny propane flame that remains continually on (burning) and lights a larger
charge of propane when the main gas valve opens. It is initially lit manually and can remain on
as long as the furnace is expected to be needed. Not used in a DSI furnace.
Sail Switch - a switch with a paddle arm that
extends into the blower air stream. It closes when
the blower reaches its rated speed (or nearly so) and
allows power to pass to the ignition circuit. This is a
safety device to assure there is adequate air
circulation in the furnace.
Thermostat – a device that senses temperature in the RV interior and sends a signal to the
furnace when the temperature drops below the “set temperature”, i.e. the temperature you wish to
maintain. This signal is termed “calling for heat or “demanding heat”. When the thermostat is not
calling for heat, it is said to be “satisfied”.
Time Delay Relay – A relay (switch) that generates a slight
time delay before the furnace start cycle proceeds. It has an
internal heater that warms up as the current from the thermostat
passes through it and the relay closes when it gets hot enough.
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RV FURNACE TROUBLESHOOTING
A delay of about 10-20 seconds is typical. When the relay closes, 12V power from the battery is
passed to the blower motor and the fans start to run.
TIPS and TECHNIQUES
Tools Needed
It is difficult to do much troubleshooting on a furnace without a voltmeter and the basic knowhow
to use it. Most tests are voltage measurements to see if adequate power is reaching various
components of the system. If you buy an inexpensive Volt-Ohm Meter (VOM), it will usually
have brief instructions with it. You don’t need a fancy one – the measurements used do not
require a lot of accuracy.
Diagnostic Light & Codes
Newer Atwood furnaces may have a small red light on the Ignition Control Circuit Board. The
light will flash a code (typically 1-5 flashes) to indicate error conditions such as High Limit
Switch open. There should be a code chart on the motor housing for that model of furnace.
Blower Motor and Fans
The fans are behind the outside access cover. The motor has a shaft running through the center
of it with one shaft (usually the left side) driving the main furnace air circulation fan and the
other driving the combustion air fan. There are two blowers (fans) but only one motor. Typically
there is a casing surrounding the motor and the air circulation fan and the Sail Switch may also
be inside that casing.
The motor is powered by the RV’s 12v system and is affected by low voltage. When the battery
charging system is operational (shore power or generator on) the DC voltage usually remains
sufficient (12.0 – 13.8VDC), but when relying on battery power alone the voltage drops and may
get quite low overnight. The motor will continue to run at lower voltages (down to around 10.5
VDC) but fan speeds may be insufficient to activate the sail switch or provide adequate
combustion air to the burner. Corrosion on the wiring or furnace power terminals may also cause
reduced voltage at the blower motor. Check the voltage at the motor itself as well as at the
batteries or your RV’s monitoring panel.
Combustion Air Intake & Exhaust
The external Combustion Air Intake & Exhaust is through the external furnace
cover panel and may be either two ports or a mesh intake grill surrounding a
single exhaust port (illustrated). Bugs such a spiders and mud dauber wasps
and even mice may nest inside and block air flow, so check for blockages.
Some furnaces do not run well with the cover removed, so it may be necessary
to replace the outside cover for testing.
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RV FURNACE TROUBLESHOOTING
Air Return and Ducts
Return air vents should be kept free of dust and lint and should not be obstructed. Vacuum the
intake area and screen (and filter, if provided) to keep good air flow through the heat exchanger
and to keep dist and dirt from building up around the fans, sail switch, limit switch, exchanger
and ducts. Dirt in these places can cause switch malfunctions and internal heat build-up. Crimps
in ducts can also reduce air flow into the room or cause the high limit switch to activate.
Lock-out & Reset
Newer furnaces may go into “lock-out” if a potential safety problem is detected. Lock-out simply
means that the Control and/or ignition circuits shut down and do not allow furnace operation.
Lock-out can be reset by turning the thermostat switch off and changing the Set Temperature to a
low enough value so there is no demand for heat and waiting 10-15 seconds. Then turn the
thermostat on and set the temperature to the desired level and the furnace should attempt to start
again.
Propane Gas Pressure
All propane appliances are designed to operate from a propane (LPG) supply that is at a standard
pressure of 11 water column inches of pressure (about 3 psi). This is measured with a tool called
a manometer. The LP tank pressure is much higher than 11” WC, so the pressure in the system is
controlled by an LP regulator at or near the main supply tank. Either high or low pressure can
cause the burner to fail to light or work improperly. Oily residue in the tank can clog the
regulator, causing it to deliver gas at an incorrect pressure or sporadically. And of course an
empty tank has little or no pressure. An LP regulator is not expensive, so if you suspect a
problem with the gas supply, replacing the regulator is usually more cost effective than hiring a
professional to test it. LP regulators are standard parts, available at any LP gas dealer and many
hardware and home stores. You do not need an RV specific part, but do not use a regulator
intended for natural gas.
SYMPTOMS AND CAUSES
Find your symptom below and see what the likely causes may be.
Thermostat On but Furnace Fan Does Not Come On
This may be caused by:
1. No voltage from thermostat to furnace (bad wire or no power to thermostat)
2. Bad Time Delay Relay
3. No 12v power from the battery to the furnace Time Delay Relay
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RV FURNACE TROUBLESHOOTING
4. Motor circuit breaker is open or manual switch is Off
5. Loose wire
6. Bad blower motor
Low battery voltage can cause excessive current through the motor and trip the circuit breaker
and so can excessive load on the motor, e.g. dirt in the fan housing. A defective motor can also
cause a breaker to trip.
Fan Runs but No Heat from Ducts
This is a classic condition with several possible causes, most of which should be obvious after
reading the Operational Overview.
1. The Sail Switch is not closing, so no power to the rest of the furnace. This may be because of
a defective switch or because the fan is turning too slowly to push the sail enough to fully
close the switch. A slow motor may result from low voltage at the motor (check voltage at
batteries and at the fan) or a motor that is simply not turning well due to dirt or corrosion. A
blockage of the Return Air Intake can also cause a lack of air flow against the Sail.
2. The High Limit Switch has failed and is not closed, so no power reaches the Controller
Circuit Board and hence no ignition. Check for 12+ volts leaving the switch and/or at the
terminal on the controller.
3. The Control Circuit Board is failing and either does not open the Gas valve or does not
trigger the Igniter. Check for 12V to the gas valve during the opening cycle.
4. The Gas Valve fails to open, so no propane in the burner. Could be a bad valve or simply no
12v power is reaching the valve (check voltage).
5. The Gas Valve is open but no propane flows. Possible causes include no propane or low
propane pressure (less than 11 water column inches). Low or no propane can be caused by a
clogged line or main propane regulator. Check other propane appliances for proper operation.
6. The Igniter fails to spark (or in a non-DSI furnace, the Pilot Light is not on). Could be a
defective Igniter or no 12v power is reaching it. It is also possible the gap at the ends of the
Igniter electrodes is too great (or that they are shorted together).
7. Igniter does not confirm that ignition was successful (no flame is sensed), so the
Control/Ignition board has shut down (lock-out)
8. Propane and spark are present but the burner doesn’t light. This can occur if the Combustion
Air Intake is clogged and there is insufficient air flow through the combustion chamber.
9. Loose or corroded connections in any of the above components
Furnace Runs for One Cycle but Won’t Restart
The typical cause is a blockage in the Combustion Air Intake or Exhaust, resulting in a lack of
air into or out of the combustion chamber. Look for spider webs, dauber nests, debris in the air
inlet and exhaust. It may also be caused by an inadequate gas supply, e.g. low gas pressure or a
partially open Gas Valve. See Propane Gas Pressure.
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RV FURNACE TROUBLESHOOTING
Short Cycling - Furnace Starts and Stops at Short Intervals
This is usually caused by warmed air from the Ducts blowing out onto the thermostat, causing it
to be satisfied, but as soon as the warm air stops flowing the thermostat cools and calls for heat
again. If the Duct outlets are adjustable, try aiming them away from the thermostat or closing the
nearest outlet. A cold draft on the thermostat can have the same effect – look for a cold air
current leaking in from somewhere. Sometimes an over-sensitive thermostat will also do this and
replacing it will cure the problem.
Fans Run and Heat is Produced but Burner Goes Off and On
This is called “cycling on the limit switch” and results from overheating in the Heat Exchanger,
which causes the High Limit Switch to engage. The usual cause for this is poor air circulation
through the furnace, which is a result of blockage of the Air Return or outlet Ducts. An
accumulation of dust and dirt in the Heat Exchanger can also cause it. And in rare cases it may
be an over-sensitive limit switch.
Furnace Runs Normally but Does Not Shut Off When Temperature is Reached
This suggests that the thermostat is still calling for heat. Check the voltage on the wire to the
Time Delay Relay to see if this is the case. If voltage is present, the thermostat is defective. If no
voltage is present, there is a short in the furnace wiring that is creating the voltage that normally
comes from the thermostat. Note, though, that it is normal for the fan to run for 20-60 seconds
after the thermostat is satisfied. This cools the furnace internally.
Soot on the outside exhaust vent
Soot is a sign of improper combustion and means there is a problem. Carbon monoxide [CO] is
another byproduct of improper combustion and is an extremely dangerous gas, so a furnace
showing soot at the outside vent should be repaired BEFORE being used again. Soot and CO
result from burning the propane without sufficient combustion air and generally means that
either the Combustion Air Intake is blocked or the Intake Blower is running too slowly. See
Blower Motor and Fans and Combustion Air Intake & Exhaust.
Works on Shore Power but Not When Disconnected
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RV FURNACE TROUBLESHOOTING
This is a sign of a weak battery or corrosion between the battery and the furnace, resulting in low
voltage at the furnace. When on shore power, the charging system keeps the voltage up high and
powers the furnace, but without it the voltage drops low enough that it cannot operate reliably.
While the furnace specs may say it will run from 10.5 to 13.8v, in practice the furnace may not
start and run much below 11.5v.
Furnace Goes Dead Overnight
This is usually just an exhausted battery. The furnace blower uses a lot of electricity and if the
RV’s charging system is not running the battery can easily go dead overnight. May be bad
battery(s) or simply insufficient batteries for the amount of power you are consuming. Low
voltage at the furnace (due to corrosion in the wiring) can contribute to this because the motor
draws more current when the voltage is very low (under 12.0 vdc).
Fan Motor Makes Screeching Noise
This is typically caused by the fan blades rubbing against the fan housing in either the two
blowers in the furnace or a bad bearing in the motor. (see Blower Motor and Fans).
Poor Air Flow from Duct Outlets
This is often caused by crimped ducts or excessive length ducts that have extra loops that slow
down air flow. Check as much of the ducting as you can reach and straighten loops (removing
excess if possible) and eliminate kinks and crimps as much as possible. Also check the outlets
themselves – many are adjustable and may be partially closed. Take the outlet off and make sure
it actually opens – broken ones are not uncommon.
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RV FURNACE TROUBLESHOOTING
PHOTOS
Atwood Two Speed Furnace (Model 8500-IV)
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RV FURNACE TROUBLESHOOTING
Older Atwood with ported intake & exhaust (circuit board not visible)
LINKS FOR MORE HELP
Atwood Video -
http://www.atwoodmobile.com/Service/...heatingvid.cfm
Atwood Troubleshoot -
http://www.atwoodmobile.com/Service/Trouble/dsi.cfm
Mark’s RV -
http://www.marksrv.com/furnace_trouble_shooting.htm
DuckTec Parts - RV Furnace Trouble Shooting - RV Furnace Trouble Shooting Page
RVers Corner - Troubleshooting the RV Furnace
RV Motorhome Answers - RV FURNACE BASICS,HELP ON RV FURNACES
Rx4RVs - Furnace Troubleshooting (the first step) - Rx4RV
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RV FURNACE TROUBLESHOOTING
Dinosaur Circuit Boards -
http://www.dinosaurelectronics.com/
SOURCES FOR MANUALS
Atwood Parts -
http://www.atwoodmobile.com/Service/Info/heatinfo.cfm
Bryant RV – Furnace and Refrigerator manuals: Service Documents and Manuals
Gary Brinck – RVForum.net 3/14/2008 Page 11.
__________________
Al & Susanne
Aka: Puss 'n Boots
Wildcat 31 TS - Silverado 2500 DuraMax Diesel
Tip of the Day: Stay safe, have fun and never leave the cat out in the rain!
Linear Mode
