If Shore Power Fails Will Rv Gas Furnace Continue to Heat
You've been out and about all day, and now the temperature is starting to drop. You turn the dial on the thermostat, expecting a rush of warm air, but then nothing is happening. You're in the middle of nowhere, everyone is wearing jumpers and the temperature is set to plummet overnight. What are you going to do? It's time for a bit of RV troubleshooting.
This is a horrible situation to find yourself in, but all is not lost, with a little bit of know how it's possible you can get the furnace back up and running within a few minutes. We'll cover some common issues and how to fix them in this article, but to start with we'll get ourselves familiar with how an RV furnace works and the parts that make it work
How Does an RV Furnace Work?
We can't account for every model and furnace type, but this applies to the most common RV furnaces you're likely to come across.
- The thermostat triggers a request for heat, sending a current to the RV furnace.
- The current powers the time delay relay which in turn passed power to the RV furnace blower.
- The blower spins up and pulls air from the Air Return creating a flow of air through the heat exchanger and out of the ducts. Another fan pulls air into the combustion chamber to feed the furnace.
- As the blower speed increased the fan air causes the sail switch to close.
- Power then flows through the High Limit Switch and onwards to the Control Circuit Board.
- As power is delivered to the control board it opens the gas supply and causes the DSI igniter to spark.
- The gas lights and creates heat within the combustion chamber.
- Heat will pass into the heat exchanger, heating the air passing through the Ducts.
- As the RV begins to warm up, the Thermostat will reach its preconfigured temperature.
- Once the correct temperature is reached power is removed the burner will shut down.
- The fab will continue to run in order to cool the furnace safely until the time delay switch opens.
If you own a furnace that operates a pilot light, the operation is similar, but the control board doesn't trigger the ignitor. Once the pilot light is lit, the furnace is controlled by the flow of gas alone.
All RV furnaces operate on battery power alone, so you don't need to plug into the mains or run a generator. As long the RVs batteries are suitably charged and you have a supply of gas, then you can operate the furnace with no issues.
RV Furnace Parts
Air Return: The air return is a vent on the inside of the RV that sucks in air to circulate through the heat exchanger and out through the ducts. This is not to be confused with the air used for combustion, there is a seperate intake for this.
Blower Motor: The blower motor drives a couple of fans that circulate air through the furnace. One fan brings air in through the Air Return, the other fan brings in air from outside into the combustion chamber and out through the exhaust.
On/Off Switch & Circuit Breaker: The circuit breaker and On/Off Switch serve to interrupt power to the blower motor. The on/off switch is commonly used for servicing, while the circuit breaker can automatically shut down in case of an issue. If the circuit breaker has triggered it can often be reset but the underlying cause of the trigger should be investigated and fixed.
Combustion Air Intake: This is the air inlet positioned on the exterior of the RV that pumps air into the combustion chamber.
Combustion Chamber: The chamber where oxygen and propane are burned to produce heat.
DSI: The DSI is an acronym (Direct Spart Ignition). It's an electronic igniter that uses a spark ignites the gas-air mix.
Ducts: The Ducts carry the heated air from the furnace to the interior of the RV.
Exhaust: The exhaust is an air outlet on the exterior of the RV that exhausts the gases produced during the combustion process. The gases are a combination of carbon dioxide and carbon monoxide, these gases must never enter the interior of the RV, doing so can result in serious injury or death.
Fan Speed Controller: The Fan Speed Controller controls the speed of the fans, increasing or decreasing the BTU output of the furnace. Lower fan speed will result in a lower BTU output.
Heat Exchanger: The heat exchanger used conduction to take the heat generated within the combustion chamber and uses it to warm the air circulating through the ducts.
Gas Valve: The gas valve is an electronically controlled valve which opens to allow gas to flow into the combustion chamber. It's controlled by the circuit board.
High Limit Switch: The high limit switch controls excess heat from damaging the heat exchanger. If the temperatures get too high the switch will cut power and avoid damage that could be caused. Damage to the heat exchanger can cause deadly gases to enter the ducts, so this is a very important safety device.
Igniter: The igniter generates a spark to ignite the propane. In a DSI furnace, the igniter takes the place of a pilot light. It will also act as a sensor to verify that ignition was successful.
Ignition Control Circuit Board: The ignition circuit board is in charge of operating the gas valve and firing the ignitor spark. In some models the ignitor will try several times to ignite the furnace, you may hear a distinct tick-tick-tick sound as the spark initiates. Some models may only attempt to ignite every 60 seconds, while others will attempt once and then shut down if the ignition is not achieved, these require the thermostat to be cycled to attempt again.
Pilot Light: The pilot light produced a small flame that continuously burns and will light the main propane source when the valve is opened. This is not present in DSI furnaces.
Sail Switch: The sail switch features a paddle arm that covers the blower air stream, as the blower reaches full speed the sail switch is pushed out of the way, this action allows for power to be passed to the ignition switch.
Thermostat: The thermostat monitors the internal temperature inside the RV and will trigger the furnace to turn on if the internal temperature has not met the specified threshold.
Time Delay Relay: The time delay relay uses a small delay before initiating the furnace cycle.
RV Furnace Troubleshooting General Tips
Tools
The most important tool in your arsenal to help you troubleshoot an RV furnace is the voltmeter. Equipped with one of these and knowledge of how to use will go a long way to helping you diagnose and fix most RV furnace issue.
We would use the voltmeter in order to ensure that sufficient power is reaching the components that make up the RV furnace. You don't need to go all out and buy an expensive voltmeter, a cheap one will usually suffice. We quite like this one.
Diagnostic Codes and Lights
Newer RV furnaces tend to have a small led attached to the ICB. This LED will flash a number of times if there is an issue present, the number of flashes will indicate the type of issue. A chart will be available somewhere on the surface to help you decipher what the flashes indicate.
Fans and Blower Motors
The furnace fans are most often located behind the outside access cover. The motor will provide power through a shaft to both the main furnace fan and to the combustion air fan. Typically the motor and fans are covered in a case which may also contain the sail switch.
The motor will be powered by the RV's 12v power system. If the RV is plugged into shore power or has a generator running, then the battery will remain fully charged and there should be no drop in voltage to the motor. However, if the RV battery is an end of life or has been powering the motor for an extended period of time, the voltage may not be sufficient power the motor.
Wear and tear or corrosion of the wiring may also impact the voltage reaching the motor, checking the voltage at the batteries as well as at the motor may help to diagnose some issues.
Combustion Exhaust and Air Intake
The external combustion exhaust and air intake are located at the external furnace cover panel. Bugs, dirt or even nesting mice can block airflow and inhibit operation, so it's worthwhile checking for blockages.
Word of warning, some furnaces will not operate with the external cover removed, so ensure to replace it after checking for blockages.
Air Return and Ducts
All air vents should be kept as clean and dust free as possible. The intake vents should be cleaned periodically to remove the build-up of dirt and to ensure the flow of air. Dirt that enters the ducting can build up on fans, switches and on the heat exchanger which can affect performance or cause a fire risk.
Resets and Lockouts
Some furnaces will enter a lockout condition if an issue is encountered. It simply means the control board won't allow the furnace to start. The lockout condition can be reset by turning the thermostat off and reducing the temperature to a value which would not cause the furnace to turn on. Wait approximately 30 seconds and then turn the thermostat back on and increase the temperature to the desired level.
Gas Pressure
All gas appliances are set to operate with a standard supply pressure of approximately 3psi. The pressure within a propane tank is much higher than this, so a propane regulator is used to control the gas pressure. If a residue is present in the propane tank then this can interfere with the regulator, causing it to fail or work intermittently. Additionally, an empty tank will not provide any pressure, so be sure to check your levels.
If you suspect an issue with the gas supply, then a gas regulator is the most likely component to fail. Thankfully they are inexpensive and can be easily replaced, check out our article of the best RV propane regulators.
Common RV Furnace Problems
RV Furnace Blower Won't Start
This can be caused by a number of reasons:
- Not enough power from the thermostat to the furnace. Can be caused by a bad wire or by power not reaching the thermostat.
- Failed Time Delay Relay
- No power from the RVs battery to the furnace relay switch.
- The circuit breaker is triggered or turned off for servicing.
- Loose wiring.
- Failed blower motor.
If the RV battery voltage is low, this can cause a spike in the current which may cause the circuit breaker to trigger. Additionally, excessive strain on the motor caused by dirt can cause the circuit breaker to trigger. If the motor has a short it can also cause the breaker to trigger.
Check the wiring, looking for corrosion, or loose wires. Check for a build up of dirt around fans and the motor. Check the voltage coming from the battery and where it reaches the motor. Reset the circuit breaker and try again.
RV Furnace Fan Runs But No Heat
This one of the most common issues you'll encounter, but its also one of the easiest ones to troubleshoot.
- The Sail Switch is failing to close fully, resulting in no power being provided to the rest of the furnace. This can be caused by a stuck or fault sail switch, or because the fan is turning to slowly to push the sail. The motor might be slow due to a lack of voltage, so check the voltage at the batteries and at the fan. The motor may also turn slowly due to corrosion or dirt. Lastly, a blockage at the air return will cause a lack of airflow.
- The high limit switch is not closed or has failed, this will cause no power to reach the controller circuit board. Check the voltage at the controller board and at the switch.
- The controller circuit board has failed and is not opening the gas valve or its not triggering the ignition. Check the voltage at the gas valve during the turn-on cycle.
- The gas valve is not opening, so a lack of gas is reaching the combustion chamber. This can be caused by a failed valve or a lack of power. Check the voltage at the gas valve.
- The gas valve opens but no propane flows. This can be caused by a line blockage, bad regulator or lack of pressure. Check other gas appliances to ensure they are working.
- The igniter is not sparking or the pilot light is not on. Can be caused by a defective igniter or a lack of power. Check the voltage at the igniter and check the gap between the igniter to ensure it's not too large or that its shorted.
- The furnace lights but the igniter fail to detect a flame and therefore initiates a shutdown.
- Both propane and spare occur, but the burner still fails to light. This can happen if the air intake is blocked or there is insufficient air flow into the combustion chamber.
- Corroded or loose fittings in any of the components listed above.
RV Furnace Runs Once But Won't Restart
This sort of issue is most often caused by a blockage in the air intake or exhaust, which causes a lack of air in the combustion chamber. Check for debris, dirt or bugs in the exhaust or inlet. It can also be caused by a lack of gas pressure or a gas valve which is only partially opening.
RV Furnace Stops and Starts
This issue is often caused by the thermostat receiving false readings. Either hot air from the duct is blowing on the thermostat, causing it to think the RV is warmer than it actually is, or cold air is blowing across the thermostat, resulting in a similar issue. If the ducts can be directed, make sure they point away from the thermostat. Also, check for any drafts.
If the issue still persists then it's likely the thermostat is faulty and should be replaced.
RV Burner Goes off and On
This problem is called 'cycling on the limit switch' and is a symptom of the heat exchanger overheating, this causes the high limit switch to act. Nine times out ten this is caused by a lack of air circulation through the furnace, normally due to a blockage of some sort.
A build-up of dust or dirt within the heat exchanger can also cause this problem, and in some circumstances, it can be caused by a faulty limit switch.
RV Furnace Doesn't Shut off When Temperature is Reached
This problem would indicate that the thermostat is still requesting more heat. You should check the voltage from the thermostat to the time delay switch, if a voltage is present, then the thermostat is faulty. If no voltage is present then it's likely there is a short somewhere that is creating the voltage that normally comes from the thermostat.
It should be noted that the fan will normally run for a couple of minutes after the required temperature is reached, this acts to cool the furnace safely.
RV Soot on the Exhaust Vent
Soot is an indication that combustion is not occurring cleanly. Carbon Monoxide is another product produced during improper combustion, this is an extremely serious and dangerous gas. If you see soot on your external exhaust you need to get the Furnace inspected by a professional immediately, you MUST NOT use the furnace until it's been inspected.
Soot and carbon monoxide are produced when there is no oxygen available during the burn, normally caused by a blocked intake or fan that's not turning fast enough.
RV Furnace Works on Shore Power but not on Battery
If your RV furnace only works when you're connected to mains power or a generator, then it's a sign your battery needs replacing or that there's corrosion. Both of these issues result in reduced voltage reaching the furnace and lack of power to the motor.
RV Furnace Stops Running Overnight
The most common cause of this problem is an exhausted battery. The motor in the furnace will use a lot of electricity and if the battery or batteries are not up to spec then they may drain overnight.
In addition, corrosion of wiring can cause excessive current draw from the battery, resulting in the battery losing power faster than expected, so it's worth checking
RV Furnace Fan Making a Noise
This issue is most likely caused by the fan blades rubbing against the fan housing, it may also be caused by a loose or bad bearing within the fan motor.
Weak Air Flow From RV Furnace
We most often see this issue with ducts that are excessively long or have crimps somewhere. Check as much of the ducts as you can, straightening or removing excess duct. It's also worth checking the outlets, many have adjustable air flow, so removing the cover will allow you to check.
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