Troubleshooting RV furnace problems

Troubleshooting RV furnace problems

By Steve Savage
Mobility RV Service

When winter arrives, most service calls I get are for furnace repair. So let’s review the do’s and don’ts.

First of all, not knowing what you are doing with a gas-fired appliance can be dangerous, no matter how easy it looks on YouTube. Knowledge of multi-meter use is essential. So is understanding how the furnace sequence of operation goes. Here is a quiz question to test your knowledge: When you turn the thermostat up on the wall of your RV, what does it do?

If you did not say, “It starts the blower motor,” your answer was incorrect. The thermostat does not light gas, it starts the motor. The motor has to spool up and close the circuit on a small switch called a sail switch behind the blower wheel cover. You cannot see it without removing the cover as it is fastened to the back. The sail switch then sends power to the high limit temperature control. That thermo switch opens if the furnace gets too hot and shuts down the gas. These two switches are safety switches and they are in series, which means power passes from one to the other and then to the module board, which sends power to the gas valve and the igniter which does the sparking and ignites the gas.

That means you will first hear the motor spool up and run for about 15 seconds. If the sail switch has closed and the high limit is good, you will have power to the board and those seconds are to exhaust any unburned gases from the combustion chamber. That delay is handled by the board and it is another safety feature. When the thermostat is satisfied, it tells the board to shut off the gas and the fan continues to run for a couple of minutes to cool down the combustion chamber and make sure all the gas is vented to the outside. That delay is either on the board or on a separate relay, depending on the model of furnace. If all of this sounds fairly complicated, it is, and even a lot of techs have trouble keeping straight who does what.

I can’t walk you through the entire troubleshooting process, but I will touch on some things for you to think about, if you choose to learn more about this topic.

If the furnace motor doesn’t start when you turn up the thermostat, the possible problems are the fuse for the furnace, the thermostat, the module board, the small fan relay that is separate on older model furnaces, or the motor itself.

If the motor runs but nothing else happens, look to the possibility of  a failed sail switch or high temperature relay. You check for this by checking for voltage on the lead to the board from the high temperature relay.

If you hear the “click, click, click” of the igniter and the “thunk” of the gas valve, you normally can think in terms of the control systems being good. You have noises which suggest you should have spark and gas. I say “should” because you now can have a cracked igniter insulator, bad igniter placement, a defective gas valve, or tah dah, the most common problem this year, stink bugs disrupting gas flow to the igniter. How many bugs does it take? Glad you asked — one!

“Buggy” combustion chambers are perhaps the most common problem I see in Suburban furnaces, although it is also a problem in Atwoods. How do you know if your furnace has bugs? One way is the telltale smell of propane out of the exhaust tube of the furnace with either erratic or no ignition.

What about the igniter? Maybe you have gas, but no spark since you will still hear a click if the spark is jumping through a cracked ceramic insulator. Good point, but bad igniter problems are really rare and I will go way out on a limb and say almost never in Suburban furnaces. It’s really infrequent in Atwoods, so if you start assuming a bad igniter, you will nearly always be wrong.

What does it mean if the furnace does not ignite and you either do not hear the thunk or you hear the thunk, but do not smell gas? You probably have a bad gas valve. Getting to the gas valve on Suburban furnaces is a real pain, not so bad on the Atwoods, but you still have to check it, not simply throw parts at it. By the way, never take a gas valve apart, unless working as part of a demolition team.

To test a Suburban gas valve, you are going to have to pull the working parts of the furnace out of its case. The gas valve sits between the blower wheels and is a pain to remove, so don’t pull it without testing it.

To test it, take your trusty multi-meter, set it to ohms, and put one lead on each terminal of the solenoids on the gas valve, so you are testing the solenoids one at a time for continuity.

You are looking for values between 30-50 ohms, but realistically I look for extremes. I do not replace a valve that reads 51 or 52 ohms, and I expect to see an “open” or OL if a solenoid is bad.

If the valve tests bad, you will need to remove it and replace it with a new gas valve. To get it out of a Suburban furnace you will need a 5/8″ crowsfoot on a 3/8 ratchet with an extension. Getting it back in place with the supply tube to the burner reconnected requires holding the valve up from the bottom with one hand and using one finger to start the inverted flare nut on the tube back into the valve. After you do about a hundred of them, you will still be asking who on earth designed this thing!

Getting the Atwood valve out will require a long 1/4″ extension along with a 1/4″ magnetic tip, which I use on my Dewalt drill. It is fairly straightforward, once you figure out which hex head screws you have to remove to get the assembly out (spoiler hint — there are three of them, plus a wing nut).

There is a good deal of information about furnace repair on the Internet, although frankly, this is one repair that is somewhat complex and I think beyond the majority of RV owners’ pay grade. If by chance you do have a furnace problem, get it taken care of as they only get worse over time, beginning as an occasional nuisance and then failing completely when you need heat most!

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