A quick guide to troubleshooting RV furnace problems

9
Steve Savage submitted this article to RVtravel.com when he was a Master Certified RV Technician with Mobility RV Service.

When winter arrives, most service calls I get are for RV furnace problems. 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.

How does a furnace run?

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.

Quick troubleshooting tips to consider

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, another common problem, stink bugs disrupting gas flow to the igniter. How many bugs does it take? Glad you asked — one! Also, furnace vents can be blocked by mud daubers.

“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 they’re 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.

Testing a gas valve

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!

Related articles:

Protect your furnace vent from LP-sniffing bugs

Maintain your RV furnace and keep warm!

You can get highly rated Camco mud dauber screens at Amazon.

##RVDT1453

Subscribe
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

9 Comments
Newest
Oldest
Inline Feedbacks
View all comments

Robert Johnson
16 days ago

On my 2006 prowler the furnace sounds like jet motor when it is on. Is there an adjustment I can do?

Don Verellen
24 days ago

I have had to clean the igniter/flame sensor a couple of times on my 2016 Altitude Atwood furnace. The last time it quit sensing about 8 hours after installing a couple of very old propane tanks. My wife noticed that the flame on the cooktop had a lot of yellow and red instead of mostly blue also. Furnace would ignite and then shut off after about 5 seconds and when I pulled the igniter it had a lot of soot on it. Cleaned up with some 320 grit emory cloth and works like a charm.

Melissa
26 days ago

I currently have an odd issue w/my 2018 KZ Durango 2500 w/a Suburban/Airxcel furnace. The limit switch keeps failing, or overheating, but only during the night. During the day, the furnace will work normally. I had been able reset it at the thermostat by dropping the switch to it’s lowesting setting & turning the thermostat to Off, wait about 1 minute & then turn the thermostat up to 75 & turning the thermostat to Heat. This worked for the most part, but I had to get up every hour during the night to do this. Since it was negative temps outside, I was willing to do this since it meant staying warm. It should be noted there’s a hole cut in my duct work in my bedroom (rear bedroom RV) &I had covered this, thinking it was a mistake, but it wasn’t &that hole acts as a return for the furnace I believe. Well, I had no issues for 1 week and then my furnace started acting up again. I’m in MT for a month, temps are dropping – if anyone has any ideas, please help!

Chris
20 days ago
Reply to  Melissa

I have the same problem. Hope someone has an answer.

Tom Herbert
30 days ago

Great article with lots of useful information. The furnace in my 2016 Cedar Creek works fine but there is something that I can’t figure out. When on shore power, if the power goes out the furnace fan comes on for about 10 seconds and then goes out. This happens even if the thermostat is off or set to A/C. What’s happening here? Thanks!

impavid
1 month ago

On two RVs my furnace wouldn’t run. The first time it happened I spoke with a tech and he told me to pull the front off the thermostat, take out the fuse, wait 5 minutes and replace the fuse of course making sure the fuse was good. Both times that solved the problem. I don’t recall the make of the first thermostat but the one I have now is Coleman. Hope this helps someone.

Nancy K Michaels
1 month ago

Good info! One other possibility is an empty propane tank, which is what happened to us last night in 30°+ weather!! The furnace worked just fine after my husband made the switch to the other tank at about 4 AM!!😖

James Wills
1 month ago

Not advertising for somebody, but I bought a couple of Tank
sensors that sit on the bottom of the tank and you can access the tanks level on your smartphone using Bluetooth. With them in place I Know what level my tanks are without the hassle of removing the cover and lifting them up to see how full they are and when the working one is low, I just switch to the other before I go to bed. Here is the Amazon link for the sensors, I am just a user, I get nothing for this endorsement. but maybe RV Travel will because it is coming off their webSite.

https://www.amazon.com/gp/product/B01C5RQKJA/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Jeff Arthur
1 month ago

Doesn’t the furnace run on 12v ? I’ve always heard that if you don’t have a good 12v source ( Battery) it’s not going to work?
I didn’t see battery listed.
Was a good informative article though
Thanks