Your RV Furnace May Be Headed for a Premature Death

Here you can see what my Suburban RV furnace looks like. There are three knockouts on the front for attaching 4″ round ducts and two on each side. The two on the left side have been made to look knocked out in this stock photo. There is a large rectangular vent on the bottom which is the only one Arctic Fox used when building my RV. I knocked out one on the left side to add the duct I did. Special collars attach to the furnace to which the ducting is then attached.

There is an insidious hidden problem with the installations of many RV furnaces putting them on a collision course with a premature death. Yours could be one of them. Mine was.

I discovered a problem with my RV furnace, a problem I know for a fact that other people have had, and one that may go undiscovered until it’s too late. Actually, the problem has to do with the ducting, specifically a failure to sufficiently duct the furnace at the RV factory or in some cases ducting that has become damaged or clogged for one reason or another. This problem was causing my furnace burner cycle on and off excessively, sending it toward an early grave, or at least a premature repair and one that should not be unnecessary.

I found these registers on Amazon. They rotate and have dampers which is why I chose them over other round vents of the same size.

It appears my RV was “under-ducted” to coin a phrase, that Arctic Fox did not install  sufficient ducting so that the furnace could properly expel the hot air it makes. I don’t see why everyone else with the same make and model RV as mine wouldn’t be experiencing the same problem and I know for a fact that other makes and models have had the issue as well. The insidious thing about all this is that you may have the problem and never know it. You may even wind up paying for repeated repairs to your furnace that should not have been necessary, and still never know what caused the problems.

I noticed that when I would turn on my RV furnace the fan would start to run first, then, a few seconds later I could hear the gas burner turn on. That’s as it should be. You may have to be close to your furnace and listen to it carefully in order to hear the burner ignite and burn. What should happen after that is that when the RV reaches the temperature you’ve set on the thermostat the burner should shut off and the fan should keep running another minute or two until the furnace has had a chance to cool down somewhat. Then the fan should shut off.

This Deflecto Supurr-Flex Aluminum Dryer Vent Duct is UL Listed and rated up to 258º F. Some other ducting I looked at was rated much lower and could have presented problems. This stuff isn’t any more costly than other ducting I looked at.

That’s not what was happening in my RV. Instead, after running a few minutes, and before the RV had reached the temperature set on the thermostat, I could hear the burner shut off. The fan would keep running. Then in a minute or so the burner would start up again. This cycle would repeat every few minutes–burner on, burner off, burner on, burner off, all the while the fan kept running.

Here is what was happening: the fan and burner would start as they should, but because there wasn’t adequate ducting for the furnace to expel the heated air into the RV the heat would back up in the furnace and the burner would overheat. This tripped a safety mechanism, the high temperature limit switch, that would shut off the burner until the fan cooled the burner down to a safe temperature when the burner would then reignite. The furnace was working perfectly but the safety mechanism, the high temperature limit switch that shuts of the burner when it overheats isn’t designed and built for frequent cycling. It was headed for a premature death. Insufficient ducting was restricting the hot air flow out of the furnace causing the safety mechanism to repeatedly activate, sending my furnace hurtling toward the repair shop prematurely.

When a 4″ knockout is removed from the Suburban RV furnace this vent collar firs into the hole and give you a neck over which the ducting slips and is clamped to with a hose clamp.

My furnace was almost certainly behaving like this for a long time before I ever noticed it. Probably since the RV was brand new. Why would I suspect anything was wrong? After all, when it got cold in the RV and I turned on the heat the furnace would in fact heat the RV, but I never noticed the on-again-off-again cycling of the burner because the fan kept running all the while, or if I did notice the burner turning on and off I never thought anything of it. As far as I knew everything seemed to be working just fine… but it wasn’t. 

When my RV furnace comes on the fan starts running and then the burner ignites. If things are working as they should the burner will stay lit until the RV reaches the temperature set on the thermostat. Then the burner will shut off and a minute or two later when the furnace has cooled off a little the fan will also shut off. When the burner cycles on and off repeatedly every few minutes it’s a sign that something is wrong. There is a good chance there is a blocked vent or duct, or the manufacturer of the RV did not build in sufficient ducting to allow the furnace to properly breathe. Don’t think your RV manufacturer is above that!

I ordered these from Amazon. I wasn’t sure if I would be installing one duct or two so I ordered four clamps.

To confirm the on-again-off-again activity of my furnace burner I put my digital meat thermometer’s probe over the floor register of the heating system nearest the furnace. When the temperature rose to about 175º I could hear the high temp limit switch trip and the burner shut off. I watched the thermometer fall to about 140º and I could hear the switch activate, the burner ignite, and I watched the thermometer rise back to about 175º. This cycle repeated over and over until the RV finally reached the temperature set on the thermostat and the burner then fan finally shut off. That ain’t supposed to happen. The burner shouldn’t cycle on and off repeatedly. It should go on once and stay lit until the RV comes to temp and then it should shut off. Once on, once off. That’s all.

I spoke with a tech at Suburban, the maker of my furnace. After explaining the repeated cycling of the furnace burner and the high and low temperatures it was bouncing back and forth between I was told my furnace safety mechanism was working as it should. I was also told that the on-again-off-again action of the burner likely pointed to a restriction in the ducting and that under-ducting by RV manufacturers was a problem Suburban had encountered before. 

The furnace in my RV sits underneath the refrigerator. That doesn’t strike me as the best spot because the furnace gets warm when the fridge is trying to keep things cold. You’d think it would be smarter to separate the two. Anyway, in this photo you can see the new vent I installed to the left of the furnace and the aluminum ducting I used/

I understand that my Suburban SF-35FQ furnace has the equivalent of 11 outlets: seven 4″ round outlets on the sides and one large rectangular outlet on the bottom that counts as four round ones. The specs for my furnace say that a minimum of four of the outlets must be used so that the furnace can properly aspirate but the tech told me using more than four is better. In my RV the manufacturer used only the rectangular outlet on the bottom of the furnace which would count as four round ones and this would meet the minimum requirement, but, they also put some 90º bends in the ducting which creates resistance in the airflow within the ducting and it appears that it effectively reduced the airflow to below the minimum requirement thereby causing my furnace to overheat and the safety mechanism to cycle the burner on and off excessively. The installation manual says “Avoid making any sharp turns in the duct system. Sharp turns will increase the static pressure in the plenum area and could cause the furnace to cycle.” That seems to be exactly what was going on in my RV. It would seem my RV was under-ducted at the factory. Thank you Arctic Fox. 

This is the ThermoPro food thermometer I used to monitor the temperature of the heated air cominog out of our furnace. It isn’t expensive, but we has found it to be pretty accurate and fast reading.

You could be having this same problem and it could be going on unbeknownst to you. It’s an under-the-radar kind of thing. I don’t say this so as to create a panic, but it would be worth paying attention to your furnace. You could test it by letting it run for 20 or 30 minutes while listening carefully to see if you can hear the burner turning on and off. If it does not it’s likely fine at least in regard to an overheating burner, that is, assuming your high temperature limit mechanism is functioning properly. If your burner does cycle on and off repeatedly then you may have a blockage in the ducting or your RV manufacturer may have failed to provide sufficient ducting for your furnace. Restrictions in the ducting, by the way, can be caused by insects, critter nests, damaged ducting or the build-up of crud over time. Your furnace would be more likely to experience the problem I have outlined on a warmer day when the air it breathes into its combustion chamber isn’t as cold as it might otherwise be. When it is breathing colder air this works against the burner overheating.

I solved the frequent cycling of my RV furnace by adding another duct. I removed one of the 4″ knockouts on the left side of the furnace and ran some flexible ducting to a register nearby. I can feel a strong airflow coming out of the new register and I tested my heater by letting it run 20-30 minutes and it didn’t cycle on and off at all. Not once. Problem solved.

Should you undertake a similar repair make sure to follow the furnace manufacturer’s instructions or hire it done by a qualified shop or individual. (Good luck with that!) Also be certain the ducting you get is up to the task. Not all of the flexible ducting on the market is suitable for the temperatures encountered in a heating system such as those you find in RVs.

I used my Black and Decker RTX rotary tool set to 30,000 RPM and a cutting bit to cut out a circle for the new register. First, in order to minimize any wood splintering I covered the area with painter’s tape which I new would peel off easily. I then drew a circle on the tape with a pencil compass.

Cutting the hole into which the new register would fit took a bit of figuring. I didn’t want to try to use a keyhole saw because I thought it would be too hard to control and may splinter the surface of the soft luan wood. I couldn’t remove the panel I was cutting into so that ruled out a coping saw. I might have been able to use a 4″ hole saw but that might have splintered the surface and would have been an expense. As it turned out there is a bit that looks like an ordinary drill bit but it’s made to be used like a router bit. (See my reply to John in the Comments section for more information about cutting bits.) You drill a hole into the surface you are cutting with the tip of the bit then move your drill sideways using the side of the bit to cut. I already had  Black and Decker RTX rotary tool like a Dremel tool and it came with one of those bits.

I’ve had one of these Black & Decker RTX rotary tools for years. I haven’t used it all that much but they come in very handy at times such as it did for cutting the 4″ round hole I needed when installing a new heating register in the luan paneling of my RV.

Below is a list of the things I used to install the new duct and register The links have my Amazon Affiliate code embedded so if you buy any of these things through the links I provide I may receive a small commission. These items appear in the photos above. Click on any photo to enlarge it.

Deflecto Supurr-Flex Aluminum Dryer Vent Duct, Flexible, Fire-Resistant, 4 Inches Dia. x 8 Feet, Silver (F0408B/4) which is UL Listed and rated to 265º.

I attached the vent duct using these hose clamps: 4″ Adjustable 304 Stainless Steel Duct Clamps Hose Clamp Pipe Clamp Air Ducting Clamp Worm Drive Hose Clamps (4Pcs)

The vent I used was a Valterra A10-3353VP Rotating/Dampered Heating and A/C Register-4″ ID x 5-3/8″ OD, Black which I chose because it fit the space, has dampers, the color and it rotates.

My furnace required this vent collar in order to attach a 4″ round duct. Your furnace may be different.

Black & Decker RTX Rotary Tool

Scotch Blue Exterior Painter’s Tape
There are many kinds of painter’s tape for different applications. I used this stuff because I had some on hand.

ThermoPro Food Thermometer
Inexpensive but fast reading and quite accurate. It allowed me to watch the rise and fall of the temperature of the air coming out of our heat ducts in real time, up and down between 140º and 175º, back and forth, over and over.

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