Ask Dave: Why does RV furnace blow warm air then turn cold?

Dear Dave,
The furnace in my 1997 Forest River Salem blows a little warm air and then only blows cold air. What should I look for? —Anon., 1997 Forest River Salem

From Dave—
This question actually came from the person that purchased what I call the Trailer From Hell (TFH) this past weekend. I have been shooting video content for the RV Lifestyle & Repair Club for more than 8 years.

When COVID hit, we purchased a 1997 Forest River Salem travel trailer to renovate the inside and add several aftermarket items. This would allow us to shoot at the facility and not travel to dealerships or other places during the lockdown, and also did not require a production crew most of the time.

Why it was the Trailer From Hell

I call it the TFH as it had a massive water leak that ruined the front walls and flooring and required a tremendous amount of work just to fix that. We did, however, get some great content for the site, as George spent more than 100 hours working on the repairs.

George worked on the unit and renovated the entire inside with a new floorplan that actually looked very nice.

After George finished the interior, I towed the unit down to my office and started shooting content on finish work and adding more accessories.

The TFH name was also appropriate because it fought us in every project we worked on. My brother has a great saying: Some days are Dragon Slaying Days, others are Hammering A Boulder Into A Rock Pile. This was a rock pile project!

Time to sell the TFH

We shot content for just over one year and decided it was time to cut bait and sell it. I can’t think of one component we did not work on, including a new door, distribution center, tires, and even a new dump valve! I listed it for $3,000 just to move it fast, and felt comfortable that everything worked. The only item left for the new owner would be to find an air conditioner cover, as the old one blew off on the tow home, and some touch up paint. Did I mention this was the TFH?

I parked the unit in the front parking lot with an info sheet and listed it on Facebook Market Place—that’s when the crazies came out! After showing it a dozen times with everyone wanting something different to be added or offering less than half, I had a group of brothers drive two hours to see it. They would pay cash if everything worked as promoted, especially the furnace.

The RV furnace did not work

So I went to the rig to verify the refrigerator, air conditioner, and furnace would work. Check, check, and then no check! The house battery was dead, so I plugged the unit in, always the first step to ensure there is sufficient 12-volt power.

Then I set the thermostat to 80 degrees and turned on the switch underneath. The fan motor started up and within about 45 seconds I heard the click of the LP valve and then a light amount of heat coming through the floor vents. After 10 minutes, the RV furnace was blowing cold air.

First step, check the LP in the two 20 lb. DOT cylinders on the tongue. I used my Level Check LP detector and one was empty and the other was about 1/2 full. That was the cylinder that the regulator was using.

Inside I verified the furnace vents in the floor were uncovered and there was a good flow of air through them so the sail switch was lifting. This was also verified by the gas valve clicking open and the spark attempt that I could hear in the furnace.

As I stated earlier, there was an initial flame that produced a small amount of heat. So I felt comfortable that the RV furnace was going through the stages of air flow, open the gas valve, spark, and ignite the burner assembly. The initial heat also verified that. The next check would be the LP pressure.

Check the LP pressure

A quick test is to light one of the stovetop burners, so I used a candle lighter and tried lighting the front burner. It took a while for the air to bleed through the LP line. Then after several “pops” it produced a very small flame. When I tried lighting one of the rear burners, it would not light and the front went out, as well. To me, this indicated low LP pressure and typically means a weak regulator.

You can adjust the LP regulator by turning the set knob clockwise, but this also only produced a very light flame. The unit had been sitting for more than 3 years without the LP being used. Typically I would have Steve, my RVIA Certified Master Technician, check the system with a water column tester, but he was not available.

Since the RV furnace did light initially and provided some heat, it appears the flame was not high enough to heat the thermal coupler, which creates a closed circuit and keeps the gas valve open. This would be due to low LP pressure. The potential new owner had another regulator at home and decided to take it “as is”.

Adios, TFH

They hooked up the truck, as it was getting dark. None of the running lights worked, even though I had tested them on my truck bringing the unit from the storage lot. I offered to test their 7-pin. They said it most likely was bad and they would take backroads home. I watched as the dark trailer disappeared down the road.

Today the check cleared and I got an email that it was the regulator and it was now working. Today was a Dragon Slaying Day!

 You might also enjoy this from Dave 

Why does RV furnace light on second attempt but not first?

Dear Dave,
I have a Suburban SF30 furnace in my 2007 Allegro Bus. When I turn it on in the morning, the fan starts but it will not ignite. I turn it off. On the second attempt, it does ignite and produces heat as it should. It shuts off when it reaches the set temperature and restarts as the room cools. I have already replaced the sail switch and the limit switch, those being the easiest and cheapest. Now what? Control board? Igniter? Gas valve? That’s about everything, isn’t it? Thanks. —Mark, 2007 Allegro Bus

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here. 

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