Dec 7, 2013 - Personal    3 Comments

Troubleshooting an RV Furnace Blowing Cold Air

I’ve been struggling with my RV furnace blowing cold air the past few days, but I haven’t had any need to troubleshoot, knowing that the arctic chill is to blame. But there are several other reasons for an RV furnace to blow cold air.

Andy Baird left some great troubleshooting info in a comment to a previous post and I wanted to make sure that readers who don’t look at comments get it. Thanks for this, Andy!

“One common reason for the furnace blowing cold air is low battery voltage.

Unlike a residential furnace, in which the blower doesn’t start until the plenum has warmed up, an RV furnace starts blowing immediately. That’s because it uses the same motor to power both the hot-air blower and a second, isolated blower that moves air through the combustion section.

If the batteries were too weak to power the blowers, or if the motor failed while the burner was lit, the furnace would quickly become a fire hazard. So as a safety feature, there’s an air-sensitive switch in the duct that has veto power over the burner’s gas valve. If this “sail switch” is not activated by sufficient airflow, the burner will not light, or if already lit, will shut off.

That situation can occur when the house batteries are weak. The blower motor spins, but not fast enough to deflect the sail switch. The result: a furnace that blows cold air. It happened to me the first time I camped in wintertime.

It’s also possible for the sail switch to get stuck, due to dust, rust, or insect nests. If you have propane and your battery situation looks good, this is a possibility.

Finally, some possible problems with the propane system include valves, hoses or regulator frozen due to moisture in the system, extreme low temperatures (propane liquifies at -42° C., so if it’s colder than that, it will never vaporize and you won’t have gas pressure), and too much butane in the propane.

Propane is typically sold mixed with butane, and in warm weather this is no problem. But butane liquifies at -1° C, so at any temperature below freezing butane won’t vaporize. Now, your local dealer’s supplier should have made seasonal adjustments to their mix… but if they have a load of propane left over from summer, it may contain so much butane that it won’t vaporize adequately in cold weather. There’s not much you can do about this except switch dealers (if there’s another one nearby) or hope that your local dealer runs through his supply of the less-volatile mix.”

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3 Comments

  • Propane may not vaporize at all at -41 but even at -20 it’s ability to vaporize slows down. With the furnace putting a constant draw on the tank the pressure may get low enough that the furnace fails to fire. When the furnace stops drawing Propane the pressure will rebuild. The solution maybe to go to a bigger tank or to hook more tanks together to maximize the ability of the propane to vaporize. Have you checked with the propane company about renting or acquiring a Larger tank? In Alberta we would call them a pig but there are tanks that are about 500 lbs. The propane company would have to come out to refill it but it would have the ability to vaporize more Propane. the other option would be 100 lb tanks.

    Don

  • Thanks Rae! This is extremely valuable information. I have had this problem on occasion but never understood what the problem was. I start camping early in spring and continue into late fall. It isn’t intuitive to think your battery might be too low if there is enough juice for the fan to blow air.

  • My furnace is diesel fueled so this may not apply to yours but if I get too much carbon buildup in it my furnace will blow cold. The tech realigned my outlets so the output is not blowing so close to the input to reduce this happening–so far so good.

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