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Ask Dave: Why does hot air come out of my furnace exhaust?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses how an RV furnace works.

Dear Dave,
Why does really hot air blow out the outside of my propane furnace? —Chiara

Dear Chiara,
Your RV furnace is designed similar to a residential furnace but on a much smaller scale. When the inside ambient temperature reaches the set thermostat temperature, it creates a closed circuit that sends 12-volt power to the module board calling for heat.

The module board then starts the blower motor, which pulls inside air into the furnace through the cold air return and runs long enough to lift the sail switch on the outgoing air shroud. At the same time, the burner assembly pulls outside air from the intake tube blowing it through the burner assembly purging any LP or old fumes through the exhaust vent.

Once the sail switch is lifted, the gas valve opens, the spark ignitor lights the LP, and the flame heats up the burner assembly. This is all enclosed in a series of tubing shown here with a high limit switch monitoring to make sure it doesn’t get too hot:

The heat and exhaust from the burner assembly are exhausted outside as there is carbon monoxide in this mixture just like in your home. The blower motor continues to pull clean inside air through the cold air return and blows it over the top of the enclosed burner assembly and out through either the plenum or hoses to the vents. This way no carbon monoxide or other fumes from a burning flame get inside the rig.

Read more from Dave here

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Joe
3 days ago

If only someone would build an RV furnace as efficient as a home furnace. New home gas and propane furnaces are around 97% efficient and use a plastic pipe for exhaust, oil can get about 87%.

Steve Hericks
2 days ago
Reply to  Joe

‘Condensing’ furnaces operate around 95% efficiency BUT several things are necessary to do it which run at odds with RV construction (not insurmountable). When hydrocarbons combust, they create CO2 and water vapor. The reason most furnaces are 80% efficient is that it is undesirable for the temperature of the exhaust to get below 212F because the water condenses. Condensing water no longer suspended in the exhaust stream, is separated (by gravity) and also creates a corrosion problem.

Condensing heat exchangers are bigger with two different sections. One extracts heat from the hotter, pre-condensing gases (plated steel construction). The lower temperature, post-condensing gasses require a corrosion resistant (316 stainless steel?) heat exchanger which needs low points to allow liquid water to be collected and separated. I don’t think the condensate can legally be dumped on the ground and would need to be pumped to the gray tank.

PerryB
3 days ago

Dave, you gave a long winded answer that could have been simplified by saying,
“RV furnaces are massively inefficient!”
The heat coming out of our exhaust is much warmer than the air from the furnace that heats the inside.

Wayne
4 days ago

It’s a furnace yes. However I think one could roast a wieners at the exhaust demonstrating that they are very inefficient.

TIM MCRAE
4 days ago

?? Because it’s a furnace?

Sorry just couldn’t resist. I know you were thinking it…

Jerry
4 days ago
Reply to  TIM MCRAE

😉