Dear Dave,
My RV furnace seems to get really hot. I was wondering what temperature I should be getting out of the ducts and the exhaust? I’m also wondering how hot the furnace itself should be? It is too hot to touch when it is running. —Ronald
Dear Ronald,
With any type of troubleshooting, I always recommend getting as much specific information as possible. In this case, that would be finding the actual temperature using either an infrared thermometer or an airflow meter with a temperature sensor. In several of the videos we conducted for the RV Repair Club, we used both to verify what the temperature of the exhaust air was as well as the interior air.
How a typical forced-air RV furnace operates
First, let’s look at how the typical forced-air RV furnace operates. The thermostat is set to a desired interior temperature and when the interior air temperature drops to +/- 2 degrees of that temperature, it closes and calls for heat. At that time, the blower motor starts and pulls in outside air through the outside vent, which is circulated through the burner assembly to purge air in the chamber.
This typically takes about 15 seconds. It also pulls interior air through the cold air return, sending it over the burner assembly, which lifts a sail switch telling the monitor board there is enough airflow. This will then open the gas valve, start the spark ignitor, and light the flame.
Interior air is then circulated over the burner assembly, providing warm air into the rig. At the end of the burner assembly is a high limit switch or temperature sensor that will shut the unit down if it gets too hot. The temperature setting of the high limit switch will vary with models, but is typically set at 160 degrees F.
In our test of a Hydroflame 30,000 BTU model, the exhaust air coming out of the burner assembly to the outside of the rig was at 230 degrees and climbing. That is extremely hot, too hot to touch, but is a normal operation.
Test the inside air temperature
Inside air temperature, however, is much lower than that and cannot surpass 160 degrees F due to the high limit switch. According to my Dometic source, the temperature coming out of the RV furnace to the ducts or plenum should be 40-70 degrees hotter than the ambient air temperature being drawn in through the cold air return. So if it is 65 degrees inside, the temperature could be 105-135 degrees.
A good tool to use for checking RV furnace and air conditioner efficiency is an airflow meter with a temperature sensor.
This will not only tell you the temperature coming out of the vents, but if you have sufficient airflow. Airflow is measured in CFM units and will vary depending on the size of the unit you have and the distance the vent is from the main unit. According to the Suburban Service Manual, the airflow of a typical 12,000 BTU unit would be 122 CFM and a larger 30,000 BTU unit would be 345 CFM max.
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.
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I have had two travel trailers recently. One a 2017 passport. The current one is 2022 twilight by heartland-thor. Both had very low air flow due to only 3 four inch flex ducts running under the belly compartment. Some of the ducts were smashed almost closed. Total c.f.m Output about a third required. I eliminated all three and blow the air freely out the front. Problem solved. I am a 42 year HVAC technician. That set up from manufacturer is dangerous.
Also the over heating of the heat exchanger can crack or split the chamber and put out c.o. not good.
They make a very efficient heater! Just like everything else in all walks if life, cost is the main factor. I looked to replace mine years ago. A regular furnace was around $260 and a high efficiency was pushing $800. Needless to say the cheap one stayed in place.
Another thing, taking temperature like shown, no piping so no resistance will greatly affect the readings. Mine( indoor air temps) was high. The furnce had only 4 outlets. I added a heat in the bedroom and in the bath. Reduced outlet temps a lot and then had warmth in the bath. {bleeped} designers.
230F from the exhaust! It’s amazing that nobody has come up with a more efficient system.