RV ‘Gremlins’, Part 5: Forced air furnaces and why they’re so finicky

I typically don’t get as many questions about furnace efficiency as other appliances, mostly because of how people are using their units and how often the furnace actually gets used.

Most RVers store their rigs in the winter, so the furnace only gets used slightly when the temperature dips in the mountains or during autumn, so it doesn’t get tested as much in extreme conditions.

When the thermostat (t/stat) is set to the desired temperature and the ambient temperature inside the rig drops below that at +/- 2 degrees, the t/stat wiring closes and sends 12-volt power to the module board of the furnace. Last week we discussed the issues with the t/stats for the air conditioner. However, there is another issue when it comes to cold temperatures and the t/stat.

RV thermostat didn’t work below freezing

Years ago I took a demo unit out to train dealers in Fargo, ND, and the temperatures dropped to zero degrees. I got to Fargo late and decided to just pull into the dealer lot and get up early to train the sales staff. I pulled in, put down the jacks, and walked back to start the furnace as it was warm in the cab area from the engine heater but brutally cold in the living area. The furnace would not start. It would not even attempt to start. I checked the batteries and they were at 12.6 volts. I also checked the gas shut-off valve to the furnace, and verified the stovetop would light. So I knew there was LP.

I left the engine running and used the dash heater and the motor aid. This was a bedroom heater Winnebago used by running the engine coolant back to the bedroom through a secondary automotive heater under the bed, which provided free heat while driving.

The next day I called back to our tech support and was told the wiring for the t/stat was a lighter gauge and that it was too cold to start the furnace. I was told: “Yeah, we’ve had some issues when the temperature drops below freezing.” What???

I do still see several issues with the t/stats used in RVs today, as they are not very good quality. Some of the “zones” they have with remote temperature sensors are not very accurate.

How an RV furnace works

When the power is sent to the module board at the furnace, it first will start the blower motor and fan. This is designed to draw outside air through the exterior vent, run it through the burner assembly to purge any stale air or LP fumes, and exhaust it out through the exterior vent. None of this air enters the coach as it is a completely sealed system within the burner assembly and exhaust chamber.

At the same time, the blower fan draws in interior air through a cold air return vent inside the rig and blows it over the top of the burner assembly, passing a sail switch into the coach. There is no heat yet, as this step is a test to ensure there is enough airflow before the gas valve opens. If the sail switch does not raise high enough to again create a closed circuit that would send power to the module board, the unit will not attempt to light.

If your batteries are sulfated, they will not have enough power to run the fan at the proper speed, and the fan will run blowing cold air until the batteries are completely depleted.

The “gremlin” culprit

Here’s the gremlin: the temperature sensor. When the furnace does light and the air passes over the burner assembly, it gets heated and flows over a high-limit switch. This temperature sensor will shut the furnace down at a predetermined temperature.

If the high-limit switch is weak, it will shut the furnace down earlier than designed and sometimes will be affected by ambient temperature, a plugged exhaust port due to mud daubers or spiders, or insufficient airflow inside the rig. If you have rugs over floor vents or pinched hoses to the vents, the airflow will be restricted and either the sail switch will not be lifted high enough, or the air cannot flow fast enough and the temperature increases at the switch.

Another gremlin is altitude. Most furnaces run on LP and need a regulator to reduce the pressure to 11” of water column. The LP goes through an orifice in-line to be mixed with air. Most LP appliances in an RV work fine below 4,000 feet sea level. However, when they get into higher altitudes, the air to gas mixture doesn’t work. Some appliance manufacturers have an optional orifice that will work. However, it does take a little bit of work to change it out every time you go up the mountain and again when you go down!

More RV “gremlins”

• Part 4: RV roof air conditioners: Sometimes they work, sometimes they don’t
• Part 3: The secrets of RV absorption refrigerators
• Part 2: The mysteries of 12-volt wiring
• ‘But it worked yesterday!’ RV ‘Gremlins’, Part 1: Lead-acid batteries

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

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