Ask Dave: My RV’s rear heater is extremely hot. What temperature should it be?

Dear Dave, 
I have a rear coach heater that resides by the stairs under the fridge. It is heated by radiator lines that also heat the water heater while you drive. The last time I drove the RV I smelled something that seemed very hot in this area of the coach. When I opened the grate to access this area and felt the metal box that houses the core and fan, it seemed very hot to me. I’m not sure how hot it should be, but it was too hot to hold your hand on! I can’t find anything online to guide me on how to troubleshoot the problem. I unplugged the only electrical plug (fan, I’m assuming) and drove it again. Still super-hot. Any thoughts on how to move forward? Thanks. —Tom, 2015 Winnebago Sightseer 35G

Dear Tom,
Your rear heater was invented by Winnebago back in the late ’70s and called “Motor Aid”. My first job at Winnebago in 1983 was aiming headlights, installing batteries, and hooking up the coolant lines from the engine heater to the Motor Aid hoses going to the back of the rigs on line three. How I got there is a long story, having owned a gas station for a couple of years making no money and just wanting a job that would allow me to pay off the bills and play softball all over the country.

How the “Motor Aid” system works

Anyway, the concept is cutting into the engine coolant line or “antifreeze” and routing it back to the automotive-style heater like the one used for the dash. The coolant will get over 200 degrees, at which time the thermostat opens and allows the coolant to flow throughout the engine and ultimately the hoses to the back of the rig. It is routed through the automotive heater and then connected to the Atwood water heater, that has specially designed piping that wraps around the 6- or 10-gallon tank.

Here is the 3D diagram from the Winnebago website of your rig. It shows the lines coming from the front, going to the heater, then to the water heater, and back to the front. The heater has a coil inside and a fan just like the dash heater, so you have basically free supplemental heat while running down the road. The coolant flowing around the water tank heats the water inside, so there is hot water when you arrive at the campground without the need to run the propane.

Keep in mind that the temperature in the area of the rear furnace and water heater will be close to 200 degrees, which is VERY hot and would most likely burn your fingers if you touched it. That is why it is typically covered and not exposed.

Use an infrared thermometer

As I have stated often, it is important to understand the operation and especially operating temperatures of appliances and components. Therefore, I would recommend getting an infrared thermometer and getting actual temperatures.

The wires you pulled off are just 12-volt power wires going to the fan motor and should not have anything to do with the “hot” smell, unless the bearings of the motor are getting dry.

I would suggest checking out the shroud area to make sure nothing has fallen into the compartment like insulation or a panel that has come loose and is getting hot. I would also check all the fittings of the hoses to verify there is no antifreeze leaking, which would give a hot smell as well.

Next, check the engine temperature on the dash gauge as a stuck thermostat would not allow proper flow of the antifreeze and the entire coolant system would be running hot. As stated earlier, the thermostat should open at somewhere between 192-198 degrees depending on the type. It is not uncommon for the coolant to be over 200 degrees. Your gauge should show the temperature and what is actually too hot.

I remember a trip to Colorado with a prototype 1989 Itasca Sunflyer in the winter with temperatures below zero. The front firewall of the rig had so many holes the engine heater could not keep up, so I had to drive with snowmobile boots on and a sleeping bag wrapped around me. My girlfriend, now my wife, and her daughters had to stay in the bedroom with the door closed. The Motor Aid was the only thing that kept them warm!

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When RV heat is on, cabinets and counters get hot. Why?

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Read Dave’s answer.

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

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