The dash heater in our motorhome works fine at idle but blows out cold air when driving

by Ron Ford
(Ocala, FL)

Purchased new in 2000, our CAT diesel powered 34 ft, one slide HR Endeavor's heater (in-dash) has never worked once the vehicle is set in motion. While stopped the dash or floor vent (not coach furnace) heat works fine and registers in excess of 100 degrees at either vents.

Once the vehicle is in motion and the outside temp is in the 40 degree range or colder the previously hot air turns cold. Factory techs have worked on problem several times including replacing the heater control valve on the fire wall, to no avail. The even reversed the inlet line, still no luck. If stopped at a traffic light or idling somewhere, the heat will blow you out of the coach however, as soon as the vehicle is in motion, it turns cold. The hot water line from the engine to the firewall is very hot to the touch. Factory techs have never been able to figure it out.

ANSWER Greetings Ron thanks for submitting your question on our Ask An RV Question Page.

This sounds like a classic problem with the vacuum system of your RV. Diesel engines do not produce a vacuum when operating, so they use vacuum pumps to produce the vacuum to operate the vacuum motors on items such as the vacuum motor that operate your heater blend door.

I think what is happening is that once you start driving the RV other vacuum items on your RV start to operate. The problem could be due to leaks in the vacuum lines or connections leading to these other vacuum operated accessories. The lack of vacuum pressure closes the heater blend door(s) and does not allow the fan to blow air through the heater core and sucks cold air in from the outside vents.

This can also be caused by a weak vacuum pump(s). The reason you get heat when you are stopped at a stop sign or idling is because there is a lower demand for the vacuum pressure by the other vacuum operated accessories

I would strongly suggest that you have the vacuum system on your RV tested for leaks and defective vacuum pump(s).

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Happy RVing

RVing Al