Hi all,
This is my first question and I'm hoping someone can help me. I have a 1996 Coleman Evcon gas heater/ac unit in my manufactured home. The problem I'm having is that the fan (not heat or ac) keeps coming on for no reason. Even when I have the heat and ac off, the fan keeps kicking on and running for several minutes - sometimes up to an hour. Then it will shut off and sometimes come on again immediately. It happens sporadically and for seemingly no reason. I had a HVAC tech out but he couldn't figure out what the problem was. And of course while he was here, it didn't come on. I'm afraid it's going to short-circuit or overheat or something from running all the time. Has anyone else had this problem? Please help!

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Originally Posted by GPeirce

Hi all,
This is my first question and I'm hoping someone can help me. I have a 1996 Coleman Evcon gas heater/ac unit in my manufactured home. The problem I'm having is that the fan (not heat or ac) keeps coming on for no reason. Even when I have the heat and ac off, the fan keeps kicking on and running for several minutes - sometimes up to an hour. Then it will shut off and sometimes come on again immediately. It happens sporadically and for seemingly no reason. I had a HVAC tech out but he couldn't figure out what the problem was. And of course while he was here, it didn't come on. I'm afraid it's going to short-circuit or overheat or something from running all the time. Has anyone else had this problem? Please help!

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If your uit is a package system and it is that od, it probably has a pilot flame for the furnace. The pilot flame can generate enough heat to activate a heat sensor that turns on the blower. The sensor is probably old and worn out as most are just a bimetal strip that is not high tech acurate.

The limit switch could be the problem even if you don't have a pilot light. It will be a little round doodad around the heat excanger or blower with 2 wires running to the control board. There may be more than one. If your heater has a schematic, it may help you identify the right one.

Relays are common trouble makers too, but usually they stick on or off.

Perhaps, just to make sure that the issue is at the unit, isolate it by disconnecting the thermostat and see if it happens again.

The fan does not start just by itself. Something is making it do it.

Perhaps working it backwards may go like this:

Fan<-Relay<-CPU (or some control)<-Safety loops (limit switches)<-Gas Valve<-Draft inducer<-Control Board<-Heat call<-Thermostat (fan on/off, heat, cold, auto).

I Hope this gives some clue...