furnace short cycle

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Dec 2, 2012
Hi guys!
I have a 1991 class c and am having furnace problems. The furnace blows hot air for about twenty minutes and then stops blowing hot air out of two of the three vents and the remaining vent is not very hot. Not sure if this is technically considered a "short cycle" since one of the vents is blowing air that is somewhat  hot. I took out the fan and checked the sail switch but didn't find anything wrong. Could my motor be bad?
If you have to turn the furnace off and then back on to get it to produce heat after it fails, you're probably hitting the overtemperature safety lockout.

This can be caused by insufficient air flowing through the furnace. It's possible the fan motor is failing to run at full speed, but you can also have a problem with the furnace's air intake or hot air output.

Make sure all of the outlets are clear and open - closing one or two down will restrict the airflow through the furnace.

If your furnace uses flexible ducts that run through the cabinetry, it's easy for one or more of these to get kinked or torn.  Trace them out while the furnace is running and make sure there aren't any air leaks or crushed spots.

Also make sure the return air intake to the furnace is not obstructed.
If the cool air is coming out for a short time prior to the furnace turning off completely, it is normal. The blower will run for a couple of minutes to cool down the system.
As Jim says, the fan runs a few minutes after the burner shuts off and of course the "hot"  air begins to cool immediately. The furthest away vents appear to cool first, simply because the air cools in the long er duct before reaching the outlet, which perhaps explains what you are observing.

So, the question is, has your room reached the "set" temperature on your thermostat? Not necessarily the whole room, but the part where the thermostat is sensing the temperature. If so, then your heating system is working as designed. If not, the furnace may be shutting off because of an internal overheating condition.
yea it looks like this is how my heater is designed to function. If I leave my heater on for two hours the burner turns off a couple of times for a few minutes and then comes back on. It stays hot in the cab. Thanks for the help guys

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