Central heating furnace circa 1985, fan comes on in longer and longer spirts then stays on

jeffrimerman

AC/heating guy fixed the system but figured that's how it runs so wanted to ask. When thermostat tells it to come on, then the pilot comes on, then the furnace. Furnace stays lit and then the fan cycles on for a quick second and after maybe 5-6 times longer and longer then stays on and runs fine. Is that how the older units work?

Jamie Hall

Well, not that I recall... it is likely that there is a thermostat in the plenum which should turn the fan on when the plenum warms up. However, it should have a differential between on and off at some lower temperature -- so that once it switches on, the fan should stay switched on until some short time after the burner turns off.

Sounds like that switch is misbehaving (no or very small differential). And should be at least looked at and checked, if not replaced. For one thing, the blower motor is not going to like that kind of quick switching on and off.

EBEBRATT-Ed

I agree with @Jamie Hall once the burner lights the fan should come on in a min or two and the fan should stay on until your upstairs thermostat is satisfied then the burner should shut down and the fan will continue to run for a short time to cool the furnace and then shut down.

Check your fan/limit control

JUGHNE

Does this furnace blow air up (as in the basement) or blow air down--counterflow- (as first floor closet with ductwork in a crawlspace)

HVACNUT

Less likely but still possible is lower BTU input.