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Heating vents malfunctioning - blowing out hot air first, but then air turns cold.?
We have central gas heat with a thermostat and furnace in the attic. The other day, we realized that after about 15 minutes of having the heater on/fan running, the vents starts blowing out cold air instead of hot air. The house had not reached the temperature that the thermostat was set for - the heater just seems to stop working. If we turn the heater off for a while, then turn it on again, it will blow warm/hot air again...but only for a few minutes, and then it switches to cold air....essentially our house isn't getting heated. What could be the cause?
4 Answers
- Anonymous1 decade agoFavorite Answer
what you describe sounds like a faulty limit switch. OR the limit switch is working fine and for some reason you are building up too much heat in the plenum. however, the fan should go off too if the limit switch is working properly. a limit switch is a normally closed switch so with the system energyzed you should read 0 voltage across the switch. if it's open, you'll get 110 or so across the switch.
- Anonymous1 decade ago
It sounds like the furnace is short cycling - meaning it stops producing heat before the thermostat is satisfied. Furnaces usually short cycle when they go off on limit - meaning they are operating at above manufactures design temperatures. There are many things that can cause the unit to run hot and go off on limit like dirty filter, dirty A coil (if you have a/c ) dirty blower squirrel cage, restricted return duct. I recommend calling a HVAC service company out to look at the unit and give it a check up. If you are in the Chicago area we would be glad to help... Best of Luck in Advance !!
- 1 decade ago
Have you tried turning the thermostat up ten degrees to see if its just the thermostat going bad. If the temp does not rise above current temp, it could be faulty flame sensor not letting the furnace run long enough. Is the duct in the attic insulated? If its not the thermostat, its probably a job for a Professional.
