Heating, Air Conditioning, Fridge, HVAC/Heat not working after AC on.


I live in california.  Some days, I go to bed with the AC on and it shuts off sometime during the night.  The outside temp drops a lot and we need to turn on the furnace in the morning. The house may be around 62 deg, outside low 40's.  Put the thermostat on 68.  The heat will not turn on.  This has happened 3 times, exactly the same way. About an hour or two later, the heat turns on. Once on, I can cycle the AC and the heat on back and forth. It works normally.  If the heat is on when I go to bed, it is fine.

Note, the furnace is in the attic.  It it cool up there, could be in the low 40's at night.  So I am trying to link this problem to the furnace being in a cool environment and the fact that the AC was on first, then we flipped to heat.  Any ideas? Do frozen evaporator coils prevent heat from working.  If yes, then maybe on these cool nights, we have the AC running, coils are frozen and remain in the cool attic environment.  After a couple hours in the morning, they thaw from the attic heating up, etc. No idea what I am talking about.  Thanks.

Hi John, It sounds like the control board is going into soft lock out mode. Next time this happens find the sight glass on the front of the furnace and see if the LED is flashing. The flash codes should be listed on the inside cover of the blower section. Let me know what it says. Most likely its going to be flame failure. Cleaning the flame sensor would be the first thing to do.

If you can send me the make and model number of the furnace, I can help you find the flame sensor.

I don't think the temperature in the attic would have any effect on the furnace. As far as a frozen evap coil, I don't think that's the problem.

Heating, Air Conditioning, Fridge, HVAC

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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