Heat doesn't work on ac

Is this an electric unit or does it run on fuel?

Sounds like high blower relay is bad

Most furnace operating on Oil or gas will heat the combustion chamber before starting fan to prevent cold air draft at start-up, can last a minute or two. Eiter the main flame does not come on or the chamber thermostat have fail. Try to modify fan option on room thermostat from auto to on. Cold air coming out: burner problem or no more oil or gas, warm air: burning chamber thermostat problem, leave room thermostat fan option to on until repair.

you probably need new control switch.

There should be a high speed blower relay that bypasses the resistor for slower speeds.
Check with an auto parts store or the dealer.

Thermostat on wall, or manual controls at AC? Did it work for you on low before now? Reason I ask, is that normally they are only a 2 speed fan ( low and high) so if it's a wall thermostat and has been changed from original, perhaps it the way the thermostat is wired.

sounds like it is miswired. actually try fan on with the heat on and see if the fan works.

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It may well be the fan switch is defective

i assume u r refering to the cooling fans,,, if this is so the freon is low ,fans come on and lower the high side ,there fore lowering the low side and the low side switch cuts it off and on,,charge it up

could be the selector switch turn off power switch low and high wire if it blows high replace selector switch if it does not work replace motor. Tom