Heater not heating ,but fan and air conditioner works fine

Outside temp too low, so outside thermostat and it's settings will not allow the heat pump to work. But insist on the ausilary heat to work instead. Heat pumps usually do not produce enough heat when below 40F outside temp. If you have Gas heat, it may be more economical than the heat pump below 40F. If electric auxilarry heat, it may compete at 32F. Talk to your dealer, installer, or manufacture.

Sounds like wiring issue to me. I've delt with units like this and there is a wire lead for fan only and a separate one to run the fan when the unit is cooling or heating. I'm only speculating here but I would bet money that this was improperly installed.

Dont get discouraged though because it sounds like its working (for the most part) if you feel up to the task find the manual and check your connections. Otherwise hire a professional.

PLEASE CHECK THERMOSTAT SETTING +29 AND ALSO MODE"HEAT". STILL PROBLEM CHECK THE HEATING SENSOR IS NOT OPENED. BY A OHMS METER CHECK THE SENSOR. iF ANY READING IN OHMS IT'S O.KAY.

lower temp setting lower then outside temp should work

check for wiring & compresor

the fan is controlled by a fan center or a stack switch. as burners light and heat up the spring sensor on the bulkhead above burners heats up and turns and a a point energizes the indoor fan if it gets to hot the burners go out on safty. sounds like a fan control is bad tap on them gently and they sometimes break loose but if not then have a pro replace fan control and test heat exchanger.

9 times out of 10 on this old of vehicle its the heater core. If your car temp gauge is working and the car is heating up fine and the blower works then it is most likely your heater core. Not sure how hard it is on that car. SOme times it just under the dash takes about 30 mins but some cars you have to pull the dash out. The part alone is cheap. I would try logging onto autozone.com put in your car find the part (heater core) then look at the how to change section of auto zone.

Goodluck.

There are 3 possibilities beginning with the most likely problem

1) If your heater heats up for a few minutes and then quits, chances are there is a blockage in the air flow, check for dust and grime blocking the air intake coarse filter , airgun blow to clear accumulated dust and otherwise clean the coarse filter .
2) If the heater fan blows but there is no warm air, the heating element needs renewal ( this happens very rarely)
3)If the fan stalls and the heater heats up and the thermostat kicks in, then this will be not be a DIY job. This is least likely as you said the air cooler works.

http://www.residential.carrier.com/support/troubleshooting.shtml
Here is some troubleshooting tips.
Also service available.

I need more info. Is this a split system, packaged, or ptac. Heat pump or electric heat strip?