I have a 1991 lennox whisper heat model #g20q3e-75-2 75,000 btu having problems with the thermostat board in the furnace do you know where i can get one and about how much do they cost?

SOURCE: heat pump auxiliary heat does not turn on

Turn up your outside thermostat in the outside unit.
You should have about 3 degrees difference in one stage to the next. So I would change the thermosat with one that has three stages of heat.

SOURCE: Replacing Round Honewell Mercury Thermostat

try to get one that is made for oil heat, the installation is easy only two wires, and the instructions make it easy.

SOURCE: Replaced mercury thermostat w/Honeywell CT87K - doesn't work!

You need a specific thermostat for a Wall Furnace it is a Millivolt Thermostat and not a 24v standard thermostat....usually it will work anyway however if the powerpile (generator produces millivolts from the pilot light) is weak it may not be producing enough...or the Pilot light may be dirty and lazy...won't produce enough millivolts ...or you can have bad or dirty contacts or connections....or a combination of any of the above.....a 24v Thermostat and particularly a non-mercury thermostat has too much resistance in it for a millivolt system....as your Wall Furnace (other than the fan) runs on less than 1 volt

SOURCE: Gas furnace is constantly blowing cold air

An indoor blower running when it is not supposed to usually indicates an overheat condition. I would guess that it has been cycling on and off for some time and finally caused the high limit to stick and not auto reset. if you tap it sometimes they reset. the real problem is that it has been overheating from lack of airflow, over sized unit, not enough return air, too many closed registers, dirty filter, dirty squirrel cage blower and or air conditioner a-coil. If you get it started again sit and watch the unit run the flames should come on and stay on for the entire heating cycle, not going on and off.
I wish I could help you more but it is difficult without seeing the problem myself. Good luck! this should get you started!

SOURCE: furnace blows cold air

Sometimes the fans kicks in by time ---or by heat...even if the heat cycles dont starts . Is this gas or electric,, name brand,.... model # ect...ect..