Air Conditioning - A/c Switches To Warm Air

I have a central A/C unit that blows cool for days at a time, then all of a sudden it starts blowing warm air. I have had a professional out to check it and they say the unit is in perfect working order. No leaks. They told me to replace my thermostat. I bought a cheapo, and it did not blow cold air at all, always warm. I replaced it with a fancy one. No problems programming it or wiring it, very simple. Seemed as though the problem was solved for about 2 weeks, then it started again. I can shut off the thermostat and turn it back on and it will be fine. Or it can take up to 2 hours fooling with it to make it start blowing cool air again. I don't know what to do. I have no problem paying a pro, but I can't afford to have them start at the beginning again and still not know what to do. Thanks

I guess that the unit did not act up while the tech was there? Intermittent problems are kinda tough sometimes.
It sounds like your compressor for some reason intermittently is failing to start. First step is to verify (the next time that it is blowing warm air) that indeed the compressor is not running in the outside part of the unit. Usually you can tell pretty easily by simply listening to the unit while it is operating normally, the compressor will have a unique sound above the fan noise, the compressor noise of course will be missing if it is not running. Once that is verified there are several common causes that will need investigation. If you have a Split system with a air handler in the attic???? it will have a safety switch on the condensate drain pan. You may simply have a partially plugged drain and as the pan fills it shuts the comporessor down then slowly drains out and the switch resets and the unit functions again until the pans fills back up. The safety switch prevents drain pan overflow which can ruin your ceiling. You could have a faulty time delay switch that keeps the compressor from short cycling. You could have a bad contactor. You could have a bad capacitor. You could have a short in your t-stat wiring. You could have a weak compressor and need a hard start kit. Post back exactly what you have there and I am sure that someone here can lead you thru a few simple checks that you can perform yourself to narrow the problem down a bit. Note: A package unit is a one piece box outside with supply and return ductwork to your crib. A split system will have a condensor unit outside with copper lines running to a air handler unit inside your crib maybe in the attic or a closet. Let us know man.

It is a two piece, unit in attic and one outside. They put in a start booster about 8 years ago. I noticed when the warm air starts to blow, this fancy t-stat starts blinking "cool on", which after looking that up, means the compressor is trying to start. When it is blowing cool air, that does not blink. I will verify that the compressor is not starting by listening to the unit when it is running correctly like you said. I will also check the safety switch on the drain pan. I hope it is easily identifiable. I will take each thing you said and by process of elimination get this thing fixed. Thank you very much.