I have a central air conditioning and heating system, there's 2 different systems one for upstairs and one for the lower level and basement. The one for the lower level and basement I'm having problems with. For the first week over night when it gets real cold it will blow all night and coolish come to find out there was frost all over the unit outside and on the condenser and the fan sounded like it was trying to turn but wasn't so I thought hey the defroster is bad. I turn the unit heat off for awhile and then kick it back on and works fine. Well today the thermostat power is off and it doesn't work at all. Can it just be a bad thermostat? Does a thermostat control the defroster for the unit outside? Someone please help I'm tired of freezing me and my kids butts off.

And by the way I checked the fuse on the thermostat, turned off all the breakers and back on. The age of this system is only 5 years old

Possibly it may be you have a short in your t-stat wiring most likely the white wire that controls the defrost cycle is shorting somewhere , and now you have a either a burnt transformer or if your lucky and your transformer is protected by a fuse that has been blown , best thing to do is call a service tech to find the short , especially if your 24v transformer is not protected by a fuse ,as you don't want to keep burning transformers , and if that's what it is have your service tech install a fuse in case there problems in the future

Sounds like you have a heat pump. You will get a vent temperature of about 90 degrees on a 45 degree day. The air feels cool when you are a few feet away from the vent is normal. The outside unit has to defrost itself from time to time, when this happens the fan turns off, the reversing valve switch's, making a funny noise and just the compressor runs until the defrost cycle is done.