For the second time in about a week our heater came on instead of the AC.

The first time I woke up at 6:00 am sweating and felt the air coming out of the vents, it was HOT, my first thought was a fire, but after waking up a bit more I checked the thermostat and saw that it was reading 84 degrees... it was set to 74, and it was in the 60's outside. I turned off the unit, went outside, looked at it, gave it a dirty finger gesture, went back inside took out the two air filters because they were dirty, and flipped it back on.

It worked fine after that for about a week. We replaced the air filters with new ones, and now this morning, I could smell the heater come on... you know that burned lint smell of a heater that hasn't been on in months? That's what it smelled like, and in about an hour it went from the low 70's to 77 degrees in the house. So we cut it off again.

I told my wife to turn on the fan, and just let the fan blow.

Any ideas? We really can't afford to have a service guy come out.

It's a Trane gas pack unit. Model number is: YCC030FLMOBH

Thanks in advance!

I forgot to say, but we have a Honeywell programmable digital thermostat... that I installed as a replacement to an old switch type. It's probably two years old.

Either you have a bad relay somewhere or you have a 24 volt connection that is being crossed.
Check for any chewed wires outside. Little rodents like to eat them. Also maybe redo all the connections at the stat and outddor unit. Might have a break somewhere in the insulation that is causing it to go to heat and not ac.

A few item can cause this

Thermostat (unlikely but possible)
Intermittent short in the low voltage thermostat wiring both inside and outside.

The thermostat and associated wiring controls the signals to the outside unit. Somehow the outside unit is being told to operate in the heat mode so check the wiring especially outside going to the unit is you have a dog that chews the wiring then check the thermostat for proper operation. If the thermostat has a auto setting for heat cool changeover make sure it is turned off so that only you control the thermostats operation.

Thank you both so much. You jogged my memory of an incident with the string trimmer a while back. That'll be my first check.

The control board on this was replaced about a year and a half ago because we were having problems with our heat. That was under a warranty agreement.

Anyhow, thanks again, and keep up the good work!