My HVAC Mechanic can't fix the problem

[palmgirl]

I have an old rooftop model and I have no idea what brand it is. We have a 2 story condo with it on the roof.
First, he replaced all the wiring in the heating unit because I couldn't afford a $5000 new unit. Originally there was a small gas leak from the supply but all the wiring was really old and it looked like the unit had a fire at one time.
The unit worked after this but then after we shut the thermostat off the unit would continue to come on and off throughout the day and night.
Second, he replaced the older thermostat in the condo thinking this may be the problem. The unit still turns itself on and blows hot air even after the thermostat is off.
Third, he went back on the roof and saw that the sensor had some corrosion or gunk on it so he replaced that. Now, the heater won't come on at all.
Can anyone help us with this. I trust my mechanic but this is getting crazy.....

[nomadpeo]

i agree with the duke. a simple test while caught in the act is to verify whether 24 volts is present at the heat signal wire at the unit while the thermostat is not calling. sounds like the problem is intermitent, but rarely is a unit going to just start a heat sequence without a signal. the signal may be bypassing the thermostat through a short between the thermostat input wire and the thermostat heat output wire.

replacing all the wires in the furnace is a little out of the ordinary. if it is necessary to do this, you may need to take your cue from your first adjective, 'old', and find a way to replace this thing. i'm concerned for your safety. at the very least, call another technician and don't trust this one.

[safetech]

I HAD A SIMILAR PROBLEM WITH A SERVICE CALL WITH THE SAME SIMPTOMS AS YOU DESCRIBE. iT WAS A YORK GAS FURNACE AND AC COMBINATION AND I CHECKED ALL THE WIRES COMING FROM THE THERMOSTAT TO THE UNIT AND THEY WERE OK I CAUGHT THE UNIT ON WITHOUT A CALL FROM THE TSTAT AND NOTICED THAT IT WAS THE FIRST CIRCUIT BOARD WHERE THE WIRES FOR THE LOW VOLTAGE GO AT THE UNIT IT WAS A TRICKY CALL BECAUSE I DID NOT HAVE THE SCHEMATIC TO REFERENCE UNTIL I GOT ON THE COMPUTER THAT EVENING.