I have a problem with a 15-year old Heil natural gas furnace that is connected to a Honeywell CT3500 thermostat. The inducer/exhaust fan cycles on and off (about 10-15 seconds on, then shuts off, then immediately cycles on again) - extremely annoying and not exactly "silent" either. This happens only with the thermostat NOT calling for heat or cooling: if the thermostat clicks on (I can hear the relay quite clearly), the cycling stops and the exhaust fan runs continuously and everything runs normally.

Apparently (so the girlfriend tells me), this started happening when the old mercury thermostat was removed and replaced with the Honeywell - before my time in this house.

I had a technician come by last week and of course, the problem didn't manifest itself while he was present. He pulled the vacuum tube off the fan motor to test that it had enough negative pressure to activate the pressure switch properly - it did. He spoke of replacing the fan, the igniter, and gas valve/control unit if he came back and could catch it doing it.

Luckily the service call was free because my suspicion is that the problem is not with the furnace and that if I had the old mercury thermostat and swapped out the Honeywell, I would find the problem would go away.

I'd like to resolve this once and for all. I'm handy with electronics and electrical work and have done tons of repair work. I've heard that an isolation relay might be needed? Or even simpler: are there programmable thermostats on the market that would be capable of handling the situation (i.e. have their own circuitry that emulates an isolation relay)?

What tests would be recommended with my meter to determine the problem lies with the thermostat? Should I pick up a manual thermostat and try it out? But really the end solution would be to have a proper programmable unit that doesn't cycle the d*mn exhaust fan - if that's the problem.

Thanks,
Stephane