Hello,

I have a two-zone forced air system using an oil furnace. A week or so ago, I lost the ability to start the furnace from the downstairs thermostat, which was a standard round Honeywell electro-mechanical. I could start and stop the fan by setting the control to "On" rather than "Auto", but with the other switch set to "Heat" the burner just wouldn't come on.

To diagnose, I shut off the furnace breaker and fan switch and tied the R and W wires together. When I turned the breaker back on, the furnace and fan both started, which I assumed meant the thermostat was bad.

I replaced both thermostats with programmables but continue to have the problem. Once again, after turning the breaker back on, the furnace and fan started, but after 10-15 minutes (not long enough to reach the set to temp) they went off, never to start again from the new downstairs thermostat.
The upstairs zone works fine with the new programmable.

I noticed the downstairs vents appear to be open because running the heat from upstairs also generates hot air downstairs. I'm not familiar with how the vent switching works and don't know if the downstairs was just left open from the initial time it ran after restoring power.

Both thermostats have R, RH, Y, G, and W wires.

Any help would be appreciated.

Thanks,
Frank

Are the vents electrically operated? If there are more than 2 wires running to them, the system may use a switch at the vent to start the heat. Of course that doesn't explain the system running with the red and white connected.

Does the thermostat show the temperature as being below the set point after it shuts off? Do you have the R and RH jumpered at both thermostats?

Labman,

Thanks for the quick response. I removed the R - RH (actually R - RC) jumper from both thermostats as per the instruction book and wired the old R to the new RC and the old RH to the new R.

I assume there's an electric switch for the vents but don't know for sure. There are two boxes above the furnace with slider switches indicating "Auto" and "Man Open", They're both set to "Auto" and I haven't touched them. I don't think the problem is with the vents per se, because I can run the fan from the downstairs thermostat and can feel (unheated) air coming through the downstairs vents.

Thanks,
Frank