More than likely the A/C powers the Thermostat when the furnace is running.
Two suggestions.. one requires technical knowledge I do not have (Specifically the diagrams for the Thermostat including some they likely do not publish)
But if this is the case (and there may be a way to test this.. Okk how to test it)
IF there is a 12 volt wire where the A/C was that is "HOT'. and another that rund down to the Thermostat, Hook a Automotive TEST LAMP between those two leads See if that makes the furnace work. Just a standard test lamp or if you wish get a license plate lamp with pigtalis (Auto store) stick a dash indicator lamp in the socket.. or a Marker light assembly works too. I use one of these as a "Self Resetting Fuse" (In the event of a short the light comes on, clear the short and the light goes out) INCANDESCENT lamps only folks, NO LED's for this job.
IF that does not work.
Identify the Furnace T-Stat leads and swap your digital T-Stat for something like this.
Honeywell Heat Only 20 bucks at True Value Hardware[/ur]
I did set the "Anticipator" setting to the maximum and still get what I consider to be short cycling on the furnace. but dang does it work well The only real issue is where I mounted it it is far too easy to ACCIDENTALLY change the setting.
Oh one more thing... I put a toggle switch in front of it (in series with it) so that I cAN turn the furnace OFF, also an indicator light (led) to let me know the switch is ON. But the indicator light is "Dressing" (100% optional).
