Flaky Furnace Thermostat

DrewE wrote:

CarnationSailor wrote:

BFL13 wrote:
The Suburban gas furnace Installation manual should explain which wires to use with the thermostat. ( Any link to the manual on Google? We could check that out.)

Of the three wires you need two with this Tstat AFAIK, but which two? IMO you can find out by touching them together.

Yes, the Tstat is just a switch and has its own battery to power it, so it does not matter if the wire ends on W and R are 12v.

Do you mean to go without the air conditioner and heat pump now, and just have the furnace? (Where we camp on the Island here we never need air conditioning, but YMMV)

From the tstat manual, the only thing that makes sense is to connect the red (+12 vdc) to the white (Furnace) via the additional tstat. But this didn't work and you seem sure that it doesn't matter that the 2nd tstat is made for a 24 vac system. Maybe that means that the tstat that I bought from Lowe's is bad?

Does it work if you manually connect the wires together (no thermostat involved)? If not, does it work to connect the white to the other (ground) wire? One pair of them should make the furnace run, and those are the two that get connected to the thermostat. Since it has a battery, ignore the 24VAC power connections entirely. If manually jumping the wires makes the furnace run but connecting them to the thermostat does not, then it would seem the thermostat is indeed bad or you're misunderstanding the connections to it--which admittedly is not at all hard to do with some thermostats as the conventions and instructions for wiring them seem to have been invented by insane Martians.

If no combination of wires makes the furnace run, then perhaps you accidentally shorted the wrong ones together at some point and blew the furnace fuse (there may well be a smallish value one on the furnace control board).

The furnace still works when controlled via the OEM tstat so no blown fuse.

Although it seems logical to me that the only wires from the furnace to use are the red (+12 vdc) and the white ("Furnace"), I will test that by shorting them together. (I'm pretty sure if I involve the one labeled "-12 vdc", I'm going to end up send 12 volts to ground and blowing a fuse somewhere.)

Since we are leaving for Palm Springs in the morning, I'll postpone any further troubleshooting until we get there. (It's upper 80's there now so I can risk accidentally disabling both my furnace and my heat pump AFTER we get there!)