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justincaseme · Dec 22, 2008, 01:52 AM

Over the summer my mechanical thermostat broke and I replaced it with a White-Rogers 1F78 (digital). I thought this was going to be easy, but in a long process of trial and error the furnace fuse blew several times. Finally we got the heater to work, but when we'd turn on the A/C, the fuse would blow. Eventually we got the A/C working, but then turning on the heater would blow the fuse... very frustrating... we stuck with the A/C since that's what we needed at the time.

Well it's winter now, and we really need the Heater instead. Having both the heater and A/C wired correctly would be great :) I can't figure out where I'm going wrong with the wiring. I've checked the manuals for both the heater and the A/C. At this point, I can't even be sure if it's the wiring on the furnace end, the thermostat end, or both, that's screwed up. Here are some pictures of both the furnace end wiring and the thermostat end wiring. Any help?

Thermostat Manual
Installation Manual Part 1
Installation Manual Part 2

I know the PCB labels are tough to read. So I've tried to clarify them where needed with purple text.

FURNACE END WIRING
On the picture of the Furnace end, the white and red wires at the top connect to a pump for removing condensation (I believe).

The purple 3-amp fuse to the right is the one I've blown many many times.

THERMOSTAT END WIRING
Notice how RH and RC are jumpered together by the manufacturer. I don't know why that is.

KC13 · Dec 22, 2008, 08:54 AM

Rc-Rh jumper is needed for single transformer systems, can be removed for 2 transformer systems. Confirm which you have, but based on what I see it looks like 1. The connections look right with one exception: remove the blue wire from the "com" terminal at the furnace and the "B" terminal at the thermostat. This circuit is causing the fuse to blow.

justincaseme · Dec 22, 2008, 05:21 PM

Originally Posted by KC13

Rc-Rh jumper is needed for single transformer systems, can be removed for 2 transformer systems. Confirm which you have, but based on what I see it looks like 1. The connections look right with one exception: remove the blue wire from the "com" terminal at the furnace and the "B" terminal at the thermostat. This circuit is causing the fuse to blow.

It worked! Thank-you! BTW, if you happen to check this again, maybe you can answer one other quick questions about the setup.

* Are the red and white wires to the condensation pump hooked up correctly? My understanding is that a float ball inside the pump-tank rises with the water level and trips the pump as needed. What are these red and white wires doing then? Are they even needed?

Thanks

KC13 · Dec 22, 2008, 05:38 PM

Post a pic. It is probably an auxiliary float switch that can be used to disable the system or activate an alarm if the pump malfunctions and the reservoir is over-filling. I encounter a lot of these that are not hooked up. Pity...