Forum Thread

Help Wiring New Thermostat

MadPup 3,539 April 3, 2016 at 12:42 PM
After I scored a Wi-Fi thermostat via this deal I am now looking at wiring it up and need some advice.

My current battery-powered thermostat has 3 wires (see photo). I believe these to be: 1 for furnace, 1 for fan alone, and a common. These wires are connected to the furnace control board as shown in another photo. None of the wires are always live, and I need a 24V supply for the new thermostat. Page 13 onward of the furnace manual [pritchardmechanical.com] covers wiring up a controller. Strangely it details using a different connector block to the one that is being used now.

At this point I have acquired 50' of 18/5 thermostat wire and intend to replace the current wire. However I need some advice on how exactly I should wire it up at either end.

Link: user manual for the new thermostat [honeywell.com].

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#2
Quote from MadPup View Post :
After I scored a Wi-Fi thermostat via this deal I am now looking at wiring it up and need some advice.

My current battery-powered thermostat has 3 wires (see photo). I believe these to be: 1 for furnace, 1 for fan alone, and a common. These wires are connected to the furnace control board as shown in another photo. None of the wires are always live, and I need a 24V supply for the new thermostat. Page 13 onward of the furnace manual [pritchardmechanical.com] covers wiring up a controller. Strangely it details using a different connector block to the one that is being used now.

At this point I have acquired 50' of 18/5 thermostat wire and intend to replace the current wire. However I need some advice on how exactly I should wire it up at either end.

Link: user manual for the new thermostat [honeywell.com].
The new 4th wire will just get connected to the "C" terminal on the furnace and the "C" terminal on the thermostat. Most likely you'll have a green wire you can use for the "G" fan terminal. That other terminal block in the manual is for a "communicating" thermostat with a serial interface. The standard Y, W, R, G, C and related terminal are documented a little farther into the manual.

I don't know how likely it is you'll ever want to add air conditioning, but the furnace supports 2 stage cooling. If you might want AC in the future, you might be better off running 6 conductor thermostat wire (you'd actually have to go to 7 conductor based on what home depot sells).
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Last edited by jkee April 3, 2016 at 01:08 PM
#3
or move G to C on both ends.
fan still works, you just won't be able to flip it from auto to on to turn the fan on when you want

err wait... you're not getting juice on the red and white?
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Last edited by stufine April 3, 2016 at 05:47 PM
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#4
Thanks for the advice guys. I returned the 18/5 wire and exchanged it for 18/7. Also...

I just checked the voltage on the wires again and discovered that I am getting 27V AC between red and white and between red and blue. For some reason I had thought the supply was DC so was looking at readings of zero previously.

I like the idea of using the fan wire as common instead. However, since we have no AC we use the fan to circulate air for the 1-2 weeks in August that it gets really hot here. But that's a few months from now so it would buy me some time to rewire.
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Last edited by MadPup April 3, 2016 at 08:12 PM
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#5
Quote from MadPup View Post :
Thanks for the advice guys. I returned the 18/5 wire and exchanged it for 18/7. Also...

I just checked the voltage on the wires again and discovered that I am getting 27V AC between red and white and between red and blue. For some reason I had thought the supply was DC so was looking at readings of zero previously.
Transformers output AC voltage, your furnace control board and thermostat use AC voltage. 27V is fine, things are speced at 24V but a little over or under is fine.
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