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Author Topic: Another Thermostat wiring question  (Read 8801 times)

Sconnieman

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Another Thermostat wiring question
« on: December 09, 2011, 01:09:16 PM »

Just about done getting the boiler all connected, just need to figure out the thermostat. I have a honeywell aquastat on the boiler that will control the fans and auto damper's on the boiler to keep the water temp where I want it. In my house I have two basic digital thermostats that are installed right next to each other. A previous owner had a pellet stove that was controlled by the second thermostat. The thermostat that is currently connected controls both my propane furnace and my central air. Is there a way to wire the currently connected thermostat to the one right next to it that is currently not connected to anything? My thought is that this disconnected thermostat (which is a heat only 2 wire model) can be our primary heat control, which will turn on just the furnace fan to maintain temp while the boiler is running, and the thermostat that is connected to our propane furnace and AC will then function as backup with the temp being set lower so it kicks on the propane if the fire goes out or something.

I think this is a fairly common setup. So where do I connect the two wires from the currently disconnected thermostat?

Second part of this is that I have a pressurized system, so I understand I should put a strap on aquastat in the house so it can monitor the incoming water temp. It would then turn on the furnace blower if the water is over a set temp such as 200 degrees or so to dump the excess heat. Anybody have any idea how that would be wired?

Thanks for any help you can offer.
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NW Wisconsin
Aqua-Therm E275
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RSI

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Re: Another Thermostat wiring question
« Reply #1 on: December 09, 2011, 02:12:24 PM »

You need to connect the W wire from the second thermostat the the wire that is currently on the G terminal in the main thermostat. (remove the wire from the G connection, don't just add the new wire)
Then run the other wire to the RH. Leave this wire on the main thermostat and add the new wire.

On the aquastat just run the wires from the common and normally open connections to the RH and G terminal in the furnace. (leave existing wires on screws and add new wires)
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Sconnieman

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Re: Another Thermostat wiring question
« Reply #2 on: December 12, 2011, 09:01:18 AM »

Thanks RSI. This worked. The only problem was that when I wired it this way the central air turned on. I turned off the breaker for the central air and now it is working perfectly. Thanks again.
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RSI

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Re: Another Thermostat wiring question
« Reply #3 on: December 12, 2011, 09:13:39 AM »

Did you leave anything on the G terminal in the original thermostat? That will turn on the AC sometimes.
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Sconnieman

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Re: Another Thermostat wiring question
« Reply #4 on: December 12, 2011, 09:44:52 AM »

Yes there was/is a wire on the G terminal. I just put the W from the new thermostat into G with the existing wire and the AC turned on. Then I just put the W from the new thermostat into G by itself and took the other wire out and then it didn't work at all. So I just put both wires into G and shut the AC breaker off, and it works. Do you think that is ok, or is there a better way?
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RSI

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Re: Another Thermostat wiring question
« Reply #5 on: December 12, 2011, 10:10:23 AM »

Take both wires off the G terminal and splice them together.
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Sconnieman

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Re: Another Thermostat wiring question
« Reply #6 on: December 12, 2011, 10:57:01 AM »

Take both wires off the G terminal and splice them together.

Ok I don't think I'm following that completely. I should just wire nut the wire coming from W of the new thermostat to the wire that is connected to the G in the existing thermostat, and then leave them disconnected from the G terminal? Can I turn my AC breaker back on then, or should I still leave that off?
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yoderheating

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Re: Another Thermostat wiring question
« Reply #7 on: December 12, 2011, 11:29:22 AM »

Sconnieman, you can either hook it up the way you have and pull the breaker or use a relay to keep power from back feeding to the old thermostat.
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Sconnieman

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Re: Another Thermostat wiring question
« Reply #8 on: December 12, 2011, 11:48:37 AM »

Sconnieman, you can either hook it up the way you have and pull the breaker or use a relay to keep power from back feeding to the old thermostat.

I've never installed a relay before, then again I've never done a lot of this stuff before and it hasn't stopped me yet, but I'm fine doing it the way I'm doing it now, because it works. The only thing I'm concerned about is causing damage to the AC unit without power going to it. I thought I read somewhere that these central ac units need power in the winter for some reason.
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yoderheating

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Re: Another Thermostat wiring question
« Reply #9 on: December 12, 2011, 11:51:05 AM »

Not sure about that, I always use a relay when I install a furnace.
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RSI

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Re: Another Thermostat wiring question
« Reply #10 on: December 12, 2011, 12:13:42 PM »

Take both wires off the G terminal and splice them together.

Ok I don't think I'm following that completely. I should just wire nut the wire coming from W of the new thermostat to the wire that is connected to the G in the existing thermostat, and then leave them disconnected from the G terminal? Can I turn my AC breaker back on then, or should I still leave that off?
Yes, just wire nut them together and leave the G terminal on the existing thermostat empty. You can turn the AC breaker back on then.
If you want the fan switch to still work then you need to do what Yoder suggested but I prefer it to not work so it doesn't get turned on and over heat the house.
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yoderheating

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Re: Another Thermostat wiring question
« Reply #11 on: December 12, 2011, 12:40:27 PM »

 So if you remove the wire from the g terminal on the old thermostat what turns on the fan when you run the ac in the summer?
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RSI

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Re: Another Thermostat wiring question
« Reply #12 on: December 12, 2011, 01:11:07 PM »

The G not being connected has no effect on the AC operation. It is only for the manual fan switch. All you need for the AC to work are the Y and R connected.
For some reason it back feeds through the thermostat if you leave the wires on the G terminal.
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yoderheating

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Re: Another Thermostat wiring question
« Reply #13 on: December 12, 2011, 01:54:41 PM »

 What turns on the fan when running the AC?
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RSI

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Re: Another Thermostat wiring question
« Reply #14 on: December 12, 2011, 02:54:36 PM »

The fan is controlled by the circuit board on the furnace when the AC is running.
I have always hooked them up this way and the AC always works like normal.
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