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Old 09-28-2011, 08:27 PM   #1
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one thermostat for 2 separate systems


Hello all, live in a 60 year old house with hot water baseboard heat on an oil furnace and retrofitted central a/c with the air handling unit in the attic. I have 2 seperate thermostats for the two systems. The heating thermostat is in front of the fire place and I'd like to relocate it to the hallway with the a/c thermostat or possibly get a thermostat that can run both systems. It seems that from what I can gather, that most dual purpose thermostats are for controlling one system that controls heat and a/c. My current a/c thermostat has a switch for heat, but i'm not sure if I could wire the furnace into the same stat. Both stats are currently the mercury "bubble" type.

I attached pictures of both thermostats and the wiring currently setup.
here is the a/c thermostat:


Here is the heat Thermostat:

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Old 09-28-2011, 09:56 PM   #2
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one thermostat for 2 separate systems


My Honeywell Visionpro controls my heatpump and my gas furnace that also acts as an air handler.

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Old 09-28-2011, 11:27 PM   #3
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one thermostat for 2 separate systems


as long as the replacement stat has both rh and rc terminals then it will control 2 systems. However, you will still have to run wire to the new location.
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Old 09-29-2011, 04:23 PM   #4
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Ok, so when I wire up the new stat from my AC the Yellow wire will go to "y" terminal, Green goes to "g" Red will go to "RC" and then from the furnace my black wire which is currently on the "r" terminal on its stat will now go to "rh" and the white wire will go to the "w" terminal?
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Old 09-29-2011, 05:55 PM   #5
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one thermostat for 2 separate systems


Also check for jumper between RC and RH
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Old 09-29-2011, 07:12 PM   #6
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one thermostat for 2 separate systems


you would want to remove that jumper if there is one for your type of setup.
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Old 09-29-2011, 09:41 PM   #7
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Great, thanks so much for the advice.
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Old 09-30-2011, 12:49 AM   #8
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It could be just a case of old eyes but it looks to me that your heating stat is line voltage and that should never be connected to a low voltage stat.
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Old 09-30-2011, 01:34 AM   #9
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It could be just a case of old eyes but it looks to me that your heating stat is line voltage and that should never be connected to a low voltage stat.
really seems low voltage for cooling and high voltage for heating
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Old 09-30-2011, 06:23 AM   #10
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It could be just a case of old eyes but it looks to me that your heating stat is line voltage and that should never be connected to a low voltage stat.
actually if you look a little harder the stats are the same just without the same terminals.... look how all the holes line up where the other terminals would be.
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Old 09-30-2011, 01:22 PM   #11
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one thermostat for 2 separate systems


to marry up the line voltage stat oil heat just relaces the stat with a relay 24V coil and pair of N.O. contacts so when the new cooling stat calls in heat the 24volts from the cooling Transformer will power up the 24V coil close the N.O. contacts(run the boiler)with R to W on the cooloing stat and run the baseboard to setpoint
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Old 10-02-2011, 10:06 PM   #12
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Ok I bought Honeywell RTH 7500. I wired up both systems to the stat and called on the heater and A/c seperately and they both worked.

I traced the lines to the old heater thermostat back to the furnace and the black and white wires are labeled: 24v Thermostat. I wired the new stat using the 5-wire stat wire with 3 wires wired to the A/C and the Black and white wires from the furnace connected to the other 2 going to the "w" and "rh" terminals with the jumper wire removed on the new stat.

When I called on heat, i shut the oil burner switch off right after it started to run because I knew that it had worked. However, I'm concerned with the fact that the stat wire is around 20. Ga. wire and the wire coming from the furnace is around 12-14, so when I connect them I suspect that the thin Ga. wire is carrying alot of voltage which could create some issues?

Biggles, you had mentioned something about a relay? I'm assuming that the 24 volt wires would connect to that and then from the other side of the relay I could run the thin thermostat wire to the stat?
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Old 10-02-2011, 10:19 PM   #13
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one thermostat for 2 separate systems


Quote:
Originally Posted by sst1226 View Post
Ok I bought Honeywell RTH 7500. I wired up both systems to the stat and called on the heater and A/c seperately and they both worked.

I traced the lines to the old heater thermostat back to the furnace and the black and white wires are labeled: 24v Thermostat. I wired the new stat using the 5-wire stat wire with 3 wires wired to the A/C and the Black and white wires from the furnace connected to the other 2 going to the "w" and "rh" terminals with the jumper wire removed on the new stat.

When I called on heat, i shut the oil burner switch off right after it started to run because I knew that it had worked. However, I'm concerned with the fact that the stat wire is around 20. Ga. wire and the wire coming from the furnace is around 12-14, so when I connect them I suspect that the thin Ga. wire is carrying alot of voltage which could create some issues?

Biggles, you had mentioned something about a relay? I'm assuming that the 24 volt wires would connect to that and then from the other side of the relay I could run the thin thermostat wire to the stat?

You don't need relay with 24vac thermostat

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