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Old 11-11-2007, 07:57 PM   #1
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Combine two thermostats?


Recently moved into an older home, that has a new (4 years old?) natural gas powered, steam radiator heating. Also has a second system, air conditioning (forced air, vents retrofitted) that is about 15 years old.

The AC is using the old Honeywell round dial thermostat. The furnace is using a Honeywell T8082 series thermostat (looks like it has a mechanical timer on it, where we only have two on snaps, and one off snap).

What we would like to do is replace both of them with a single programmable thermostat.

I am comfortable swapping either of them to a new thermostat. Done it before in other homes. What I am not sure is how I would combine the two into one, and what is involved?

Would be nice to have one set of controls, or is my best bet to use two programmables?

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Old 11-11-2007, 11:55 PM   #2
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Combine two thermostats?


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Originally Posted by geoffc View Post
Recently moved into an older home, that has a new (4 years old?) natural gas powered, steam radiator heating. Also has a second system, air conditioning (forced air, vents retrofitted) that is about 15 years old.

The AC is using the old Honeywell round dial thermostat. The furnace is using a Honeywell T8082 series thermostat (looks like it has a mechanical timer on it, where we only have two on snaps, and one off snap).

What we would like to do is replace both of them with a single programmable thermostat.

I am comfortable swapping either of them to a new thermostat. Done it before in other homes. What I am not sure is how I would combine the two into one, and what is involved?

Would be nice to have one set of controls, or is my best bet to use two programmables?
If both systems possess like voltage (preferably low voltage) to control their function(s), this can be a relatively simple problem to solve. assuming either system has high voltage a relay and a transformer can be incorperated to achieve 1 control point (I don't recall a programmable line voltage 'stat).
IF YOU DO DECIDE TO APPROACH THIS CONCEPT AND ARE NOT QAUALIFIED, HIRE SOMEONE WHO IS!

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Old 11-12-2007, 08:23 AM   #3
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Combine two thermostats?


Should be no problem. A thermostat will likely have following terminals: W, R, RC, Y, G, C, and possibly others, which you need not worry about. R and RC will most likely be connected with a short wire. You want that piece removed.
W and R go to your boiler.
Y and RC go to your AC. If AC has 'fan only' terminal, that's where G goes to.

If your thermostat is 100% battery-powered, you don't need C. Of course, it is better to get a combined-power unit, which uses batteries for backup only. C is a common (ground) and if neither AC nor furnace has a special terminal for it, you can just hook it to the nearest ground screw.
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Old 05-26-2012, 02:07 PM   #4
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Combine two thermostats?


Great advice! If only the directions in the manuel would have included what each wire did I would have been done an hour ago. Thanks to these posts I finished fixing my installation in less than a minute.
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Old 05-26-2012, 03:51 PM   #5
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Combine two thermostats?


if you go with a new digital stat with a C to power the stat on a tue 24v set up you would need to add and isolating relay for Rh-W HEAT closure on N.O. contacts from the new stat with the cooling as normal Rc-Rh,Y,G C into the subbase.so you would run Rh and W to the coil on the relay then run the 2-wire steam heat boiler to the contacts....when heat is called in from the new stat the col closes the contacts and the boiler runs to setpoint for heating on the new stat.by having Rc-Rh jumper it gives you automatic setting and powers the boiler relay coil as the heating point as a closure.
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