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Can I combine two thermostats into one?
March 23, 2007 7:15 AM   Subscribe

Do I need to have separate thermostats for my central air conditioning & my radiator heat?

The previous owners of our house installed central air not long before we bought it, though they left the radiator heating in (so we don't have a forced-air furnace, just radiators & AC). When they installed the air conditioning, they installed another thermostat for that on the wall opposite the existing thermostat for the radiators. My question is, do I really need two separate thermostats for those? I'd like to remove one & just run the wires from it to the other one if possible, as everywhere else I've ever lived has just had one thermostat to control both heating and cooling & I'd like to free up the wall space.

I should note that it's not such a big deal that I feel like hiring someone to come out & fix it, but I figure if I can do it myself I might as well.
posted by zempf to Home & Garden (5 answers total)
How is your boiler fired or are your radiators electric?
posted by Mitheral at 8:23 AM on March 23, 2007

Yes, I have a single thermostat controlling central air and radiators - that sounds like a lazy AC installer, or perhaps they split them because the location of the radiator thermostat was non-optimal for AC (for example, it gets direct sunlight in the summer but not in the winter).

Most thermostats are quite clear about what gets wired to what and radiator heat is among the simplest - you will probably have one or at most two wires for the heat and two or three for the AC. Just keep them separate and labeled and you shouldn't have any trouble combining them into one unit. As far as what you have now - the radiator thermostat may not support AC, the AC thermostat will most likely support both. You may want a new thermostat to take care of both if you don't have a programmable one as now you'll be able to use the programming part for both heat and AC.
posted by true at 8:26 AM on March 23, 2007

Yeah, I should've noted that getting a programmable thermostat was my other impetus for asking the question. As for the radiators, they're gas-heated water (not steam). I wouldn't be surprised if the dual thermostats were just a result of a lazy installer, as a bunch of other repairs were done in a somewhat halfass manner. They're on opposite walls of a hallway that doesn't get any direct sunlight, so I don't imagine the atmospheric conditions are all that different between the two.
posted by zempf at 8:53 AM on March 23, 2007

I think the key is that before you go out and get a new thermostat, you need to take the old ones off and see how many wires / what type they are. If they're different types (one uses a different number of wires than the other) you just need to get a compatible thermostat for both.

And of course you'll need to somehow run the wires to one place in the hallway, which may be a greater or lesser challenge depending on how they ran them in the first place.

But I think it's definitely a doable DIY project.
posted by Kadin2048 at 9:31 AM on March 23, 2007

You should be fine, the only gotcha to be aware of is heating systems with a standing pilot may be of the millivolt type instead of the standard 24V which requires a different thermostat. If you take both your stats into a heating supply place they should be able to determine what kind of control system you have and supply the appropriate thermostat. Oil furnaces need thermostats that have minimum cycle times but you don't have to worry about that with gas.

Alternatively you could just mount your existing stats side by side. You may find stringing the wire across the hall may be non trivial which would explain the current locations.
posted by Mitheral at 9:34 AM on March 23, 2007

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