combining two old units to one new thermostat?

Old 01-20-04, 08:00 PM
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Question combining two old units to one new thermostat?


Our home has forced-air heat from a gas furnace and central air using the same ductwork (The central air was added much later after the home was built). For some reason, we have separate thermostats for the heating and a/c, mounted about four inches from each other on the first floor. Is it possible to upgrade to a single electronic thermostat to control both systems, somehow combining the wiring from the two separate thermostats into the new electronic control unit?

Thanks in advance.
Old 01-20-04, 08:46 PM
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Should be pretty easy to do with a heat/cool thermostat. Just connect the wires to the correct terminals and you should be good to go.

Just check that your old thermostats are low voltage ones (24V) - not mains voltage. Most electronic thermostats work off 24V.
Old 01-21-04, 07:30 AM
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With 2 tstat now you chould be wired with 2 transformer. So with the new tstat make sure that there is no jumper for R to Rh to Rc. On the new tstat then put the R from the AC on Rc and the R from the furnace to Rh.

If the new tstat just has R on it then you will have to rewire the 24V set up on both units.

Old 01-21-04, 08:36 AM
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Ed hit it right on the Nail

So you have an outdoor package unit tied into your ductwork, or just a regular unit, with an A-Coil on top of the furnace?

Will type this to help you a bit.
*Remove jumper wire on T-stat Subase between RH and RC

A/C wire to T-Stat
Red to RC
Yellow to Y
Green to G

Heat wire to T-stat
Red to RH
White, to W

NOTE- If the heat stat has a fan switch (green wire), you may have to decide if you want the furnace fan to be "ON" when you turn the new t-stat fan switch to "ON" or you rather use the package unit's fan for the "ON"?

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