Unify Radiators and Forced Air Cooling into One Thermostat

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Old 03-02-18, 09:27 AM
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Unify Radiators and Forced Air Cooling into One Thermostat

I'm working on a new house that has hot water radiator heat throughout, plus 2 zones of forced air cooling. Right now there are three thermostats. I'm going to keep the upper floor a/c thermostat, but I'd like to take the old heating thermostat and the new cooling thermostat on the first floor (right next to each other) and unify them into one new wifi thermostat.

The problem is that most thermostats don't seem to handle this situation innately. It will want to run the fan every time it calls for heat, even though they're separate systems. Any simple solution here?

I would like to have the option to leave the forced air system on circulate even during the winter.

I'd prefer to use a Honeywell RTH9580 but I also have an original Nest on hand.
 
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Old 03-02-18, 11:37 AM
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Seems like most thermostats with separate R and RC connections should be able to get the job done if the existing heat stat is low voltage.

The 24 volt common must come from the cooling transformer.

Configure for gas heat if you do not want the fan to run with a heating demand. Configure for electric heat if you do want the fan to run with a heating demand.
 
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Old 03-02-18, 12:24 PM
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So "gas" inherently means radiators and not forced air heat? I wouldn't have guessed that.

Maybe I'll test it out with a multimeter first before installing.
 
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Old 03-02-18, 02:15 PM
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"gas" means the furnace controls the blower (if it has one), not the thermostat. Older systems had a fan/limit switch above burner which would turn blower on when it got hot enough. Then would keep in on a bit after the flame went out and was still warm enough to heat the house. With that the thermostat didn't have to know anything about what was going on.
 
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Old 03-02-18, 03:18 PM
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I would like to have the option to leave the forced air system on circulate even during the winter.
That wouldn't be a problem. The thermostat should have a "fan on" setting where you can manually run it when desired.
 
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