Can I pair two thermostats to work with a single zone system?

Question

I have a southern California two-story home built in the late 70's. Its a single zone system with both heating and AC. The thermostat is located downstairs. In the summer the upstairs gets too hot and in winter the downstairs is too cold. Normally the system is either set to heat or cool depending on the season.

I'd like to place a second thermostat upstairs (wireless) and pair it with the hard-wired one downstairs. That way the "system" can tell the actually max/min of the house temperature.

Is there any thermostats out there to do this? Would it even work?

Answer 1

What you actually need is a single thermostat that has support for one or more secondary remote thermometers. I believe the Honeywell TB8220U1003 supports this via this part:

Remote Thermometer http://customer.honeywell.com/Honeywell/getliterature.axd?literatureID=C7189U_color_3in.jpg

As a side note, it is often necessary to properly balance your HVAC system depending on the season by adjusting the dampers to increase/reduce the airflow to certain areas based on the season. Also remember to open your cold air returns in the winter. You might not need a different thermostat at all!

Answer 2

You can't wire the thermostats in parallel, because the thermal anticipator resistors do not draw the correct amount of current when both thermostats are calling for heat. This will cause wild temperature swings.

Instead, I used a relay to select which thermostat has control. It comes on when thermostat A wants service and switches control to thermostat A. When thermostat A is not calling for service, thermostat B has control.

The relay can also operate dampers to control the direction of the air.

This does not work with electronic thermostats.