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I have a southern California two-story home built in the late '70s. It's a single-zone system with both heating and cooling. The thermostat is located downstairs. In the summer the upstairs gets too hot and in winter the downstairs is too cold.
I'd like to put a wireless second thermostat upstairs and pair it with the hard-wired one downstairs. That way the system can tell the actual max./min. of the house temperature.
Are there any thermostats out there to do this? Would it even work?
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:
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!
My two story home plus basement in the US Mid-West like many others used to often get a little warmer upstairs in summer even with some of the downstairs returns blocked. What I did is add a 2nd thermostat upstairs that just triggered the blower fan when it got above a certain temperature. I just ran a 2 pair thermostat cable from the furnace blower relay in the basement. So if the downstairs is set at 75, I set the upstairs thermostat to 76 and the running of the blower evens out the temperature difference. This saves the electricity cost and wear and tear, (and avoids adding the few hundred watts of heat a PSC motor gives off) of having the fan on all the time as it only runs if temperature is above 76 upstairs so it only runs when needed. This will also bring warmer air from upstairs to the downstairs which may then trigger the downstairs thermostat to cool the air that otherwise would have remained un-cooled. As the thermostat has a 4 event timer for both hot and cold, I set a 15 minute time window for fan on right about bed time and at wake up to circulate air in the house regardless of temperature load. This can be done by setting the temp to, say 60, during the 15 min time window to force it on. Also I set the downstairs thermostat to 74 (1 degree cooler) just before bedtime which will run the A/C for 10 to 30 minutes to cool/dehumidify the air to enable ease of going to sleep. The upstairs thermostat is then set at 75 (1 degree lower) during sleeping hours. I do the converse in the winter but with temp settings appropriate/efficient for heating. Everything is now automatic other than only having to switch the upstairs thermostat from cooling to heating in the fall and vice versa in the spring (2 actions per year). In the spring and Fall, the 15 minutes force fan circulate in the morning and evening keeps the house air from going stale when neither heat or cooling demand would cause the blower to come on.
Total cost for cable and thermostat was < $40 and since I did this, I very seldom wake at night due to being too warm. My neighbors who also have a single zone system spent $15,000 to make a bedroom in their basement for sleeping in the summer, as their HVAC tech said nothing could be done without building a 2 zone system. My solution works for me.
Note I only connected the two fan terminals (the jumpered Rs and the G) on the 2nd upstairs thermostat as these will be switched on (closed) when either heat or cooling is activated and switched off (open) when temperature set point is satisfied.
My 4 event schedule for summer on upstairs Aux Thermostat that controls fan only:
6:15am to 6:30am 60F,
6:30am to 10:00pm 76F,
10:00pm to 10:15pm, 60F,
10:15pm to 6:15am, 75F
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.
