Thermostat that can time use of two different furnaces?
I’m thinking about installing a heat pump in my home’s forced air system, but electricity from my utility is prohibitively expensive between 3pm and midnight. Assuming that I leave in place the existing natural gas furnace, are there thermostats that can be programmed to, for instance, use only the gas furnace between 3pm and midnight, and to use the heat pump during other hours?
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The Ecobee thermostat supports the concept of "time of day" electric rates, but I'm not sure if it can be configured to run a different furnace during certain times. You might need two thermostats, one for each furnace, and adjust the setback times to make the gas furnace run during the expensive electricity time.
Another possibility would be a timer and a relay. The timer would trigger the relay to switch the "W" (heat) terminal from one furnace to the other. This would mean one thermostat would operate normally, and the timer would control which of the two furnaces that thermostat would be controlling and when. Note that my "switch the "W" terminal" is a simplification, you'd need to do a little more than just that to make this work.
What if you put the heat pump on a timer so it's off from 3pm to midnight. Then use a two-stage thermostat with the heat pump as the first stage and the gas furnace as the second stage. The way a two-stage works is if the temperature drops below the set point, the first stage is turned on. If it drops more than two degrees (or whatever it's set for) below the set point then the second stage is turned on. If the first stage can't turn on because of the timer, the temperature will fall to the point where the second stage turns on.
What this would mean in effect is that from 3pm to midnight your thermostat set point is 2 degrees lower than the actual set point. If that is not acceptable, get a programmable thermostat and have it boost the set point during those hours.
A side benefit is if your heat pump can't keep up on the coldest days the gas furnace will automatically kick in to help out.
All of this is disregarding whether it's a good idea to have the two units in the same ductwork.
Another option to consider: Use an air-to-water heat pump, and then a hydronic coil in the air handler. Include a big tank (some hundreds of gallons). Heat the tank to the maximum temperature (maybe 120 F) before 3 PM, and then shut off the heat pump from 3 PM to midnight, and draw down the temperature on the tank. Maybe around 8 PM, you will have depleted the tank but then you let the house cool down some until midnight.
Insulated tanks are pretty expensive so it might not be that great an option but it would be interesting to consider.