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Heat pumps, aux heat, multi stage and dual fuel
Updated
A heat pump changes two things. First, the "no C wire" promise mostly goes away. Second, the number of stages your thermostat can run starts to matter. If you see a wire on O/B, you have a heat pump, whatever the last owner told you.
C wire rules on a heat pump
| Pick | Heat pump with no C wire |
|---|---|
| ecobee Smart Thermostat Premium | Fits with the adapter in the box |
| ecobee Smart Thermostat Essential | Needs a C wire or an adapter sold separately |
| Google Nest Learning Thermostat (4th gen) | Probably fits, confirm with the maker's checker |
| Google Nest Thermostat | Needs a C wire or an adapter sold separately |
| Sensi Smart Thermostat ST55 | Needs a C wire or an adapter sold separately |
| Amazon Smart Thermostat | Needs a C wire or an adapter sold separately |
That is the checker's answer for a five wire heat pump with aux heat and no C wire. Sensi says heat pump systems require a C wire. The Nest Thermostat listing says heat pump systems need a C wire or a compatible power accessory. ecobee's kit covers it as long as you have 4 or more wires with G and Y.
Stage limits we could confirm
| Pick | Conventional | Heat pump |
|---|---|---|
| ecobee Smart Thermostat Premium | 2 heat, 2 cool | 2 compressor stages plus 2 aux stages |
| ecobee Smart Thermostat Essential | 2 heat and 1 cool, or 1 heat and 2 cool | 1 compressor stage plus 1 aux stage |
| Google Nest Learning Thermostat (4th gen) | 3 heat, 2 cool | 2 compressor stages plus 2 alternate heat stages |
The first two rows come from ecobee's compatibility page, the third from Google's. We did not find a stage limit we could pin to the Nest Thermostat, Sensi ST55 or Amazon Smart Thermostat, so run the maker's checker if you see W2 or Y2. If you have W3 or Y3, ecobee recommends a pro.
- ecobee Premium4 stages
- ecobee Essential2 stages
- Nest Learning 4th gen4 stages
Dual fuel: get a pro
Dual fuel means a heat pump outside and a gas or oil furnace inside. Google says you need a trained professional to set up the thermostat, because choosing incorrect options can damage the system. ecobee supports dual fuel and gives one firm warning: during setup, select Disable on the screen that asks whether the heat pump and aux heat may run at the same time. Our checker marks dual fuel as a pro install for every pick.
Systems that do not fit any pick here
- Communicating or proprietary systems. Sensi says its thermostats cannot talk to serial protocols. ENERGY STAR advises that for the very highest efficiency equipment you may want a controller from the same company.
- Millivolt heaters. Wall and floor heaters. ecobee and Google say they cannot deliver the power a smart thermostat needs.
- RV and other 12 volt DC systems. Sensi says it requires 20 to 30 VAC.
- Solid fuel. ecobee and Google list wood, coal and pellet systems as not compatible.
The old L wire on some heat pump thermostats, which lit an emergency heat lamp, is not needed. Google and ecobee say to leave it off.
Safety first. Turn off power at the breaker or the system switch before you pull a thermostat off the wall, then change the set temperature by 5 degrees and wait 5 minutes to confirm nothing starts. If you see thick wires, wire nuts, L1 or L2, or a 120 V or 240 V label, stop. That is line voltage. Put the cover back and call a licensed electrician.
Tick your own wires in the wire checker, or compare all seven picks on the roundup page.