Most smart thermostats want a C wire, the common leg of the 24-volt transformer, for steady power. Many older homes don't have one at the thermostat. Every model here has a maker statement that it works without a C wire on most systems, or includes an adapter that does the same job.
'Works without' isn't the same everywhere. Some draw power through the heat and cooling wires and recommend a C wire or power accessory if the system misbehaves; some depend on the adapter being installed at the furnace. Read each note for the maker's wording, and run your wires through the wiring checker.
ENERGY STAR labels smart thermostats on field data from real homes: at least 8% less heating and 10% less cooling run time, with an average saving EPA puts at about 8% of heating and cooling bills. The label matters less than the fit: a thermostat that can't run your system saves nothing.
Sensi ST55 Smart Thermostat
On power, its maker says no C wire is needed ("Common wire (c-wire) is not required in most applications"). Heat pump support isn't stated, so check the maker's compatibility checker if you have an O/B wire. Systems: Works with HVAC equipment found in most homes. It amazon Alexa, Apple HomeKit, Google Assistant, SmartThings, Vera. Power: Corded Electric. Its listing shows 23,126 buyer reviews. It suits phone control and scheduling and systems the maker's checker confirms. Where it gives ground: needs Wi-Fi for smart features. Warranty: 3 years.
Google Nest CE911542 Learning Thermostat
On power, its maker says no C wire is needed ("electric, oil, forced air, heat pump, and radiant, and no C wire is re"). Heat pump support isn't stated, so check the maker's compatibility checker if you have an O/B wire. It amazon Alexa, Android, Apple HomeKit, Google Assistant, iOS. Power: Battery Powered. Its listing shows 2,639 buyer reviews. It suits phone control and scheduling and systems the maker's checker confirms. Where it gives ground: needs Wi-Fi for smart features.
Sensi ST25 Smart Thermostat
On power, its maker says no C wire is needed ("COMMON WIRE REQUIREMENTS: Designed to work without a C-wire in most ho"). Heat pump support isn't stated, so check the maker's compatibility checker if you have an O/B wire. Systems: Works with the HVAC equipment found in most homes. It amazon Alexa, Google Assistant, Hand Control, SmartThings. Power: Battery or C-wire. Its listing shows 2,398 buyer reviews. It suits phone control and scheduling and systems the maker's checker confirms. Where it gives ground: needs Wi-Fi for smart features. Warranty: 3 years.
ecobee EB-STATE7-01 Smart Thermostat
On power, the listing does not say whether it needs a C wire. Its maker states heat pump support. Stages, per the maker: 2h/1c. It amazon Alexa, Android, Apple HomeKit, Google Assistant, iOS. Power: Battery Powered. Its listing shows 1,947 buyer reviews. It suits phone control and scheduling and heat pump systems. Where it gives ground: c-wire need not stated; needs Wi-Fi for smart features. If your system has a heat pump, confirm the adapter or power-stealing method supports it.
meross MTS300 Smart Thermostat
On power, a C-wire adapter comes in the box ("Some wiring configurations require a C wire (not included)"). Heat pump support isn't stated, so check the maker's compatibility checker if you have an O/B wire. It amazon Alexa, Android, Apple HomeKit, Google Assistant, SmartThings. Power: Corded Electric. Its listing shows 749 buyer reviews. It suits phone control and scheduling and systems the maker's checker confirms. Where it gives ground: needs Wi-Fi for smart features. Warranty: 3 year.
Aqara W200 Smart Thermostat
On power, the listing does not say whether it needs a C wire. Its maker states heat pump support. Systems: works with about 85% of HVAC systems, including furnaces, air conditioners, heat pumps, boilers, and PTAC unit. It apple HomeKit. Power: Corded Electric. Its listing shows 258 buyer reviews. It suits phone control and scheduling and heat pump systems. Where it gives ground: c-wire need not stated; needs Wi-Fi for smart features.
The ranking in one table
| Model | C wire | Heat pump | Stages | Sensors |
|---|---|---|---|---|
| Sensi ST55 Smart Thermostat | Common wire (c-wire) is not required in most app | Not stated | Not stated | Not stated |
| Google Nest CE911542 Learning Thermostat | electric, oil, forced air, heat pump, and radian | Not stated | Not stated | Not stated |
| Sensi ST25 Smart Thermostat | COMMON WIRE REQUIREMENTS: Designed to work witho | Not stated | Not stated | Not stated |
| ecobee EB-STATE7-01 Smart Thermostat | No C Wire, no problem | s, oil, electric, dual fuel, forced air Conventi | 2h/1c | Not stated |
| meross MTS300 Smart Thermostat | Some wiring configurations require a C wire (not | Not stated | Not stated | Not stated |
| Aqara W200 Smart Thermostat | No C-wire | with about 85% of HVAC systems, including furnac | Not stated | Not stated |
How to choose
Start behind the old thermostat: note every terminal letter with a wire in it. A C wire settles power; O/B means heat pump; W2 or Y2 means a second stage. Then read each maker's statements on exactly those points before features like screens, voice assistants or sensors.
Where a listing is silent on something your system needs, treat it as unconfirmed and use the maker's online checker. Keep the old thermostat's wiring photo until the new one has run heating, cooling and fan.
When another list fits better
If you have a C wire, the full smart thermostat table opens up every model.
For baseboard heaters, none of these apply: you need a line-voltage thermostat.
Questions shoppers ask
Can a smart thermostat work without a C wire?
Is a C-wire adapter hard to install?
Do these work with heat pumps?
How much does a smart thermostat save?
How do I check compatibility?
Our editors choose and order the picks from the makers' own statements; every pick meets the rule printed above. How we read a thermostat · Affiliate disclosure