The short answer
A Google Nest runs most Carrier systems in Northern Virginia. A regular Carrier gas furnace, air conditioner or heat pump works on a Nest once a C wire gives it steady power. A Carrier Infinity system is the exception. It talks to its own wall control over four wires marked A, B, C and D, and Carrier designs it to run with that control, not a third-party thermostat. Put a Nest on it and you give up the variable speed and humidity control you paid for. Keep the Infinity control on an Infinity system. On everything else, a Nest, an ecobee or a Honeywell Home will do, and the right one depends on your wiring.
Infinity or regular: how to tell in two minutes
Start at the wall. If the thermostat says Infinity on it, you have an Infinity system. If it’s something else, pull it off the wall plate and look at the wires. Take a photo before you unhook anything.
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| What you find | What it means |
|---|---|
| Four wires on terminals marked A, B, C and D | Carrier Infinity, or Bryant Evolution, which is built the same way. Keep the Infinity control |
| Letters like R, C, W, Y, G and O | A regular 24 volt Carrier. A Nest works with a C wire |
| Letters, but no C and no spare wire in the bundle | A regular Carrier, but a Nest will run short on power. Plan on a C wire |
| The outdoor label says Infinity or Greenspeed | Variable speed equipment. It belongs on the Infinity control |
Why A, B, C and D? On an Infinity system two of those wires carry power and two carry data between the control and the equipment. A regular thermostat has nowhere to put the data. Carrier’s own buying guide says Infinity systems are designed to run with the Infinity System Control rather than a third-party smart thermostat. That control is what manages the humidity, the airflow and how fast variable speed equipment runs. Carrier’s Comfort and Performance lines are regular 24 volt equipment, and a Nest runs those fine.
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We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
Nest on a regular Carrier: the good and the catch
The good: a Nest Learning Thermostat learns your schedule, shows your heating and cooling hours in the app, and on a heat pump its Heat Pump Balance setting decides how much to lean on the backup heat. If you use Google Home, it’s the easy fit.
The catch is the same on every brand: power. A lot of Carrier furnaces in older Northern Virginia houses were wired with no C wire to the thermostat, because the old thermostat didn’t need one. A Nest does. Without it, the Nest borrows power through the heat and cool wires, and the signs show up as a low battery warning, a dark screen, Wi-Fi dropping, a fan that won’t quit, or the system clicking on and off on its own.
What goes wrong with a Nest on a Carrier, and what to do
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| What you see | What it usually is | What to do |
|---|---|---|
| Nest says Delayed, warns about the battery, or goes dark | No C wire. The Nest is borrowing power from the heat and cool wires | Connect a spare wire as C at both ends, or add Google’s Nest Power Connector |
| E4, E24 or E74: no power to Rh | The furnace switch is off, the blower door isn’t seated, a fuse on the board blew, or a float switch tripped | Check the switch and the door. A blown fuse means something shorted, so call us. More in the E74 guide |
| Nest says it’s heating, the furnace light is blinking, no heat | The furnace locked itself out | Count the blinks before you reset. Our Carrier furnace codes guide reads them |
| Heat pump blows cold in heat mode | The reversing valve setting is backwards | Google Home app: Settings, Thermostat, Heat pump. Switch O to B, or back |
| Fan runs nonstop | Fan set to On, or a stuck relay on the furnace board | Set the fan to Auto. If it keeps running, call us |
| E52 to E58: overcurrent | A shorted wire or a failing part | Stop resetting it and call us |
The small window on a Carrier furnace’s lower door is where it blinks its status code. That code names what the furnace saw, which the Nest can’t. Write it down before you turn anything off, because shutting the power off can clear it. All of Google’s codes are in our Nest error code guide.
Nest vs the Infinity control vs Honeywell vs ecobee on a Carrier
Is Google Nest the best thermostat? On a Carrier, it depends on which Carrier.
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| Thermostat | Runs Infinity? | Runs a regular Carrier? | Pick it if |
|---|---|---|---|
| Google Nest Learning Thermostat | No | Yes, with a C wire | You want it to learn your schedule and you use Google Home |
| Google Nest Thermostat | No | Yes, basic systems | You want simple and lower priced |
| Carrier Infinity control | Yes, every speed plus the humidity settings | Built for Infinity equipment | You have Infinity or Greenspeed equipment |
| Honeywell Home smart thermostat | No | Yes | You want room sensors or lots of models to pick from |
| ecobee | No | Yes, with a C wire or its power adapter | You want room sensors and Alexa or Apple Home. We install it for a flat price |
Our pick by system
- Carrier Infinity or Greenspeed: keep the Infinity control. If the control itself is failing, replace it with another Infinity control, not a Nest. The Infinity control has Wi-Fi, so you still run it from your phone.
- Regular Carrier furnace and AC: a Nest if you use Google Home, an ecobee or Honeywell Home if you want a sensor in the bedroom that runs cold. Have a C wire run either way.
- Carrier heat pump: a Nest Learning Thermostat or an ecobee, with the backup heat on AUX and the reversing valve setting checked on the first cold night. If yours ices up, read heat pump covered in ice and our guide to aux heat vs emergency heat.
- Replacing the system soon: wait on the thermostat. If the new system is variable speed, it comes with its own control. Here’s how Carrier and Trane compare.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
When to call us
Call when the Nest code keeps coming back, when the furnace light is blinking, or when the house won’t hold temperature. We work on Carrier furnaces, ACs, heat pumps and Infinity systems across Northern Virginia, and we check the furnace code and the thermostat wiring on the same visit, so you hear whether the problem is on the wall or in the basement. A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we start. Heat pump acting up? See heat pump repair.
Want the smart thermostat done for you? Our ecobee install is a flat $549 for the thermostat and the install: wired, mounted and set up on your phone before we leave. Heading into winter, our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
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Questions people ask
Does Google Nest work with Carrier?
Yes on regular 24 volt Carrier furnaces, air conditioners and heat pumps once there’s a C wire. Not on a Carrier Infinity system, which runs on its own control over A, B, C and D wires.
Can I replace my Carrier Infinity thermostat with a Nest?
Carrier designs Infinity systems to run with the Infinity System Control, not a third-party thermostat. A regular thermostat costs you the variable speed and humidity control, so replace a failing one with another Infinity control.
Why does my Nest say no power to Rh on my Carrier furnace?
Usually the furnace switch or breaker is off, the blower door isn’t seated, a fuse on the furnace board blew, or a float switch tripped. If the switch and door are fine, call a tech.
Is ecobee better than Nest on a Carrier?
On a regular Carrier both work. ecobee offers room sensors and works with Alexa and Apple Home; Nest learns your schedule and fits Google Home. On Infinity, neither: keep the Infinity control.
What do the A, B, C and D wires on a Carrier thermostat mean?
They’re Infinity’s wires. Two carry power and two carry data between the control and the equipment. A regular thermostat, Nest included, can’t use the data wires.
