C-wire power connector
Common fix for smart thermostat power, Wi-Fi, and delayed-start problems.
If you see W5 on your Nest thermostat status display, don’t panic. W5 most commonly means your fifth stage of heating is active — on heat pump systems, that’s usually emergency heat or backup electric strips running. In most cases this is normal behavior during very cold weather. But W5 running constantly when it shouldn’t be can signal a heat pump problem.
What W5 Means
Nest uses the letter W followed by a number to indicate which stage of heating is active:
- W1 — First stage heat (normal heat pump operation)
- W2 — Second stage heat (more heat demand, compressor running harder or aux heat starting)
- W3 — Third stage heat (auxiliary heat, electric strips or backup furnace)
- W4 — Fourth stage heat (more backup heat)
- W5 — Fifth stage heat (maximum backup heat, often emergency heat)
W5 visible in the Nest status ring or equipment screen means the fifth heat stage is calling. The thermostat is telling the system to use its maximum heating resource.
When W5 Is Normal
Cold weather: Heat pumps lose efficiency below about 25–30°F (some down to 15°F). Below the system’s “balance point,” the backup heat source takes over. If the outdoor temperature is very cold and W5 is on, this is correct operation.
Manual emergency heat mode: If you switched to Emergency Heat in the Nest settings, W5 will show constantly. Emergency heat mode bypasses the heat pump entirely and runs only the backup electric heat — useful if the heat pump is broken, but expensive to run long-term.
When W5 Is a Problem
- Mild weather, W5 always on: If W5 is running when it’s 50°F outside, the heat pump isn’t working correctly and the system is over-relying on backup heat. Your electric bill will be much higher than normal.
- House not reaching setpoint despite W5: If all stages including W5 are calling and the house is still cold, there’s a heating system failure — either the backup heat elements are burned out, the heat pump compressor has failed, or there’s a refrigerant problem.
- W5 on a gas system: If you have a gas furnace (not a heat pump), W5 might indicate a stage isn’t modulating correctly or a staging control issue.
The “Delayed” Message — Not an Error
Many Nest owners confuse the “Delayed” message with an error code. This is actually the compressor protection timer — a 5-minute delay between cooling or heating calls that protects the compressor from short-cycling. It’s completely normal and not an error.
Step-by-Step Diagnosis
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Check the outdoor temperature. If it’s below 30°F and W5 is running, that’s normal.
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Check if Emergency Heat mode is on. In the Nest app: Settings → Equipment → Emergency Heat. If it’s enabled and you didn’t intentionally set it, turn it off.
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Observe whether the heat pump is running. Go outside and look at the heat pump unit. If W5 is showing but the outdoor unit isn’t running at all, the heat pump has a problem (frozen, failed contactor, refrigerant issue).
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Check your electric bill. A heat pump on W5/backup heat constantly uses 3–5x more electricity than normal heat pump operation. If your bills spiked, something is wrong with the heat pump.
Parts You May Need
When to Call a Technician
If W5 is running in mild weather and your house still isn’t reaching setpoint, call an HVAC technician. The heat pump likely needs diagnosis — low refrigerant, failed capacitor, and failed compressor are the most common heat pump failures.