Switching a Nest Thermostat to Emergency Heat: How and When

If your home runs on a heat pump, your Nest thermostat has a setting called Emergency Heat. Most homeowners never touch it, and that is usually the right instinct. But on the morning your heat pump stops blowing warm air, it is the switch that keeps the house comfortable until we can get out to you. Here is what it actually does, how to turn it on, and the part most guides leave out: when you should not.

What Emergency Heat really means

A heat pump is your main source of heat. In winter it runs the refrigeration cycle backward, pulling warmth out of the outdoor air and moving it inside. Modern units do this efficiently well below freezing, so in the Bay Area the heat pump carries the load on all but the rare cold snap.

Every heat-pump system also has a second, backup heat source for the moments the heat pump cannot keep up or is out of service. There are two common setups:

• Dual fuel. A gas furnace is the backup. This is the setup in my own house, and it is common on Bay Area retrofits where a heat pump was added to an existing furnace.
• All electric. Electric resistance strips inside the air handler are the backup.

Emergency Heat tells the system to stop using the heat pump and run only on that backup. That is the key distinction. It is not the heat pump working harder. It is the heat pump sitting out while the furnace or the strips do all the work.

Emergency Heat vs. Auxiliary Heat

People mix these up, so it is worth being precise. Auxiliary heat is automatic. When you bump the setpoint up several degrees, or on an unusually cold morning, the thermostat fires the backup alongside the heat pump for a while, then shuts it off once the house catches up. The heat pump keeps running the whole time.

Emergency Heat is manual and exclusive. You turn it on yourself, and it locks out the heat pump entirely. You reach for it when the heat pump is broken, not when it is simply busy.

How to turn it on with a Nest

The screens can read slightly differently depending on your Nest model and wiring, but the path is the same.

1. Wake the thermostat and press the ring to open the menu. Turn to Settings.
2. Press to open Settings and scroll to Emergency Heat.
3. Select Emergency Heat and toggle it On.
4. Confirm when prompted. Nest reminds you this mode uses more energy.

5. Set your target temperature. The dial now reads Emergency Heat, so you always know the backup is the one carrying the load.
6. To go back to normal, repeat the steps and switch Emergency Heat off.

If you scroll through Settings and there is no Emergency Heat option, your Nest is almost certainly not configured as a heat pump with backup. That usually means a standard furnace-and-AC system, where the switch does not apply.

When you should actually use it

In our climate, you are rarely turning this on because it got too cold for the heat pump. The honest reason a Bay Area homeowner flips to Emergency Heat is that the heat pump quit. The outdoor unit is iced over, the fan is not spinning, you hear the compressor struggle, or the system runs and the air coming out is not warm. Emergency Heat lets the furnace keep the house comfortable while you wait for service. That is exactly what it is for.

When you should not

Do not run Emergency Heat as your normal winter setting. The backup costs more to run than the heat pump, every day it is on. If you find yourself reaching for it again and again, that is not a habit to settle into, it is a signal. A heat pump that keeps leaving you cold has a fault underneath it: a low refrigerant charge, a reversing valve stuck in cooling, a defrost board that is letting the coil freeze up, or a failed outdoor fan motor. None of those fix themselves, and all of them are quieter on your wallet than a winter of backup heat.

If your heat pump is putting you on Emergency Heat more than once, that is the time to call. We diagnose the actual fault, get the heat pump carrying the load again, and stop the meter running on the expensive side of your system. We work across the Bay Area, seven days a week, with a flat diagnostic fee that we credit toward the repair.

Key takeaways

• Emergency Heat tells a heat-pump system to skip the heat pump and run only on the backup source: a gas furnace on a dual-fuel system, or electric strips on an all-electric one.
• It is meant for when the heat pump itself has failed or its outdoor unit is down, not for a normal cold night.
• Backup heat costs more to run than the heat pump, which is why Nest warns you about higher energy use and why you should switch it back once the heat pump is fixed.
• The Emergency Heat option only shows up on a Nest that is wired and configured for a heat pump with backup heat. If you do not see it, your system is probably not set up that way.
• If you keep ending up on Emergency Heat, or the heat pump runs but the house stays cold, that points to a fault worth a service call: low charge, a stuck reversing valve, a failed defrost board, or a dead outdoor fan.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

• Home Energy Score in the Bay Area: What the DOE Audit Tells You and Which Rebates Pay for It , DOE Home Energy Score is a 1 to 10 rating from a one-hour on-site audit. BayREN pays a $200 incentive for the score plus $50 for the Electrification Checklist when cycles are open. Here is what the audit covers and how we run it.
• Why We Offer 10-Year Parts + 10-Year Labor on HVAC Installations , Most HVAC contractors in the Bay Area offer 10-year parts (because the manufacturer requires it) and 1-2 years on labor. We offer 10-year labor too. This isn't marketing: it's a math decision that tells you something about how we build. Here's what the labor warranty actually covers, why most shops won't write it, and how it changes the install in ways that matter to you.
• AC Sizing Rules of Thumb (And Why They're Wrong) , The 500 square feet per ton rule, the 'match your existing tonnage' rule, the 'add half a ton for upstairs' rule, all of these have their fans, and all of them are wrong often enough to cost Bay Area homeowners thousands of dollars in oversized equipment, short-cycling, and shortened equipment life. Here's what each rule misses and what to use instead.