Heat Pump Backup Heat: What You Need for Cold Massachusetts Nights

Heat pump backup heat is a second heat source that helps or replaces the heat pump on very cold nights or when the heat pump needs service. In Massachusetts it is usually an existing boiler or furnace, or electric heat strips in a ducted air handler. You may not need it at all, but on the North Shore many homeowners keep it for the handful of single-digit nights each winter.

Key Takeaways

• Backup heat is optional. A tight, well-sized system can heat on its own.
• Your old boiler or furnace can be the backup. It runs only when the heat pump needs help.
• Electric strips work but cost more to run. They turn electricity into heat one-for-one.
• Controls make or break it. A poor setup can run the backup far more than needed.
• "AUX" and "EM HEAT" are different. One is automatic help; the other shuts off the heat pump.

Do You Need Backup Heat With a Heat Pump?

You need backup heat if your heat pump cannot cover your home's heat loss on the coldest nights. That depends on sizing, insulation and how cold your area gets. Our pillar guide on whether heat pumps work in cold weather explains how output drops as temperatures fall.

• Likely no backup: newer or well-insulated homes with a system sized for the full heating load.
• Backup recommended: drafty older homes, triple-deckers with thin walls, or partial heat pump installs that cover only some rooms.
• Backup for peace of mind: homeowners who want heat if the heat pump fails during a storm.

Not sure where your home falls? Request a backup heat assessment and we will check whether your current boiler or furnace should stay.

What Are the Types of Heat Pump Backup Heat?

There are three common setups in North Shore homes. Each has trade-offs in cost, comfort and control.

Electric resistance heat has a coefficient of performance of exactly 1. A heat pump does better. According to DOE guidance on air-source heat pumps, a heat pump can reduce electricity use for heating by about 50% compared with resistance heat. That is why strips should only run when truly needed.

What Is a Switchover Temperature?

A switchover temperature is the outdoor temperature where the backup takes over or starts helping. Installers often set it somewhere in the single digits or low teens for cold-climate units. The right number depends on your heat pump's 5°F capacity, your heat loss and your fuel costs.

• Set too high: the boiler or furnace runs on days the heat pump could handle cheaply.
• Set too low: the house may drift below the set temperature on the coldest nights.
• Integrated controls: one control decides which system runs, based on outdoor temperature.

ENERGY STAR cold-climate models must still deliver a COP of at least 1.75 at 5°F, per ENERGY STAR's heat pump criteria. So with a good unit, the switchover can sit quite low.

Auxiliary Heat vs Emergency Heat

Auxiliary heat and emergency heat use the same backup source but work in very different ways.

• Auxiliary (AUX) heat turns on automatically when the heat pump needs help. It runs alongside the heat pump or during defrost.
• Emergency (EM) heat is a manual thermostat setting. It shuts off the heat pump and runs only the backup.

Use emergency heat only when the heat pump is broken, like when the outdoor unit is buried in ice or damaged. Leaving it on by mistake can spike your electric or fuel bill. If "AUX" shows often on mild days, something may be wrong. See heat pump blowing cold air for common causes.

How Do You Keep the Heat Pump and Boiler From Fighting?

This is the most common problem we see when a heat pump is added to a North Shore home with an existing boiler. If both systems have their own thermostats set to the same temperature, the boiler often wins and the heat pump barely runs.

1. Set the boiler or furnace thermostat a few degrees lower than the heat pump.
2. Or use an integrated control that locks out the boiler above a set outdoor temperature.
3. Avoid deep nighttime setbacks that make the backup kick on every morning.
4. Keep the heat pump in heat mode, not auto, all winter.

Thermostat habits matter too. Our guide to the best temperature to set a heat pump in winter shows how to avoid triggering backup heat. As of 2026, some Mass Save heat pump programs have rules about how backup heat is controlled, so check Mass Save for current requirements.

Should You Remove Your Old Boiler or Furnace?

Not right away, in most cases. Many homeowners run one full winter with the old system as backup, then decide. If the heat pump covers every cold snap without help, removing the old system can free up space and end oil deliveries.

Keep in mind that an old oil or gas system still needs yearly service if it stays. If it burns fuel, keep working carbon monoxide detectors on every floor. If you ever smell gas, leave the house and call the gas company or 911.

Compare the bigger picture in our heat pumps in Massachusetts guide.

Set Up Backup Heat the Right Way

Heat pump backup heat is cheap insurance when it is set up well, and an expensive habit when it is not. The right switchover, the right controls and a correctly sized heat pump keep the backup quiet most of the winter.

Jose has worked in HVAC since 2015 and works on heat pumps, boilers and furnaces. HomePros can tie your systems together so each runs when it makes sense. Get a written estimate for heat pump and backup heat controls, or call (781) 426-8428.