Clement Boston Heat Pumps plans for one question on every estimate: what happens to a heat pump during a power outage? The short answer is that it stops, and so does nearly every other central heating system. What matters is what keeps the house warm until the power returns, and what each option takes.
Every central heating system needs power
A heat pump is not uniquely exposed. Furnaces and boilers burn gas or oil, but they can’t run without electricity either.
| System | What needs electricity | Runs in an outage? |
|---|---|---|
| Ducted heat pump | Compressor, fans, controls | No |
| Gas furnace | Igniter, gas valve, controls, blower | No |
| Oil furnace or boiler | Burner motor, ignition, controls, blower or circulator | No |
| Gas boiler with radiators | Ignition, controls, circulator pump | No |
| Electric baseboard | Everything | No |
The difference is how much power each one needs. A furnace blower and controls draw a few hundred watts; a heat pump draws a few kilowatts at full output. That decides the size of any generator.
How long the house stays warm
A well-sealed, insulated house loses heat slowly, so a winter outage of a few hours is usually uncomfortable rather than dangerous. A drafty house cools much faster.
Slowing the heat loss
- Keep doors and windows closed, and close off rooms you don’t need.
- Draw curtains at night; open south-facing ones on sunny days.
- In a long outage in deep cold, protect pipes on outside walls and in unheated spaces.
Air sealing does double duty here: it shrinks the heat pump you need and buys hours in an outage. See heat pumps in old houses.
Sizing a generator for a heat pump
Size for the heat pump without its electric backup strips. Strips are large resistance heaters that can draw several times what the compressor does.
What goes into the number
- The outdoor unit: the nameplate lists minimum circuit ampacity; at full output in cold weather, a ducted heat pump draws a few kilowatts.
- The air handler blower and controls: a smaller load on its own circuit.
- Backup heat strips: commonly 5 to 15 kW or more; most generator plans lock these out.
- Everything else: refrigerator, lights, well pump, sump pump, outlets.
Many cold-climate units are variable speed, with inverter-driven compressors that start gently rather than with a large surge. That makes them easier for a generator to start than older single-stage units. See variable speed heat pumps.
Transfer switches and safety
A generator must connect to the house through a transfer switch or an approved interlock, installed by an electrician under an electrical permit.
That switch keeps generator power from flowing back out onto the utility’s lines, where it can injure line workers, and keeps utility power from hitting the generator when service returns. Never feed a house by plugging a generator into an outlet.
Running a generator
Portable generators run outdoors only, well away from doors, windows and vents, as their manufacturers direct, because their exhaust contains carbon monoxide. Working carbon monoxide alarms belong in every home with a generator or fuel-burning equipment.
Dual fuel as outage backup
A dual fuel heat pump keeps a gas furnace that can carry the house on far less generator power than the heat pump needs.
Where it fits
In a house on natural gas, keeping the furnace as backup lets a small generator run the furnace’s blower and controls through a long outage. The heat pump carries most of the winter; the furnace covers the coldest hours and outages. Our gas comparison covers the running cost side.
Where it doesn’t
In Cambridge, Arlington, Brookline, Newton, Lexington and Concord, a new building or major renovation can’t add fossil-fuel equipment, so dual fuel isn’t an option for that work. Dual fuel also earns Mass Save’s partial-home rebate, not the larger whole-home one.
Restarting after the power returns
Most heat pumps come back on their own, but not always instantly, and the first hour can look odd.
- Wait a few minutes. Many units hold the compressor off briefly after power returns to protect it.
- Follow the manual after a long, cold outage. Some manufacturers ask for the outdoor unit to sit powered for a period before the compressor runs, so it can warm.
- Expect a recovery. If the house cooled, the thermostat may call backup heat to catch up. That’s normal for one recovery, not every day.
- Check the basics. Breakers, the outdoor disconnect, and the thermostat’s mode and schedule.
If the thermostat stays on auxiliary or emergency heat after the house recovers, see heat pump aux heat and thermostat settings, or arrange a repair visit.
After a storm, check the outdoor unit
Winter outages here usually come with a nor’easter, so the outdoor unit may be buried or iced when the power returns.
- Clear snow from around and on top of the unit by hand or with a broom, never by chipping ice off the coil.
- Look for bent fins, a damaged fan guard or a fallen branch before the unit starts.
- If floodwater reached the outdoor unit or air handler, leave it off and have it inspected first.
A stand high enough for the site keeps most storms from burying it; see snow stands and placement.
Planning for outages at the first visit
Tell us at the first visit if outages worry you. It changes the backup heat plan and what we write on the estimate.
We note the heat pump’s electrical figures so an electrician can size a generator and transfer switch, and we lay out the trade-offs between whole-home and dual fuel for your house. The electrical upgrades page covers panel work. To talk through your house, call us or send the form.