HOW-TO GUIDE
Why Does My Heat Pump Use Too Much Electricity in Cold Weather?
By the OhmSavvy Research Team ·
Leave the outdoor unit and air handler closed
- Heat pumps run on 240 V circuits and contain capacitors that can stay charged after power is off. Opening the outdoor unit, the air handler or the electric heat strips, and clamp-meter readings on their wiring, are jobs for a qualified HVAC technician.
- Refrigerant work requires certification. Never add refrigerant or open service ports yourself.
- A burning smell from the vents when the aux heat first runs in autumn is often dust on the strips. A smell that persists, smoke, or a tripped heat-strip breaker that trips again: switch the system off and call a technician.
Why efficiency falls as it gets colder
A heat pump doesn't make heat from electricity; it uses electricity to move heat from outdoors to indoors. The ratio of heat delivered to electricity used is the coefficient of performance, or COP. In mild weather, a typical air-source unit might deliver three or more units of heat per unit of electricity. As the outdoor air gets colder, there is less heat to collect and the compressor works harder, so COP falls, often toward two or below in severe cold, depending on the model. Cold-climate units hold up better, but every air-source heat pump follows the same trend.
At the same time, the house loses heat faster the bigger the gap between indoor and outdoor temperature. Double the gap and you roughly double the heat needed. Lower efficiency multiplied by higher demand is why a cold snap can triple or quadruple daily heating energy without anything being wrong. Below the balance point, where the heat pump alone can no longer keep up, supplementary heat covers the shortfall.
Symptoms and likely causes
| Symptom | Likely cause | What to check | Who should fix it |
|---|---|---|---|
| Usage rises steadily as temperatures drop | Normal COP decline plus higher heat loss | Usage vs outdoor temperature over several days | No fix needed; insulation helps |
| Big spikes every morning | Setback recovery triggering aux heat | Thermostat schedule and recovery setting | Owner settings |
| High usage on mild days, 'aux' or 'em heat' showing | Emergency heat left on, or aux staged too early | Thermostat mode and aux lockout setting | Owner; HVAC tech |
| Outdoor unit frosted solid, frequent defrost | Airflow, sensor or refrigerant problem | Look at the coil; note defrost frequency | HVAC technician |
| Weak, lukewarm air and long run times | Dirty filter, blocked returns, low charge | Filter, return grilles, supply air temperature | Owner (filter); tech (charge) |
| Usage jumped this year at the same temperatures | Developing fault or a changed setting | Compare with last winter's interval data | HVAC technician |
Auxiliary and emergency heat
Most ducted heat pumps in the US include electric resistance strips in the air handler. Resistance heat has a COP of 1: every kilowatt-hour in becomes one kilowatt-hour of heat. A set of strips can draw as much power as the heat pump several times over, so a few extra hours of aux heat per day show up clearly on the bill. Auxiliary heat is meant to supplement the heat pump below the balance point and during defrost. Emergency heat is a manual mode that runs the strips without the heat pump, intended for when the outdoor unit has failed.
Check the thermostat display. If it shows emergency heat, someone switched it on, often during a cold snap, and it was never switched back. If aux heat comes on frequently on days well above the balance point, the staging may be too aggressive: many thermostats let the installer set an outdoor-temperature lockout or a delay before aux heat is allowed. Those installer settings are worth reviewing with your technician rather than changing blind.
Defrost cycles
In cold, damp weather, frost builds on the outdoor coil and blocks airflow. The heat pump clears it by briefly reversing into cooling mode, warming the coil while the outdoor fan stops. During defrost the house would get cold air, so many systems bring on the aux heat to temper it. A few short defrosts an hour in freezing fog are normal. Defrosting far more often, or failing to clear the ice, wastes energy and points to a sensor, control or refrigerant fault.
Keep the outdoor unit clear of snow drifts, leaves and anything stacked against it, and make sure meltwater can drain away rather than refreezing under the unit. You can look; don't chip ice off the coil, because the fins bend easily and the refrigerant tubing can be punctured.
Thermostat settings that cost money
Heat pumps prefer steady temperatures. A deep overnight setback means the morning recovery asks for a large temperature rise at the coldest time of day, which is exactly the condition that calls in aux heat. A thermostat configured for a furnace rather than a heat pump may stage heat wrongly. Smart thermostats sometimes ship with recovery settings that prioritize speed over efficiency. Check the equipment type in the installer menu if you are comfortable doing so, try a smaller setback or none for a week, and compare daily usage at similar temperatures. If the thermostat keeps rebooting, our guide to a smart thermostat that keeps losing power covers the wiring side.
1Safe homeowner checks
- Filter. A clogged filter cuts airflow, raises run time and can trip the system into aux heat. Replace it on the schedule the maker recommends, more often in heavy-use months.
- Supply and return grilles. Keep them open and unblocked by furniture or rugs. Closing rooms off doesn't save energy on most ducted systems.
- Supply air temperature. A thermometer at a supply register near the air handler, compared with the return air, tells you how much the air is being heated. Log it with the heat pump running alone and again when aux heat engages; a technician will want both numbers.
- Outdoor unit. From a step back, look for ice that never clears, a fan that doesn't spin when the compressor runs, or unusual noises. Note what you see and when.
- Breakers. If the heat-strip breaker or the outdoor unit's breaker has tripped, reset it once. If it trips again, leave it off and call the technician.
2Measuring what it really uses
The most useful number is daily heating energy plotted against outdoor temperature. Your smart meter's interval data from the utility, or a whole-home energy monitor installed by an electrician, shows consumption by the hour. Many monitors can track the heat pump and the strip heat circuits separately, which reveals immediately how much of a cold day was aux heat. A thermostat runtime report, showing hours of heat pump versus aux heat, gives the same insight for free on many smart thermostats.
Gather a couple of weeks of data and compare days with similar outdoor temperatures. A steady curve is healthy. Scattered high days at mild temperatures usually trace back to settings or aux staging. A whole winter that sits above last winter at the same temperatures suggests a developing fault, such as a slow refrigerant leak, that a technician should diagnose. Reading interval data is covered in our guide to smart meter usage at night.
Clamp-meter readings on the outdoor unit's supply, the compressor or the strips are where a technician confirms the picture: running current, start-up inrush, and, with a power clamp, actual watts and power factor. Those conductors are inside the unit and the disconnect, so leave them to the professional. For how clamp measurements work in general, see measuring AC current with a clamp meter.
When to call an HVAC technician
Meters HVAC technicians use on heat pumps
Watts, power factor, inrush and temperature in one HVACR clamp. CAT IV 600V / CAT III 1000V.
HVAC clamp with inrush mode, temperature and a flame-sensor µA range. CAT IV 600V / CAT III 750V.
Measures two thermocouples at once and logs 20,000 readings, handy for supply vs return air over time.
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Frequently asked questions
How much more electricity should a heat pump use in very cold weather?
It depends on the house, the climate and the equipment, but it is normal for daily use to rise steeply as temperatures fall, because heat loss rises and efficiency drops at the same time. Compare days with similar outdoor temperatures rather than comparing winter with autumn. A sudden jump at the same temperature is the sign of a fault or a settings change.
Is it cheaper to turn the heat pump down at night?
Small setbacks are usually fine. Large ones can cost more, because recovering several degrees on a cold morning often makes the thermostat call for auxiliary heat. Many heat pump thermostats offer a smart or adaptive recovery setting that avoids this; check its documentation.
Why is steam coming off my outdoor unit?
That is usually a normal defrost cycle melting frost off the outdoor coil, and it typically lasts a few minutes. Defrost that happens very often, ice that keeps building on the coil, or a unit encased in ice is a reason to call a technician.
Does this apply to air-to-water heat pumps in Europe?
The same physics applies. An air-to-water unit also loses efficiency as outdoor air cools and as the flow temperature it has to produce rises, so a high radiator flow temperature setting raises consumption. Many have a backup immersion or inline electric heater that behaves like auxiliary strip heat.
Is it normal for a heat pump to run nonstop in very cold weather?
Often, yes. Variable-speed heat pumps are designed to run for long periods at low speed, and near or below the balance point even a single-stage unit may run continuously. Running all the time is only a concern when the house cannot hold the thermostat setting, the supply air feels lukewarm, or the thermostat shows auxiliary heat on for most of the day.
