A heat pump in cold weather should run for long stretches, blow air that feels warm but not hot, and occasionally stop heating briefly to melt frost from its outdoor coil. Your thermostat may also show auxiliary heat during cold snaps. What matters is whether the house holds temperature and whether the outdoor unit clears its frost. This article covers how the system operates. If you are still choosing equipment, see our comparison of a heat pump vs. a furnace in Oklahoma.
Why a heat pump acts differently from a furnace
A furnace creates heat by burning fuel. A heat pump moves heat from the outdoor air into the house using refrigerant, and there is still usable heat in cold air. As the temperature falls, there is less heat to collect while the house loses heat faster. The system's capacity drops just as demand rises.
That leads to two things that surprise people who are used to gas heat. The first is long run times. In cold weather, a heat pump may run for long periods, sometimes nearly continuously, and that is not a sign of a fault when the house is staying comfortable. The second is air temperature. The air from your vents is warmer than the room, but it is milder than furnace air and can feel cool on your hand. Check the thermostat reading rather than judging by the feel of the vent.
What the defrost cycle looks and sounds like
In heating mode the outdoor coil gets colder than the air around it, so moisture can freeze on it. Frost blocks airflow through the coil, and the system clears it by running a defrost cycle. During that cycle the heat pump briefly reverses direction and sends heat to the outdoor coil.
While this happens, it is normal to notice the following:
- The outdoor fan stops while the compressor keeps running.
- Steam rises from the unit. This is melting frost and is often mistaken for smoke.
- You hear a whoosh or a change in sound as the system switches over and again when it switches back.
- Water drains from under the unit.
- Auxiliary heat comes on so that the air indoors does not feel cold in the meantime.
A defrost cycle usually lasts a matter of minutes. Frost shows up more readily in damp conditions near freezing than on very cold, dry days, so a foggy or drizzly morning can bring more defrost cycles than a colder afternoon.
Auxiliary heat and emergency heat are different settings
Many heat pumps have a backup source, commonly electric heat strips in the indoor unit. In a dual-fuel system, a gas furnace plays that role. Two thermostat terms cover this equipment, and they do not mean the same thing.
Auxiliary heat comes on automatically. The system may use it during defrost, when the room is falling behind the setting, or when you raise the thermostat by several degrees at once. Seeing it now and then is normal, including during a cold snap.
Emergency heat is a setting you select yourself. It shuts out the heat pump and uses only the backup source. It is meant for when the outdoor unit cannot run, such as after a failure or when it is encased in ice. Electric strips generally cost more to run than the heat pump, so it is not a setting to use because the weather is cold. Check with your installer before changing any configuration settings, including temperature lockouts. Those are set for your particular system.
Thermostat habits make a difference here. A deep nighttime setback followed by a big morning increase can bring on backup heat that a steady setting would have avoided. A thermostat with adaptive recovery, set up for heat pump operation, can reduce this. Our thermostat services page covers compatibility and upgrades, and our guide to thermostat settings for Oklahoma covers everyday settings.
What is normal and what is worth a call
| What you notice | Usually normal | Worth a service call |
|---|---|---|
| Run time | Long cycles in cold weather while the house holds temperature | Runs constantly in mild weather, or the house keeps losing ground |
| Frost on the outdoor unit | A light, even coating that clears after defrost | Heavy ice on the coil, the top, or the fan that does not clear |
| Auxiliary heat | Comes on during defrost or the coldest periods | Stays on most of the time, or bills jump sharply without a cold spell to explain them |
| Vent air | Warm but milder than furnace air | Air stays cool for long periods outside of defrost |
| Sounds | A whoosh at the start and end of defrost | Grinding, banging, or a fan that does not restart after defrost |
What you can safely check
- Clear snow, ice-laden branches, and leaves from around the outdoor unit so that air can move freely.
- Check whether a gutter or roof edge is dripping on the unit, because that water can freeze on the coil.
- Replace a dirty indoor filter, since restricted airflow can hurt heating performance.
- Confirm the thermostat is set to heat and not locked in emergency heat by mistake.
Oklahoma ice storms deserve a specific word. Freezing rain can coat a unit in a way normal defrost cannot clear. If that happens, switch to emergency heat if your thermostat offers it, and call for service. Please do not chip at the ice or pour anything on the coil, because the fins and tubing are easily damaged. Do not open electrical panels, handle refrigerant, or force repeated restarts. Low refrigerant can reduce heating performance, but that needs a certified technician to diagnose and correct.
If your system has a gas furnace as backup and you smell gas or a carbon monoxide alarm sounds, leave the house and call 911 and your gas utility from outside.
When to have it looked at
Call if the house cannot hold temperature, heavy ice does not clear, or backup heat seems to run all the time. Before you call, note what the thermostat displays, what the outdoor unit is doing, and when the problem began. Our heat pump services include diagnosis and repair, and you can reach us around the clock at (405) 413-0583.