The most common heat pump problems in a Minneapolis winter show up as one of six symptoms: cold air from the vents, an outdoor unit caked in ice, a system that never shuts off, backup heat that runs all day, a new noise, or a unit that won’t start. Some of these are the heat pump doing exactly what it was built to do. Others take five minutes and cost nothing to fix. The rest need a technician, and they tend to get worse the longer a cold snap lasts.
Need a technician now? Contact us and we’ll send one out.
The 6 Problems at a Glance
| Problem | Can be normal? | Fix it yourself? | Usual cause when it isn’t normal |
|---|---|---|---|
| Blowing cold air | Yes, during defrost | Sometimes | Thermostat setting, low refrigerant, stuck reversing valve |
| Outdoor unit iced over | Light frost only | Clear snow, then call | Failed defrost control, blocked airflow, low refrigerant |
| Running constantly | Yes, on the coldest days | Often | Dirty filter, fan set to ON, buried outdoor unit |
| Aux heat runs all the time | Yes, in cold weather | Thermostat settings only | Big setbacks, wrong lockout setting, failing heat pump |
| Strange noises | A short whoosh during defrost | Rarely | Ice on the fan, loose panel, worn motor, refrigerant leak |
| Won’t turn on | No | Often | Thermostat mode, power switch, breaker, failed capacitor |
1. Heat Pump Blowing Cold Air
When homeowners call us about cold air, the cause is one of three things:
- The heat pump is in defrost mode.
- The air is warm enough to heat the house but feels cool on your hand.
- The unit has a fault that needs a technician.
Every winter we get calls about the first two. Nothing is wrong in those cases, and the homeowner pays for a service call they didn’t need.
About defrost mode
In cold, damp weather, frost builds up on the outdoor coil. To clear it, the heat pump switches to defrost mode and runs in reverse, warming the outdoor coil so the frost melts. A defrost cycle usually lasts 2 to 10 minutes and repeats every 30 to 90 minutes while frost keeps forming. While it runs, the vents can blow cool air. This is normal, and it stops on its own once the coil is clear.
How your body can trick you
A heat pump in heating mode sends out air at roughly 85 to 100 degrees. A gas furnace runs closer to 120 to 140. Your skin sits around 90 degrees, so heat pump air can feel cool when you hold your hand over a vent, even while it warms the room. If you grew up with a furnace, this is the part that takes getting used to.
The vent is a poor test. The thermostat is a better one. If the house reaches and holds the temperature you set, the heat pump is working.
What to do
Before you call, look for the signs below. If you see them, your heat pump is working the way it should.
Your heat pump is probably in defrost mode if:
- The outdoor fan stops for a few minutes.
- Steam rises from the outdoor unit.
- Cool air from the vents lasts a few minutes and comes back every 30 to 90 minutes.
Your body might be tricking you if:
- It’s an especially cold day or night.
- Your heat pump reaches and holds the temperature you set.
Also check that the thermostat fan is set to AUTO. On ON, the blower keeps running between heating cycles and pushes room-temperature air through the vents.
If cool air lasts longer than 15 minutes at a stretch, or the house keeps getting colder, have a technician inspect the unit. Low refrigerant and a stuck reversing valve both cause this, and neither is a homeowner fix.
2. Outdoor Unit Covered in Ice
A thin layer of white frost on the coil is normal in damp winter weather, and the next defrost cycle clears it. Ice that coats the coil fins solid, builds up on the top grille, or locks the fan blades in place means something has gone wrong. The heat pump can’t pull heat through a coil sealed in ice, so the backup heat ends up carrying the house.
The usual causes:
- Failed defrost. A bad defrost control board, sensor, or reversing valve stops the unit from melting its own frost.
- Blocked airflow. Snowdrifts, plow piles, and shrubs against the cabinet choke off the air the coil needs.
- Dripping from above. A leaking gutter or roof edge drips meltwater onto the unit, and it refreezes faster than defrost can clear it. It happens most often after a midwinter thaw.
- Low refrigerant. A leak drops the coil temperature and makes icing worse.
What to do
- Clear snow and ice from around the unit by hand or with a broom. Keep about two feet open on every side, and keep the unit sitting above the snow line.
- Never chip ice off the coil with a shovel, screwdriver, or ice pick. The aluminum fins bend and puncture easily, and a punctured coil is an expensive repair.
- If the fan blades are frozen in place, set the thermostat to Emergency Heat so the heat pump stops trying to run. Our guide on when to use the Emergency Heat setting explains what that switch does.
- Fix any gutter or roof drip over the unit.
If ice covers the coil again within a day, the defrost system or refrigerant charge needs a heat pump repair visit.
3. Heat Pump Running Constantly
If the heat pump runs constantly and keeps your home at the set temperature, nothing is wrong. Heat pumps are built to run for long stretches at lower output. On a zero-degree January night they may run almost nonstop, and newer variable-speed models are designed to do exactly that.
If it runs nonstop and your home still isn’t comfortable, something is wrong.
What to do
Check these before you call:
- Look at the air filter and replace it if it’s dirty. Hold it up to a light. If no light comes through, it’s overdue.
- Make sure the thermostat fan is set to AUTO, not ON.
- Check that the set temperature is realistic. A setting of 85 degrees in winter or below 65 in summer will keep the system running.
- Clear snow, leaves, and plants away from the outdoor unit so air can move around it.
If you’ve checked all four and it still runs nonstop, call a technician to find the cause.
4. Aux Heat Running All the Time
Most heat pumps in the Twin Cities have a backup heat source. It’s either electric resistance strips in the air handler or a gas furnace in a dual-fuel system. When the thermostat shows “AUX” or “Auxiliary Heat,” that backup is running.
Some aux heat is normal. Below the system’s balance point (often somewhere around 25 to 40 degrees for a standard heat pump, and much lower for a cold-climate model), the heat pump can’t keep up alone and the backup fills the gap. Defrost cycles also bring on aux heat for a few minutes to keep the vents from blowing cold.
Electric strips cost the most to run. They turn one unit of electricity into one unit of heat, while a heat pump in mild weather moves two to three units of heat for each unit of electricity it uses. When strips run all day, the electric bill shows it.
Aux heat running constantly on a 40-degree day is a problem. The common causes:
- Big thermostat setbacks. Raising the set temperature more than 2 or 3 degrees at once tells many thermostats to bring on aux heat to catch up.
- Wrong lockout setting. Many thermostats let the installer set an outdoor temperature above which aux heat can’t run. If it was never set, or set too high, the strips run in weather the heat pump could handle alone.
- A failing heat pump. Low refrigerant, a bad compressor, or an iced coil leaves the backup doing all the work.
- Emergency Heat left on. Someone switched to Emergency Heat during a cold snap and never switched back.
What to do
Keep setbacks to a degree or two, or skip them in the coldest weeks. Make sure the thermostat is on HEAT and not EM HEAT. Then watch it on a mild day. If AUX shows most of the time above 35 degrees, have a technician check the refrigerant charge and the thermostat’s lockout settings. A heat pump tune-up covers both.
5. Strange Noises
Some sounds are part of normal operation. A whoosh followed by a hiss at the start or end of defrost is refrigerant changing direction. Steam and dripping meltwater come with it. A click when the system starts and a low hum from the outdoor unit are normal too.
These sounds mean it’s time to call:
- Grinding or screeching. Usually worn fan motor bearings.
- Rattling or banging. A loose panel, debris in the cabinet, or fan blades hitting ice.
- Loud, steady buzzing. Often an electrical part such as a contactor or capacitor.
- Constant hissing or gurgling. A possible refrigerant leak.
Check for ice or sticks around the fan first. If you hear metal hitting metal, set the thermostat to OFF and call. Running a unit with a damaged fan or motor turns a small repair into a big one.
6. Heat Pump Won’t Turn On
A heat pump that won’t turn on usually has an electrical cause, and you can rule out a few of those yourself.
What to do
Try these first:
- Check the thermostat. Make sure it’s set to HEAT, not COOL or OFF, and replace the batteries if the screen is blank or dim.
- Check the power switch to the heat pump. Some heat pumps have a power switch on a nearby wall that looks like an ordinary light switch, and people turn it off by mistake.
- Check and reset the circuit breaker. Even if the heat pump’s breaker doesn’t look tripped, flip it to OFF, then back to ON, and try starting the heat pump again. If the breaker trips again right away, call a technician and stop resetting it.
If the outdoor unit hums but the fan doesn’t spin, or it still won’t start after these steps, a technician needs to find the fault. A failed capacitor or contactor is the most common culprit, and both are quick repairs.
When to Call a Technician
Call us if the breaker trips twice, ice keeps coming back on the outdoor coil, the house keeps getting colder, AUX runs constantly in mild weather, or you hear grinding, banging, or steady hissing. MSP’s heat pump diagnostic visit is $88, and it’s free for Total Comfort Club members. Most heat pump repairs we do in the Twin Cities cost between $259 and $1,500, and we quote a fixed price before any work starts. You can read more about our heat pump repair service or schedule a visit.
An annual fall tune-up catches most of these problems before the first cold snap. Our guide to heat pump upkeep covers what you can do between visits, and our guide to heat pumps in Minnesota explains what to expect from one in a Twin Cities winter.