Heat pumps are everywhere in the Triangle, and the most common question I get about them every winter is some version of "why is it blowing cold air?" Sometimes the answer is that nothing is wrong. Sometimes it really does need a repair. Here's how to tell the difference.
First, the big one: a heat pump doesn't make air as hot as a gas furnace. A gas furnace often puts out air well over 100 degrees. A heat pump in cold weather puts out air that's usually somewhere in the 90s at the vent. That's warmer than the room, but it's below your body temperature, so it can feel cool on your hand. If the house is holding temperature, that's the heat pump doing its job.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Feel the air for a full minute, not a few seconds
Put your hand at a supply vent near the indoor unit and leave it there. Lukewarm, steady air with the house slowly warming is normal. Air that's the same temperature as the room, or actually cold, while the thermostat is calling for heat is not.
Check for a defrost cycle
In cold, damp weather, frost builds on the outdoor coil. The heat pump fixes that by briefly running in reverse. During defrost the outdoor fan stops, the unit may hiss or whoosh, and you might see steam rising from it. This usually lasts several minutes and can happen every 30 to 90 minutes when it's cold and wet.
Inside, you may feel cooler air during defrost unless the backup heat comes on to cover it. If it goes back to warm air after a few minutes, that's normal.
Make sure the thermostat is set to Heat and Auto fan
If the fan is set to On instead of Auto, the blower runs all the time, including between heating cycles, and pushes room-temperature air that feels cold. Set the fan to Auto so it only runs while the system is heating.
Look at the outdoor unit
The outdoor fan should be spinning and the unit humming when the thermostat calls for heat (except during defrost). A light frost is fine. A thick layer of ice covering the coil, ice on top of the unit, or a fan that won't spin means something isn't right. See heat pump iced up or frozen.
Check the filter and the indoor coil airflow
A heat pump needs a lot of airflow. A clogged filter or closed-off vents make it run hot inside the air handler, and some systems will shut down or blow weak, lukewarm air. Change the filter and open the supply vents and returns.
Watch what the auxiliary heat does
Most heat pumps here have electric heat strips as backup ("Aux" on the thermostat). They help on very cold mornings and during defrost. If the thermostat shows Aux for a little while on a cold morning, that's normal. If it shows Emergency Heat or Aux runs all day, read what emergency heat means, because that can get expensive.
Check both breakers
A heat pump usually has one breaker for the outdoor unit and one for the indoor air handler. If the outdoor breaker is tripped, the indoor blower may still run, pushing unheated air (or only running on heat strips). Reset a tripped breaker once. If it trips again, leave it off and call.
Why heat pump air feels different
A heat pump doesn't burn anything. In winter it pulls heat out of the outdoor air (even cold air has heat in it) and moves it inside using refrigerant, the same way your AC moves heat out of the house in summer. Because it's moving heat instead of making it, it's very efficient, but the air it delivers is gentler than a gas furnace: longer run times and warm air instead of hot bursts.
That's why heat pumps can seem to run constantly on cold days. Long, steady cycles are normal and usually more efficient than short ones. What matters is whether the house is holding the temperature you set. If it is, the system is doing its job, even if the air at the vent doesn't feel hot.
When the temperature drops into the 20s, most heat pumps start to lose capacity, and that's where the backup heat strips come in. In the Triangle we only get a handful of mornings that cold each winter, so a heat pump that leans on backup heat every day in 40-degree weather has a problem worth fixing: low refrigerant, a failing outdoor fan or compressor, a stuck reversing valve, or a defrost control that isn't working right.
A quick seasonal check (refrigerant charge, outdoor coil, defrost sensor and backup heat) keeps a heat pump heating efficiently and keeps your power bill where it should be.
When to call a pro
- The air never gets warmer than the room while the thermostat is calling for heat.
- The outdoor unit isn't running, or the fan is stopped while the unit is buzzing.
- The outdoor coil stays covered in solid ice through several defrost cycles.
- The thermostat runs on Aux or Emergency Heat all the time, or your electric bill jumps.
- A breaker keeps tripping.
Still not working?
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Is it normal for my heat pump to blow cool air?
It's normal for heat pump air to feel cooler than a gas furnace, and for it to blow cooler for a few minutes during defrost. It's not normal for the air to stay at room temperature while the house gets colder.
Why is there steam coming from my heat pump?
That's usually the defrost cycle melting frost off the outdoor coil. It looks alarming but it's normal, and it should stop after several minutes.
Should I switch to emergency heat?
Only if the heat pump itself has failed and you're waiting on a repair. Emergency heat runs only the electric heat strips, which use much more electricity. Call to get the heat pump fixed rather than running emergency heat for days.
Do heat pumps work in North Carolina winters?
Yes. Our winters are mild enough that a properly sized, working heat pump handles most days on its own, with backup heat covering the coldest mornings.
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