Heat Pump Blowing Cold Air? Defrost, Aux Heat or a Problem

It's 20°F outside, the heat pump is running, and the register air feels barely warm, or cold for ten minutes before going back to lukewarm. If you grew up with a gas furnace, that feels wrong. Some of it is simply how heat pumps work; some of it means a part has failed. Here's how to tell.

Quick diagnosis

Why heat pump air feels cool

A gas furnace burns fuel and sends out air that is far hotter than the room. A heat pump moves heat from outdoors to indoors and delivers a larger volume of gentler warm air for longer stretches. Carrier gives typical heat pump supply temperatures of about 85–100°F. Your skin is in the low 90s, so air at the lower end of that range can feel cool against a wet or warm hand even though it is well above room temperature. As it gets colder outside, many heat pumps deliver air toward the bottom of that range and run longer cycles to keep up.

The better test is the house, not your hand. If rooms hold the thermostat setting, the system is doing its job. If the indoor temperature keeps slipping below the setpoint, keep reading.

Defrost, aux heat and emergency heat

Defrost. When the outdoor coil collects frost, the heat pump switches into cooling mode for a few minutes to warm the coil and melt it. Carrier puts a typical cycle at about 5 to 10 minutes; Lennox says 5 to 15, and notes that some units defrost on demand while others run on a 30, 60 or 90 minute timer. You may hear a whoosh, see the outdoor fan stop and watch steam rise from the unit. All of that is normal.

Auxiliary heat. Trane describes aux heat as supplementary heat the thermostat brings on automatically, usually electric heat strips inside the air handler, or a furnace. It runs during defrost to temper the air, when the outdoor temperature drops below a set point, and when you raise the thermostat several degrees at once. If you feel cold air during defrost, the aux heat probably isn't coming on.

Emergency heat. This is a manual mode. Trane explains that switching to it shuts the heat pump off and makes the backup the only heat source. Use it when the heat pump has failed, not to "help" a working system on a cold night; electric strips cost much more to run, and Carrier warns emergency heat can raise electric bills significantly. If you've turned it on, call for heat pump repair.

Step-by-step checks

1. Check the thermostat

The mode should be Heat (or Auto) and the fan Auto. With the fan on On, the blower keeps pushing air between heating cycles, and that air feels cool. Make sure it isn't in Cool or Emergency Heat by mistake. A new thermostat installed with the wrong reversing-valve setting can run cooling when it calls for heat; that's a job for a technician or our thermostat troubleshooting service.

2. Measure instead of guessing

Hold a kitchen thermometer in the airflow at a register near the indoor unit for a few minutes once the system has been running a while. A reading in the 85–100°F range matches Carrier's typical heat pump numbers. A reading near room temperature for more than 15 minutes is a real problem.

3. Wait out a defrost

If the outdoor fan has stopped and the unit is steaming, give it 15 minutes. Normal air should return afterward.

4. Look at the outdoor unit

Is it running? Is it buried in snow, blocked by a drift, or sitting under a roof edge that drips onto it? Light frost that clears during defrost is normal. Heavy ice that stays is not. Clear snow from around the unit by hand or with a broom, and leave ice for a technician.

5. Check all the breakers

Heat pump systems with electric backup often have a separate breaker, or two, for the heat strips in the air handler, in addition to the breakers for the air handler and outdoor unit. A tripped strip-heat breaker is easy to miss because the heat pump still runs; you only notice when defrost blows cold or the house can't keep up. One reset is fine; a second trip means stop and call.

6. Replace a dirty filter

Low airflow makes the heat pump less effective and can trip safety controls. Lennox recommends changing filters every 1 to 3 months.

7. Go easy on big setbacks

Trane notes that raising the setpoint more than 3 to 4 degrees can bring on aux heat. Smaller overnight setbacks, or a thermostat with smart recovery, avoid leaning on costly backup heat every morning.

Problems that need a technician

• Stuck reversing valve. Carrier and Lennox both list it: the system cools when it should heat. Often cooling works fine in summer and heat never comes in winter, or the reverse.
• Low refrigerant. A leak reduces heating capacity and can make the outdoor coil ice up. A technician needs to find and fix the leak.
• Defrost board or sensor faults. The unit either never defrosts and ices over, or gets stuck in defrost and blows cool air.
• Failed heat strips, sequencers or relays. Backup heat doesn't come on even with the breaker on.
• Dual-fuel furnace not lighting. On a heat pump paired with a gas furnace, cold air in a cold snap can mean the furnace side has a problem. Our furnace blowing cold air guide covers that side.
• Duct leaks. Carrier estimates a typical duct system can lose 20–30% of its air to leaks; with cooler heat pump air, that loss is felt more.

Colorado specifics

Snow and the outdoor unit

Denver averages roughly 57 inches of snow per season, according to National Weather Service records. Snow that drifts around a heat pump blocks airflow and gives melt water from defrost nowhere to go, so it refreezes into the coil. Outdoor units here should sit on a stand or pad above typical snow depth and out of roof drip lines. If yours doesn't, that's worth fixing before the next storm.

Arctic outbreaks and older equipment

Carrier notes that older heat pumps can struggle below about 25–30°F, while modern cold-climate models keep running well below zero. During a Front Range arctic outbreak, an older single-stage heat pump will lean heavily on backup heat, which is expected. A cold-climate model that blows cold air in the same weather deserves a service call. For what to expect, read do heat pumps work in Denver winters?

Dual-fuel switchover

Many Denver-area homes pair a heat pump with a gas furnace. The installer sets an outdoor temperature where the furnace takes over. Set too low, you'll feel cooler air longer; set too high, you give up savings. Thin air at altitude also trims capacity, as our altitude guide explains. See our dual-fuel guide for how that balance point is chosen.

When to call

• Register air stays near room temperature for more than 15 minutes outside of defrost.
• The house keeps losing ground on the setpoint even with backup heat available.
• You've had to switch to emergency heat.
• The outdoor unit stays iced, won't run, or a breaker trips twice.

No heat during a deep freeze? Our emergency HVAC repair team can help. If the system is old and needs repeated repairs, our Denver HVAC system cost guide can help with the repair-or-replace decision. Check our service areas to confirm we cover your town.

Frequently asked questions

Should heat pump air feel this cool?

Yes, to a point. Heat pump air is typically about 85–100°F, which can feel cool on your hand, and it blows cooler for a few minutes during defrost. If the house holds its temperature, the system is working.

How long is a normal defrost?

Carrier puts it at about 5 to 10 minutes and Lennox at 5 to 15. The outdoor fan usually stops and you may see steam from the unit.

Why is my heat pump blowing cold air during defrost?

Backup heat normally warms the air during defrost. If the air is cold, the heat strips may be off on a tripped breaker, or a strip, relay or thermostat setting may have failed.

What's the difference between aux heat and emergency heat?

Aux heat comes on automatically to help the heat pump. Emergency heat is a manual setting that turns the heat pump off and runs only the backup, for use when the heat pump has failed.

Should I use emergency heat when it's very cold?

Not if the heat pump is working. Its aux heat will help automatically. Emergency heat costs more to run and is meant for a failed heat pump.

Why does my heat pump blow cold air all the time?

Check that the thermostat is on Heat with the fan on Auto. If it is, a stuck reversing valve, low refrigerant or a defrost control fault are likely, and a technician should diagnose it.