Heat pump blowing cool air while the thermostat says heat

Is cool air from a heat pump normal?

Often, yes, and the first reason has nothing to do with a fault. A heat pump does not make air as hot as a furnace does. Trane says heat pump air is usually between 85 and 93°F, and that it feels cool to us because body temperature is 98.6°F. Carrier gives a similar range of about 85 to 100°F, against roughly 120 to 140°F from a fuel-burning furnace.

If you are used to furnace heat, the air from a heat pump can feel lukewarm at the register even while it is heating the house exactly as designed. The test that matters is whether the room temperature climbs toward the setpoint, not how the air feels on your hand.

What is the defrost cycle, and why does it blow cool air?

In cold, damp weather, frost builds up on the heat pump's outdoor coil. To clear it, the heat pump briefly runs its cooling cycle to warm that coil. Trane describes this as putting the heat pump into the cooling cycle for a few minutes, after which it goes back to heating.

What you feel indoors depends on your backup heat:

• With electric backup heat, which Trane says most homeowners have, the thermostat brings the backup heat on during defrost and the house keeps heating normally. You may see auxiliary heat (AUX) on the screen for those minutes.
• Without backup heat, or with a dual fuel system where the furnace and heat pump do not run together, Trane says you may notice cool air from the vents during defrost, and calls that perfectly normal.

Defrost is short. Trane's demand defrost cycles usually run 3 to 5 minutes, and Carrier puts a typical cycle at about 5 to 10 minutes. Trane's troubleshooting page adds that if your thermostat reports Defrost mode, cool air is expected, and the answer is to wait for heating to resume. Defrost does not need freezing weather: Trane says it can easily happen in the low 40s outside.

Which thermostat settings make the air feel cold?

Three settings on the thermostat can produce cool air while you think it is heating:

• The mode is Cool, not Heat. Both Trane and Carrier put this first. On Auto, the thermostat may also have switched to cooling.
• The fan is set to On. Trane says that with the fan on, the system blows air even when the heat pump is not actively heating. Between heating cycles that air is room temperature, which feels cool. Set the fan to Auto; fan On vs Auto explains the difference. After you change the mode or fan, allow a few minutes: a heat pump may be held off by compressor protection before it starts heating.
• Heating and cooling are reversed. A heat pump switches between heating and cooling with a reversing valve, and the thermostat has to be told which way that valve works, a choice labeled O or B. Google says that if a Nest blows cool air in heat mode, or warm air in cool mode, the heat pump orientation setting is the thing to change. On a Honeywell Home T6 Pro this is installer option 218, "Reversing Valve O/B", which comes set to O. It is part of the system type setup, and on most thermostats it is a job for the installer.

Google also describes a smaller version of the same thing on Nest thermostats. If the heat pump terminal is set to activate on each cycle when the equipment expects seasonal activation, you may feel a brief burst of cold air at the end of a heating cycle, or hear a loud whoosh.

What can you check on the thermostat?

Is the mode Heat, with the fan on Auto and the setpoint above the room temperature?

• Yes

  Does the cool air stop within about 10 minutes, or does the room temperature still rise over the next hour?

  • Yes

    That fits normal lukewarm heat pump air or a defrost cycle. Nothing needs changing.

  • No

    The thermostat is set correctly but the system is not heating. If heating and cooling seem reversed, ask your installer to check the O/B setting. Otherwise check the signs below and call a licensed HVAC technician.

• No

  Set the mode to Heat and the fan to Auto, raise the setpoint a couple of degrees above the room temperature, and wait a few minutes.

If the screen shows AUX during these cool spells, see why a thermostat shows AUX. Trane's own line for when to call is clear: if the heat pump seems to be defrosting constantly, or the cycles keep getting longer, call for service.