Quick answer
Most of the time it is not cold air, it is heat pump air. A heat pump delivers supply air around 30–40°C, well below the 50–60°C a gas furnace pushes and close to your skin's surface temperature, so it can feel cool on your hand while still heating the house. Air that stays cold while the house loses temperature points to a defrost cycle, a thermostat set wrong, low refrigerant or a stuck reversing valve.
When a heat pump is blowing cold air in heating mode, the first thing to check is whether the house is actually losing temperature. If the thermostat is holding its setpoint, you are feeling the normal character of heat pump air. If the rooms are cooling off, something is wrong, and the causes below run from the harmless ones to the ones that need a technician.
Lukewarm supply air is normal for a heat pump
A gas furnace heats air to roughly 50–60°C at the register. A heat pump heats it to something closer to 30–40°C. The surface of your skin sits at roughly 32–34°C, so moving air at the low end of that range feels neutral or even cool on your hand even though it is carrying plenty of heat into the room. Homeowners switching from a furnace in a 1990s Millcroft two-storey notice this on the first cold week and then stop noticing.
Long duct runs make it more obvious. Supply air travelling through an unheated garage bulkhead, or through the crawl space of a 1970s side-split, loses a few degrees before it reaches the far bedroom register, and 30°C air that arrives at 26°C feels distinctly cool.
A defrost cycle blows cool air for a few minutes
Every heat pump periodically runs in reverse to melt frost off the outdoor coil. During those minutes the indoor coil is absorbing heat instead of releasing it, and the system either runs backup heat to cover the gap or lets the register go cool. You will see the outdoor fan stop and a cloud of steam rise off the unit. If the cool air ends within about 10 minutes and the house holds temperature, nothing is wrong. Our explainer on the heat pump defrost cycle covers how often it should happen.
Thermostat settings that cause cold air
- Mode set to Cool, or the fan set to On. With the fan running continuously, the blower moves room-temperature air between calls for heat, and room-temperature air out of a register feels cold.
- O/B terminal configured wrong. Most brands energize the reversing valve in cooling (the O setting); Rheem and Ruud energize it in heating (the B setting). A thermostat set the wrong way cools the house when it should heat it, and this shows up often after a do-it-yourself smart thermostat swap.
- Compressor lockout set too high. If the thermostat is told to shut the heat pump off at -5°C and the backup stage is not wired or configured, you get moving air with nothing heating it.
- Emergency heat selected on a system that has no backup connected.
Our guide to heat pump thermostat settings lists the specific menus to check on the common Ecobee, Honeywell and Nest models.
Faults that need a technician
- Low refrigerant from a leak. The outdoor unit runs but pulls little heat from the air; supply air is barely warmer than the room and the outdoor coil may ice over. The fix is finding and repairing the leak, then recharging — typically $300–$800+ — not a top-up.
- Stuck reversing valve. The valve that switches between heating and cooling sticks midway or in the cooling position, so the system cools the house in January. You hear the compressor running and the air stays cold no matter how long it runs.
- Outdoor unit not running at all. A tripped breaker, a failed capacitor or a burnt contactor leaves only the indoor blower moving. A capacitor or contactor is one of the cheaper repairs at $150–$350.
- Failed outdoor fan motor, or a coil buried in drifted snow. In heating mode the outdoor coil is the one absorbing heat, so either one starves it of air, ices it over and eventually trips the unit on low pressure — leaving the indoor blower circulating unheated air.
What to check before you call
- Thermostat shows Heat, not Cool, and the fan is on Auto.
- The breaker for the outdoor unit and the disconnect beside it are both on.
- The outdoor unit is running, the fan is turning, and the coil is clear of snow and ice.
- The filter is not clogged; a heat pump is more sensitive to low airflow than a furnace is.
- You have waited 15 minutes to rule out a defrost cycle.
If the outdoor unit is running and the house is still losing temperature, switch the thermostat to emergency heat if you have a backup stage and book heat pump repair. ZK Mechanical diagnoses heat pumps across Burlington, Oakville and Hamilton for a $90–$150 visit that is credited toward the repair — book online or call (647) 801-1252, with no surcharge for evenings or weekends.