Heat Pump Blowing Cold Air — Normal Defrost, a Thermostat Setting, or a Real Repair

The first cold morning of the year is when we get this call most. Half the time nothing is broken. The other half, a part that sat unused since March didn't wake up. Here's how to tell which one you have before you pay anyone.

First: is it actually cold, or just not hot?

A heat pump doesn't make air as hot as a gas furnace. Furnace air comes out of the register around 120° to 140°. Heat pump air comes out around 90° to 100°. Your hand is about 92°, so heat pump air can feel cool to the touch even when it's working perfectly.

The real test: hold a kitchen thermometer in a supply register for a minute. If it reads 25° or more above the room temperature, the heat pump is doing its job. If it reads within a few degrees of the room, something is wrong.

Normal cold air: the defrost cycle

On a cold morning, frost builds on the outdoor coil. Every 30 to 90 minutes the system runs a defrost cycle: it flips into cooling mode for a few minutes to melt it. You may hear a whoosh, see steam rising off the outdoor unit, and feel cool air inside for 2 to 10 minutes. Most systems turn on the heat strips during defrost to cover it, but not all do.

That's normal. Cold air that lasts longer than about 10 minutes is not.

Two checks before you call

1. Thermostat set to Heat, not Auto or Cool, and the fan on Auto. With the fan set to On, the blower keeps running between heating cycles and pushes room-temperature air that feels cold. This one fix ends a lot of service calls.
2. Look at the outdoor unit. Is the big fan on top spinning and the unit humming? If it's silent while the indoor blower runs, the outdoor unit isn't running at all: check the breaker and the disconnect box next to the unit. If it's buried in solid ice, skip to the causes below.

If you just had a new thermostat installed and it's blowing cold on Heat and cool on Cool, the O/B reversing valve setting is backwards. It's a one-menu fix in the thermostat's installer settings, and the most common problem we see after a DIY thermostat swap.

Real causes, most to least common

• Reversing valve stuck or its solenoid failed. The valve is what switches the unit from cooling to heating. It sat in cooling position since March. If the solenoid coil burned out or the valve sticks, the unit cools all winter. A solenoid coil is a cheap fix; a stuck valve is sealed-system work.
• Bent or crushed outdoor coil fins. Hail or yard tools flatten the fins, and the heat pump can't absorb enough heat from the outside air. It runs longer and leans on the heat strips. Bent fins explained.
• Defrost control board or sensor failed. The system either can't come out of defrost (stuck blowing cold) or never goes into it (outdoor coil ices solid and heat output drops to nothing).
• Low refrigerant from a leak. A heat pump low on charge loses heat output first, because heating asks more of the refrigerant than cooling does. If it's low, it's leaking. We find the leak; we don't top it off.
• Outdoor fan motor or capacitor. No fan, no heat exchange, and the unit trips off on high pressure or ices over.
• Heat strips or sequencer dead. The strips cover defrost and very cold mornings. If they're out, defrost cycles feel freezing, and you'll notice it most before sunrise.
• Compressor not running. The indoor fan blows, the outdoor unit hums or clicks but the compressor doesn't start. Usually the capacitor first, the compressor second.

What the repairs cost

Installed, parts and labor, East Valley residential. These match our Tempe heat pump repair pricing.

Anything over $1,000 on a system past 12 years gets both numbers on one sheet: the repair and the replacement. The tech doesn't earn commission on either.

Aux heat and emergency heat: what they mean

Most Valley heat pumps have electric heat strips in the air handler as backup. Your thermostat calls them two different names.

• Aux heat comes on by itself. The thermostat adds the strips when the heat pump can't keep up alone: during a defrost cycle, on the coldest mornings, or when you raise the setpoint several degrees at once. A few minutes of "Aux" on a cold morning is normal.
• Emergency heat (EM Heat) is a switch you turn on. It shuts the heat pump off and runs the strips only. Use it only when the outdoor unit is broken: iced solid, not running, or damaged by hail. Use it until the repair, then switch back.

Strips cost three to four times more per hour than the heat pump. Leaving EM Heat on, or running on Aux all day, shows up on the next SRP or APS bill.

When Aux heat means a problem:

• "Aux" stays on for hours on a mild day, above about 40° outside.
• "Aux" comes on every time the system runs, not just on cold mornings.
• The December bill jumps and the thermostat settings didn't change.

Any of those usually means the heat pump itself isn't producing heat (reversing valve, low refrigerant, defrost board, from the causes above) and the strips are covering for it. The house feels fine and the bill doesn't.

Two things that cause Aux heat with nothing broken:

1. Big setbacks. Dropping to 62° overnight and jumping back to 70° at 6 a.m. forces the strips on to close the gap. Keep setbacks to 2° or 3°, or use the thermostat's heat pump "smart recovery" setting.
2. Thermostat staging. Many smart thermostats have an "aux heat lockout" or "balance point" setting. Set too high, the strips run on 50° days. A heat-pump-aware setting locks the strips out above about 35° to 40° outside, except for defrost.

When the strips don't work at all, defrost cycles feel freezing and the house can't keep up before sunrise. That's a heat strip or sequencer, about $495. In Tempe? Tempe heating repair covers heat pumps, furnaces and heat strips.

Why it happens in October and November

Your heat pump ran 2,500-plus hours this summer at 110°. The reversing valve, defrost board, and heat strips then sat unused until the first 40° morning asked them to work. A fall tune-up runs the system in heat mode, forces a defrost cycle, tests each heat strip, and checks the reversing valve before you need it. $59.95 per system, the same tune-up we run in spring. More on heating repair and maintenance.