My AC Guy Says My Heat Pump Is Just in Defrost
Your heat is running.
Then suddenly:
The outdoor fan stops.
You hear a weird WHOOSH.
The air from the vents doesn’t feel as warm.
And when you look outside, your AC appears to be:
SMOKING.
You call your AC guy.
He says:
“It’s just in defrost.”
Yep.
He’s probably right.
Heat pumps do some pretty weird-looking stuff in the winter.
Why Does a Heat Pump Need to Defrost?
When your heat pump is heating the house, the outdoor coil gets cold.
Really cold.
Under the right outdoor temperature and humidity conditions, moisture can freeze onto that coil.
A little frost can be normal.
But if it continues accumulating, that ice begins restricting airflow through the outdoor coil.
So the heat pump periodically needs to get rid of it.
That’s the defrost cycle.
What Happens During Defrost?
For a short period, the heat pump essentially reverses operation so it can send hot refrigerant through the outdoor coil and melt the ice.
And that’s when homeowners start thinking:
Something has gone terribly wrong.
😂
Several strange things can happen.
“My Outdoor Unit Is Smoking!”
Probably not.
As the hot coil melts frost and ice, you can get a big cloud of water vapor or steam rising from the outdoor unit.
On a cold morning, it can look dramatic.
Very dramatic.
Like:
Should I call Klein Cooling or the fire department?
If it’s occurring during defrost and disappears shortly afterward?
That’s usually steam.
Not your heat pump catching fire.
“The Outdoor Fan Stopped!”
That can be normal during defrost too.
Many heat pumps stop the outdoor fan during the defrost cycle.
Why?
Because we’re trying to warm that outdoor coil.
Blowing cold outdoor air across it wouldn’t exactly help.
So:
Compressor running + outdoor fan stopped
doesn’t automatically mean the condenser fan motor failed when the system is in defrost.
Context matters.
“I Heard a Huge WHOOSH.”
Also potentially normal.
A heat pump uses a reversing valve to change the direction refrigerant flows through the system.
When the system enters or exits defrost, you may hear pressure equalization and refrigerant movement.
Sometimes:
WHOOSH.
Sometimes:
PSHHHHHH.
Sometimes:
What in God’s name was that?
Heat pumps aren’t always subtle.
“The Air From My Vents Got Cooler.”
Also possible.
During defrost, the refrigeration system temporarily isn’t operating in its normal heating mode.
Many systems use auxiliary electric heat during defrost to help prevent noticeably cold air from being delivered into the house.
But depending on the system and conditions, you may still notice the supply air doesn’t feel as warm for a short period.
Then defrost ends and normal heating resumes.
How Long Should This Last?
Usually we’re talking about a relatively short cycle.
The exact timing and operation depend on the equipment and conditions.
The system should complete defrost and return to normal heating.
If it stays in defrost, repeatedly goes into defrost, remains heavily iced over, or isn’t heating the house?
Now we may have a problem.
When Should I Actually Call Someone?
Call if:
- the outdoor unit remains heavily iced
- defrost doesn’t appear to complete
- the system isn’t heating the house
- breakers are tripping
- you smell actual electrical burning
- you see actual smoke rather than temporary steam
- something simply doesn’t return to normal
There’s a difference between:
Heat pump doing weird heat-pump things
and
Heat pump actually broken.
The Klein Answer
If your heat pump occasionally:
steams,
makes a WHOOSH,
stops the outdoor fan,
and blows slightly cooler air for a few minutes
during cold weather?
Your AC guy may be completely right.
It’s probably defrost.
The machine is temporarily warming the outdoor coil to remove accumulated frost and ice.
Then it switches back and keeps heating your house.
So yes:
Your outdoor unit may occasionally look like it’s smoking.
Your brand-new heat pump probably isn’t electing a new pope.
It’s just defrosting.