It’s one of the two most common calls we get when the first cold front reaches Volusia County. A homeowner looks out the window and sees a cloud rising off the top of their outdoor unit.
“We’ll get calls saying, ‘My outdoor unit is on fire. I see smoke coming out of the top of the unit.’ So we explain that it’s part of the defrost cycle on the unit, and it’s completely normal.”
Jason Healy, Operations Manager
Why a heat pump needs to defrost
A heat pump works in two directions. In summer it pulls heat and moisture out of your home and dumps it outside. In winter, a small part called the reversing valve flips the flow of refrigerant, so the system pulls heat from the outside air and moves it in.
That flip means the coil that normally sits inside, collecting moisture, is now effectively outside.
“With a heat pump, that little reversing valve switches the polarity of the refrigerant. Now that evaporator coil that was on the inside is on the outside. So the outdoor coil starts to condensate, because it’s doing the reverse. When it gets down into freezing temperatures, which is not too often in Florida, it’ll start to ice that coil over.”
Jason Healy
Ice on the coil blocks airflow and cuts the system’s ability to heat, so the heat pump is built to clear it automatically.
What happens during a defrost cycle
A sensor spots ice
A sensor on the outdoor unit detects that the coil is icing over and signals the defrost board.
The system briefly switches to cooling
For about 30 seconds, the heat pump reverses back into cooling mode, sending warm refrigerant to the outdoor coil to melt the ice. Your indoor blower keeps running, so you won’t notice much inside.
You hear a whoosh and see steam
The valve switching makes a whooshing, gasping sound. The melting ice turns to vapor and rises off the top of the unit, which is what looks like smoke.
Heating picks back up
Once the ice is gone, the system switches back to heat and everything returns to normal.
“The customer really doesn’t notice anything different from the inside, but they hear this gasping sound outside. If they look out the window, they’ll see what looks like smoke. All that is, is the ice melting off. It’s vapor going up into the air, and after that 30 seconds everything goes back to normal.”
Jason Healy
How we set it up: Most defrost boards can be set to check every 30, 60 or 90 minutes. Total Comfort sets them to 30 minutes, so ice gets cleared before it builds up and costs you heat.
Is it defrost, or a real problem?
| What you notice | Normal? | What to do |
|---|---|---|
| Steam rising from the outdoor unit on a cold, damp day | Yes | That’s defrost. It clears in about 30 seconds. |
| A whooshing or gasping sound from the outdoor unit | Yes | The reversing valve switching. Normal. |
| Light frost on the coil that comes and goes | Yes | The defrost cycle is doing its job. |
| The unit stays covered in thick ice for hours | No | Defrost may not be working. Call. |
| The outdoor unit stops running | No | Switch to Emergency Heat and call. |
| Dark smoke, or a burning or electrical smell | No | Turn the system off and call right away. |
Call us if
- Thick ice stays on the outdoor unit and doesn’t clear
- The outdoor unit stops running, or the house isn’t warming
- Your thermostat shows emergency heat and you didn’t set it
- You see dark smoke or smell burning
Keep your heat pump ready for cold nights
The reversing valve that makes defrost possible sits idle most of the year in Florida, and that’s when it’s most likely to stick. A maintenance visit at the start of winter tests the system in heat, including the switchover, before the first cold front arrives. Learn more about heat pump maintenance or our maintenance plans.
Heat not working at all? Start with our 6-step heating checklist.
Not sure what you’re seeing?
Flat $129 diagnostic. Technician generally there in under two hours.
Frequently asked questions
Why is smoke coming out of my heat pump?
It’s almost always steam from the defrost cycle. In cold, damp weather, ice forms on the outdoor coil. The system briefly reverses to melt it, and the melting ice rises as vapor for about 30 seconds.
Why does my heat pump make a whooshing sound?
That’s the reversing valve switching the system between heating and defrost. It’s a normal sound during a defrost cycle.
How often does a heat pump defrost?
Defrost boards can be set to 30, 60 or 90 minutes. Total Comfort sets them to 30 minutes. Each defrost lasts about 30 seconds, and only happens when the coil is icing up.
Is ice on my heat pump normal?
A light frost that clears during defrost is normal in cold weather. A unit that stays covered in thick ice is not. Have it checked.
Will my house get cold during defrost?
No. The cycle lasts about 30 seconds and your indoor blower keeps running, so most homeowners don’t notice it from inside.
