When your heat pump’s outdoor unit starts steaming like a kettle and the air inside goes lukewarm for a few minutes, the system is almost certainly fine. That’s defrost mode doing its job. Here’s what’s actually happening and how to tell normal from a real problem.
What Defrost Mode Actually Does
Heat pumps pull warmth from outdoor air, even in cold weather. As the refrigerant in the outdoor coil gets colder than the air around it, moisture condenses and freezes on the coil fins. A thin layer of frost is normal. A thick shell of ice is not. It blocks airflow and kills efficiency.
To clear the ice, the unit temporarily reverses itself. The reversing valve flips, hot refrigerant runs through the outdoor coil instead of the indoor one, and the ice melts off. You see steam because ice is melting fast. You feel cool air indoors because the system is briefly pushing heat outward. The outdoor fan usually shuts off during this cycle to let the coil heat up faster.
The whole thing takes roughly 2 to 10 minutes. Then the reversing valve flips back, the outdoor fan kicks on again, and the system returns to heating. Normal.
How Often Should It Happen
In cold, damp conditions (30s and 40s°F with high humidity), defrost cycles every 30 to 90 minutes are common. On a dry 45°F day, you might not see one all day. Most modern heat pumps use a combination of temperature sensors and a timer to decide when to run defrost. They don’t just run it on a fixed schedule.
If defrost is running every 10 to 15 minutes, or if it never seems to finish, that’s worth paying attention to.
When It’s Not Normal
Three things can go wrong with defrost, in rough order of how often I see them:
The outdoor coil is heavily iced over and stays that way. If you walk out and the unit is a solid block of ice between cycles, defrost isn’t clearing it. This usually means airflow is restricted (dirty coil, blocked by leaves or shrubs, low refrigerant), or the defrost cycle isn’t running long enough.
The defrost board or sensor has failed. The defrost control board reads signals from a temperature sensor on the outdoor coil and decides when to initiate and end the defrost cycle. If the sensor is reading wrong, defrost might never start, or it might start and not know when to stop. A stuck-on defrost cycle means you’re stuck blowing cool air indefinitely. That’s often how homeowners notice it: the house just won’t heat up.
The reversing valve is stuck or sluggish. The reversing valve is a solenoid-operated component that switches the refrigerant flow direction. It can stick partially, which means the system never fully commits to either heating or defrost. Symptoms: the unit seems to heat weakly all the time, or it goes into defrost and the indoor air never really warms back up afterward. Less common than a defrost board issue, but it’s a real failure mode, and the part costs more.
What a Tech Checks
When we get a “heat pump not heating” call, defrost is one of the first things to look at:
- Is the outdoor coil iced over, and is there enough clearance for airflow?
- What do refrigerant pressures show? Low charge causes excessive icing and won’t fix itself.
- Is the defrost sensor reading within spec? Each manufacturer publishes a resistance range at a given temperature, and a meter tells the story fast.
- Does the defrost cycle actually trigger and complete when forced?
- Does the reversing valve fully switch? Suction and discharge line temperatures confirm it one way or the other.
A bad defrost board gets replaced. A stuck reversing valve may need replacement too, which means recovering the refrigerant and brazing in a new valve. Both jobs require EPA Section 608 certification, recovery equipment, and familiarity with live electrical at the outdoor unit. None of that is homeowner territory, and getting it wrong means a bigger repair bill, not a smaller one.
What You Can Check Before Calling
A few things are worth looking at first:
Clear the area around the outdoor unit. Leaves, grass clippings, or anything blocking the coil panels can cause excess icing. Most manufacturers specify at least 12 to 24 inches of clearance on the sides. Check your unit’s installation manual if you’re not sure.
Check the air filter inside. A clogged filter reduces airflow across the indoor coil, which affects system pressures and makes icing worse outdoors.
Look at the outdoor unit on a cold morning. A little frost on the coil fins is fine. Ice climbing up the refrigerant lines or covering the top grille is not.
Don’t pour hot water on a frozen coil. It works temporarily, but if there’s an underlying cause, it’ll be back within the hour.
Call Us
If the unit is running but the house won’t get warm, defrost is cycling constantly, or the outdoor coil is staying iced through multiple cycles, it needs a tech, not a wait-and-see.
Getting it diagnosed early usually means a simpler fix. A coil that stays iced for days puts real stress on the compressor, and compressor replacement is a much bigger job than a defrost board.
We serve the Tri-Valley and East Bay. Book at adriumservice.com or give us a call. We’ll get you on the schedule fast, often same or next day when we can.