Ice on a heat pump means different things in different seasons. A thin layer of frost on the outdoor coil on a cold morning is normal, and the unit is designed to clear it. A unit encased in ice, or a heat pump freezing up in July, is a problem. Here is how to tell which one you are looking at.
Winter: some frost is normal
In heating mode, the outdoor coil runs colder than the outside air so it can pull heat from it. When the air is cool and damp, which describes many Sacramento winter mornings, moisture freezes on the coil. The heat pump handles this with a defrost cycle. It briefly reverses, warms the outdoor coil, and melts the frost. You may see steam and water pouring off the unit and hear a whoosh when it switches.
Normal looks like:
- A light, even frost on the coil that comes and goes
- Periodic defrost cycles of a few minutes, then back to heating
- Steam and dripping water during defrost
Winter: frozen solid is not normal
If the coil is covered in thick ice, the top or sides of the unit are iced over, or the ice does not clear after defrost, something is preventing the system from defrosting properly. Common causes:
- A failed defrost control board or sensor. The unit never enters defrost, or ends it too early.
- A stuck reversing valve. The unit cannot switch into defrost.
- A failed outdoor fan motor. Without airflow the coil ices quickly.
- Low refrigerant. Lower pressure means a colder coil and more ice. Low charge means a leak.
- Water falling on the unit. A roof without gutters, a leaking downspout or a sprinkler dumping water onto the unit freezes on contact. Look up before you call.
- Snow or debris blocking airflow, rare in the valley but common in the foothills above Folsom and El Dorado Hills.
Summer: a frozen heat pump is always a problem
In cooling mode, a heat pump works exactly like an air conditioner, and the indoor coil or the refrigerant line at the outdoor unit can freeze. That is never normal. The causes are the same as a frozen AC:
- A clogged filter, the most common cause
- Closed or blocked vents and furniture over the return
- A dirty indoor coil
- A failing blower motor
- Low refrigerant from a leak
- Running cooling when it is cool outside, below about 60 degrees
Our article on why an AC freezes up covers the summer case in detail.
What to do right now
- Do not chip or pour hot water on the ice. The fins and coil are thin and easy to puncture. A punctured coil is an expensive refrigerant leak.
- In winter, if your thermostat allows it, switch to emergency heat to keep the house warm while the outdoor unit is off. Then turn the heat pump off at the thermostat so it stops fighting the ice.
- In summer, turn cooling off and set the fan to ON. The coil thaws in a few hours.
- Check the filter, and check for water falling on the outdoor unit.
Emergency heat runs on electric resistance heat and costs more to operate, so use it as a bridge until the unit is repaired, not as a new normal.
Why not to ignore it
A heat pump that runs iced up loses most of its heating ability, runs longer, leans on expensive backup heat, and can send liquid refrigerant back to the compressor. That last one is how a defrost sensor repair turns into a compressor replacement.
What we do
We check the defrost board, sensors and reversing valve, test the outdoor fan, verify the refrigerant charge and look at airflow indoors. Our diagnostic is a flat $69 and it is credited toward the repair. Related: heat pump not heating, heat pump short cycling, and heat pump services.
Frequently asked questions
Is it normal for a heat pump to freeze up in winter?
Light frost on the outdoor coil is normal and the unit clears it with defrost cycles. Thick ice that covers the coil or does not melt after defrost is not normal.
Why is my heat pump freezing up in summer?
In cooling mode a heat pump works like an AC, and freezing usually means a clogged filter, restricted airflow, a dirty coil, a failing blower or low refrigerant.
How do I defrost my heat pump?
Do not chip or pour hot water on it. Turn the heat pump off at the thermostat, use emergency heat in winter if available, and let the ice melt. Then have the cause checked.
What does emergency heat do?
It runs the electric backup heat strips instead of the heat pump. It keeps the house warm while the outdoor unit is off, but it costs more to run.
Can water from the roof freeze my heat pump?
Yes. Water dripping from a roof without gutters or a leaking downspout onto the outdoor unit can freeze on contact and ice up the unit.
How often should a heat pump defrost?
It depends on outdoor temperature and humidity. On cold, damp mornings it may defrost periodically for a few minutes each time. Constant defrosting or no defrosting at all both need checking.