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Problem: My heat pump outdoor unit is frozen up in the wintertime. What causes this?

frozen heatpump

Problem: My heat pump outdoor unit is frozen up in the wintertime. What causes the outdoor unit to freeze up like this? Does this hurt the unit? Does this hurt my electric bill?

Answer: It is normal to have a thin coating of frost on the outside of a heat pump periodically, but never for long periods of time. If you are getting ice on the outside of your heat pump that is heavy and never goes away even during the defrost cycle then you have a problem that needs to be attended to. The answer to your question is Yes long periods of time with an iced-up heat pump can harm the compressor and fan motor in a heat pump unit. Yes, having a frozen-up heat pump will cause your electric bill to go way up and cause you to feel cold in your home. Having a frozen-up heat pump is not good and the problem needs to be fixed. It is very hard on the heat pump to heat your home with ice on the outdoor unit. The heat pump extracts heat from the outdoor air even in the wintertime and if it is covered in ice the heat pump can not extract any heat out of the air and you will get cool air blowing when the heat pump is on and warm air when your auxiliary heat, electric or gas kicks on. The heat pump that is frozen up with ice never goes off because the indoor thermostat is never satisfied.  When a heat pump is frozen the thermostat is set in a vicious energy-eating cycle because it is very hard to satisfy the thermostat (get your home warm enough) unless you are lucky enough to get a warm day. Most heat pump thermostats are two-stage thermostats. The first stage is the heat pump only stage that depends on the outdoor heat pump to produce heat to heat the home. If it is cold outside the heat pump can not remove enough heat from the outdoor air to satisfy the need for heating the home so the thermostat kicks into the 2nd stage which is usually some kind of auxiliary heat like electric heat, gas, oil, etc. The auxiliary heat will usually satisfy the 2nd stage heat on the thermostat, but then the thermostat kicks back down into 1st stage heat (the heat pump heat) where the heat pump can not satisfy the need for heat because it is frozen up. You get a constant run time where the heat pump hardly ever turns off thus causing strain on your heat pump and also causes your electric bill to be very high. Below we have several really good YouTube videos that show and discuss how a heat pump works. Another video shows how the defrost on a heat pump works. This YouTube video was made by HVAC School. Thanks so much to HVAC School for making this video!  We also have a really good YouTube video made by AC Service Tech LLC that shows and explains how a heat pump and an air handler work. Thanks so very much to AC Service Tech for making this excellent video! The last YouTube video explains how a reversing valve works. This just amazes me how someone could think up how to make heating, air conditioning, and heat pump equipment! When I first heard how air conditioners and heat pumps worked I was overwhelmed with a fascination with how this could work! I told myself this is the career path that I want to follow! Amazing! How could anyone be smart enough to come up with this? Here is another really good Heat Pump Not Defrosting Guide. Please email us anytime if you have any questions or want us to look up parts. Our email address is We Would Love to Help You Out and Earn Your Business! 

Here is a simple 4-minute video that does an excellent job in explaining how a heat pump works in both cooling and heating modes: 

Some things that are not good when a heat pump freezes up are:

  • This makes the homeowner feel cold and makes for a very high electric bill. The homeowner feels warm when the auxiliary heat is on, but cold when in heat pump mode.
  • This constant run time is very hard on the heat pump because it never gets a break from running. This shortens the life of the heat pump!
  • A heat pump compressor’s valves can be damaged if the liquid refrigerant is allowed to enter the compressor.  Many heat pump units have accumulators to prevent this from happening, but a frozen heat pump can overwhelm an accumulator and cause damage to the compressor.  New unit or compressor time and you do not want that!
  • I have seen some units that have been frozen so bad that the outdoor fan blade is stopped by the ice and this can burn up the outdoor fan motor!  New outdoor fan motor time! This can be expensive.
  • If you see that your heat pump is frozen I would recommend switching your thermostat to Emergency heat if your thermostat has this option. Emergency heat should cut the outdoor heat pump off and use only your Auxiliary backup heat for heating your home. This will save your heat pump from being damaged until you can get it fixed.

Things that can cause a heat pump to freeze up are:

  1. Low refrigerant level or refrigerant leaks. To determine this you would need to attach refrigerant manifold gauges and use the superheat or subcooling method to check the charge.
  2. A bad heat pump defrost control board. The defrost control board might be bad and not activating the defrost.  Please click here if you are interested in the defrost control boards that we sell.
  3. A bad heat pump thermostat or thermistor. Please click here if you would like to see the heat pump defrost thermostats that we sell. 
  4. A bad or sticking reversing valve. We have a really good YouTube video below that shows how a reversing valve works.
  5. A slow or dragging outdoor or indoor fan motor. Make sure the motor capacitors are in good condition. Please click here if you would like to see the capacitors that we sell.
  6. Any kind of restricted airflow on the outdoor unit like maybe a garbage bag has blown up against the outside of the unit. Maybe leaves or a dirty outdoor coil that has become partially stopped up with lint from the dryer vent.
  7. Broken gutters that allow water to drop down on the heat pump. Also, a heat pump where the pad sinks below grade that allows the heat pump to sit in water. Not good! The heat pump produces lots of water when it defrosts if the water does not have any place to go it accumulates inside the heat pump and causes ice to form inside the heat pump which is not good. Make sure that your heat pump is setting above ground level.

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