What Causes a Heat Pump to Freeze Up?

In the winter, cold water, insufficient airflow, a stuck reversing valve, and defrost mode failure can cause the heat pump to freeze up. In the summer, you will see ice on the evaporator coil, in case the system is low on refrigerant, has a dirty coil, and has poor airflow.

Let’s take a closer look at all these issues and figure out what exactly you should do to prevent your heat pump from freezing up.

What Causes a Heat Pump to Freeze Up in Winter?

Dripping Water

Rain, snow, or water vapour can fall on the unit and create an icy layer on top of the heat pump.

This is absolutely normal and these heating units were designed to withstand freezing temperatures.

Hint: water dripping from your gutters can also become a problem. So, make sure to take care of the leaks, if there are any.

The heat pump’s defrost mode should be able to deal with this problem. If you ever feel like you want to manually ‘help’ the unit, please do not use any sharp objects to remove the ice from the heat pump – those can easily damage the coils and the fins.

You can try removing the ice with a garden hose. Also, keep leaves and snow away from the unit (including underneath the heat pump).

Insufficient Airflow

Airflow allows the unit to capture heat from the outside air and moves the heat inside the house. But if there is not enough air passing through the coils, the evaporator coil will cool to a very low temperature and, consequently, cause the heat pump to freeze up.

There are quite a few things that can restrict the airflow in your unit:

  • Branches, leaves, snow, and debris can build up around the heat pump. You have to make sure that not only the actual unit, but the area around the heat pump is clear of any objects.
  • A clogged air filter might be to blame. 
  • The supply and return ducts may be blocked by furniture, for example.
  • There might also be an obstruction in the ductwork.

Read: Why Does My Furnace Filter Get Dirty So Quickly?

Slab Settling

A lot of heat pumps are placed on concrete slabs.

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You shouldn’t put the unit directly on the ground as insects can easily make their way right into the unit. Moreover, such slabs simply make your backyard (or any other place where you have decided to place the unit) look a bit neater.

The problem with the slab is that over time it can settle into the ground or even tip over. This can restrict the airflow and block the drainage path.

In such a case, you would have to level the sinking slab.

Issues with the Compressor

The compressor is, basically, the heart of the heat pump as it makes the refrigerant travel through the system by changing the pressure.

Of course, if the unit is not able to extract heat, then it will freeze up in winter and fail to function.

These things can cause a compressor to go bad:

  • A motor fail

The motor is the part that keeps the compressor going. If you can’t hear the compressor whenever you turn the unit on, then the chances are high that the motor has failed.

  • A compressor can wear down

This usually happens because of poor maintenance or dirt. This will cause the outdoor fan to start, but everything else won’t be working.

The compressor’s bearing and contacts can also go bad. In such a case, the heat pump won’t be able to start at all.

Read: Why Window Air Conditioner Compressor Not Turning On?

A Stuck Reversing Valve

The reversing valve is one of the elements in the system that makes the unit switch from cooling to heating and back again.

If the valve is stuck, your heat pump won’t be able to switch to heating mode (or cooling during the defrost mode).

In case the reversing valve is physically stuck, you can try gently tapping something on either side of the valve.

The reversing valve might not be energized or leaking, but it is better to have a professional check these things.

Read: Why Heat Pump Heats But Does Not Cool?

Defrost Mode Failure

Your heat pump is going to go into defrost mode once the temperature of the outdoor coil gets too low in comparison to the outside temperature (this can happen at 30 F or even 60 F).

This mode can also be initiated by high pressure – when the coil has ice on it, the space between the elements will become tight and this will lead to a higher pressure.

When this happens, the heat pump quickly switches to cooling mode, in order to send the hot refrigerant to the outdoor unit and melt the ice.

Unfortunately, defrost mode does not always work. Here are a few things that can go wrong:

  • Water dripping on the unit – in such a case the ice build-up accelerates.
  • Lack of drainage – if the heat pump is not able to get rid of the melted ice, the water will get stored inside the unit, and it will freeze.
  • Wrong installation – if the unit is positioned too low in the ground or is tilted, the defrost mode might not be able to work correctly.
  • All the reasons mentioned above, like a stuck reversing valve, insufficient airflow, and so on, can become the root cause of the defrost mode failure.

Read: What Are HVAC Most Common Failures?

What Causes a Heat Pump to Freeze Up in Summer?

Did you know that your heat pump can freeze up not only in winter but also during the summer? You might find ice on the evaporator coil or refrigerant lines even on the warmest days.

What are the reasons for that?

A Dirty Coil

The more dust and debris build up on the coil, the more insulated it becomes from the warm air that is passing through the system. Basically, a layer of dirt will keep the coil cool and might even cause it to freeze up.

Poor Airflow

We already know that insufficient airflow can cause quite a lot of damage to the heat pump. If there is not enough warm air traveling through the coil, the element can easily freeze up, even if it’s boiling hot outside.

Low Refrigerant Levels

The lower the refrigerant level, the lower the pressure in the system. Moreover, the remaining amount of refrigerant won’t be able to absorb heat as efficiently.

All these things will cause the evaporator coil to freeze.

Fact: the most commonly used refrigerants boil at temperatures between 40 and 50 F.

What Do You Do When Your Heat Pump Freezes Up?

You shouldn’t try to manually get rid of the ice as the chances are high that the problem is not the ice itself, but a malfunction of the system.

If the unit has a low refrigerant level, a dirty coil, or a reversing valve that has gone bad, then you should definitely call a professional to deal with the problem.

Fortunately, there are a few things that you can do to prevent such a situation from occurring in the first place.

Change the Air Filter

Set a reminder on your phone that is going to tell you to change your air filter.

The frequency would depend on the type of filter, on how often you use the heat pump, and a few other factors, but, in general, experts recommend getting a new air filter every 1-3 months.

Make Sure the Vents Aren’t Blocked

If you have a ducted heat pump system, you have to make sure that there are at least 10 inches between the vents and your furniture.

Blocking a few vents on purpose might also not be the best decision as it will increase the pressure inside the system and may potentially lead to a breakdown.

Relevel the Heat Pump Slab

Simply work a longboard under one end of the slab, lift it up, and pack enough sand under it. You would have to do the same on the other end. 

Tip: lift the slab as slowly as you can to avoid cracking the concrete pad.

You can also consider replacing the slab with a plastic pad.

Don’t forget to make sure that the unit is level. 

When the unit is resting on an even surface, the refrigerant is able to travel smoothly. But if the heat pump is tilted, the substance can get trapped in the tubing.

Keep Debris Away from the Heat Pump

Regularly weed or mow the area around the heat pump, clear away any snow that builds up around the unit, and clean any dirt with a garden hose if needed.