Heat pumps are quickly becoming an increasingly popular option for homes with central cooling and heating. Although heat pumps have long been popular in warmer climates, the issue was always that most units weren’t capable of heating efficiently or effectively in below-freezing temperatures. However, most modern heat pumps are far better and can still provide effective heat even in temperatures as low as -20 to -30 degrees.

The heat transfer process that heat pumps use to add or remove heat from the home means that moisture in the air constantly condenses on either the indoor coil or the outdoor coil depending on whether the unit is cooling or heating. The fact that cold refrigerant is constantly flowing through one of these coils whenever the unit is running means that there is always a chance that ice could start to form on the coil and the system freeze up. If you do ever find that your heat pump is frozen, here are the potential reasons why it happened and also what you should do.

How to Know if Your Heat Pump Is Frozen

A frozen heat pump is usually a problem you’ll notice quite quickly as it will prevent your heating or cooling from working properly. When a heat pump is cooling, the indoor evaporator coil can potentially freeze for a number of reasons, and this will quickly result in hot air blowing out of your vents. In the winter, the coil in the outdoor unit can start to freeze quite often whenever the outdoor temperature is colder.

If your system does ever blow hot air when your heat pump is cooling, you will need to open up your air handler and see if the evaporator coil has ice on it. If the system starts blowing cold air in the winter, you’ll need to shut it off and then look inside the outdoor unit to see if there is ice on the coil or the refrigerant lines.

What to Do When Your Indoor Evaporator Coil Is Frozen

The very first thing you should do if the indoor coil is frozen is to immediately turn off your HVAC system at the thermostat. Although you’ll want to turn the heat pump itself off, you may want to turn the fan setting on your thermostat to “on” as this will circulate warm air through the air handler and help the coil to thaw more quickly.

The main reason why you should immediately shut the heat pump off is that serious damage can occur if the unit continues to run when the indoor coil is frozen. If the unit does run when frozen, it can lead to liquid refrigerant entering the unit’s compressor and this can damage the compressor motor or potentially cause it to burn out.

Once you’ve shut the system off, you will then need to wait however long it takes for the ice to melt and the coil to completely thaw. If the indoor coil freezes, it may take up to a few hours to thaw. As it thaws, it could overwhelm the condensate drain system and cause water to start leaking out of the air handler. For this reason, we would always recommend placing towels on the floor around the air handler to prevent possible water damage.

What to Do if Your Outdoor Heat Pump Unit Freezes

It is normal for your outdoor heat pump to occasionally start to freeze up during the winter, especially in places like Wisconsin where the temperature is often below freezing. The reason this happens is that the refrigerant flowing through the outdoor coil is typically at least 10 degrees cooler than the air temperature, and this temperature difference leads to moisture in the air condensing on the coil. If the outdoor temperature is below freezing, this condensation will lead to frost starting to build up.

A heat pump that temporarily starts to freeze is often nothing you need to worry about as the unit will eventually switch over to the defrost cycle to melt any frost that has formed on the coil. If you ever notice that the heat pump’s internal fan isn’t running when the unit should be on, it usually means that it is currently defrosting as this fan always shut off during the defrost cycle. In this case, we would always recommend waiting for three or four hours to give the unit plenty of time to defrost on its own.

If your heat pump still is still frozen after four hours, then it might be time to have a technician inspect it. However, there may still be a few alternatives that you can try first depending on the specific model you have. Some units allow you to manually turn on the defrost cycle at your thermostat, and this is definitely something to try if your unit has this feature. There are also units where you can set it so that only the fan in the outdoor unit will run, and this can also be worth trying as the fan will blow air over the coil and help to thaw the unit out more quickly. If your unit doesn’t have either of these features and your heat pump won’t defrost, you will usually need to have a technician determine what the issue is.

Possible Reasons Your Heat Pump Won’t Defrost

When defrosting, a heat pump switches modes and reverses so that hot refrigerant flows through the outdoor coil to melt any ice that has started to form. The most common reason that a heat pump will fail to defrost is that its reversing valve is stuck in one position or broken, which prevents the unit from being able to switch so that the refrigerant flows in the opposite direction.

The outdoor unit can also freeze if the coil is too dirty or the sides of the unit are clogged with debris. If the unit is clogged, the fan won’t be able to circulate much air through it. As a result, the refrigerant may remain cold enough that the coil starts to freeze since there isn’t enough air blowing over the coil. If the unit is clogged or the coil is too dirty, you will need to have a technician clean it as the only way to do so is to partially dismantle it and remove the fan assembly.

All heat pumps have an internal temperature sensor that measures the temperature of the coil and signals the unit to defrost whenever the coil gets colder than 32 degrees. In some cases, the problem may be that the sensor is positioned where it receives direct sunlight. The heat from the sun can prevent the sensor from working properly and lead to the unit not defrosting when it starts to freeze up or the defrost cycle not running long enough for the unit to fully thaw out. In this case, all you may need to do to get your unit working again is have a technician relocate the sensor.

At Healthy Home Heating & Cooling LLC, our technicians can help if you have a frozen heat pump or need any other cooling or heating repairs or maintenance services. We also install and service most other HVAC equipment including furnaces, air conditioners, ductless mini-splits, boilers and indoor air quality units. If you’re dealing with a frozen heat pump or need any other HVAC service in De Pere, give us a call today.

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