Houses covered in snow

Heat pumps are made to run in both winter and summer. Despite the outside temperatures dropping, a heat pump can still transfer heat from the ground or air when it’s below zero degrees. Using a compression system and electricity, the heat can be transferred to your home, providing hot water and warm radiators.

However, some air source heat pumps can begin to ice up during the winter, this is quite normal as the energy is taken out of the air the water vapour in the air freezes on contact with the cold surface.

What causes a heat pump to ice up?

Because your heat pump is located outside, it’s normal for it to get covered in snow and ice during winter and this isn't usually a problem. Snow resting on top of the unit isn’t anything to worry about. However, if you notice that the whole unit is covered in ice as well as the insides of the coil, there could be a problem that isn’t just related to the cold weather.

Heat pumps work using a refrigerant that absorbs the heat from the air, ground or water. Moisture that’s present in the air can freeze and cause ice to form around the unit. Heat pumps are either hung on an external wall or situated on a concrete slab. As the heat is taken out of the air water is left behind as a result of the condensation process, this happens when the heat pump evaporator is colder than the air around it, causing condensation (or water) to form. In colder weather this water can then freeze.

If the drain from the heat pump (pipe or drain hole) is blocked or freezes over this can also cause the heat pump to be frozen. For this reason most manufacturers will have a trace heater at the base of the heat pump.

To prevent the ice causing any severe damage, a heat pump will have a defrost cycle. This usually kicks in automatically and works to de-ice the coils. However, there could be something that is preventing the pump from running this cycle, such as low refrigerant levels, broken thermostats/sensors or a sticking reversing valve. Or it could be something as simple as leaves or debris blocking the vents on the heat pump.

It is also important to consider the area around the unit, blocking a heat pump by parking too close or placing a bin or fence around to hide it could cause issues with airflow into the unit. You should check minimum clearance guidelines in the manufacturers handbook.

If the sensor on the heat pump is faulty, it may not be able to detect the ice build up, and this could be another reason the pump isn’t activating the defrost cycle.

How to defrost a heat pump in winter

If you’ve noticed that your air source heat pump unit is excessively icing over, you should contact a HVAC professional who can help determine the cause. When this problem is ignored, it can result in problems with your unit in the future, so it’s best to get it sorted sooner rather than later.

The defrost cycle is a normal part of heat pump operation. A defrost setting works by turning your heat pump into an air conditioner. When it heats your home, the heat pump takes heat from outside and heats your home. When it works in reverse, it can use the heat from your home to thaw out the unit, this is usually why buffer vessels are fitted. After a certain amount of time, the system will switch back to heating your home. You shouldn’t feel any temperature difference within your house while this process is happening.

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