Long spells of cold weather may temporarily cause ice to build up on your heat pump in winter. This isn’t a sign that your heat pump is malfunctioning though; it’s actually quite normal. Should the temperature drop below a certain level, the heat pump will run a defrost cycle to fix the problem.


Why does an air source heat pump freeze?

We’ll start off by taking you through the process on a warm summer’s day…


Air source heat pump in summer

An air source heat pump extracts heat from warm air (for instance 20oC) by passing very cold refrigerant liquid (e.g. -25oC) through heat exchanger coils.

  • The fan draws warm air over these refrigerant-filled coils and, due to the large heat differential, heat passes into the much colder refrigerant liquid. This causes the refrigerant to evaporate into low-grade gas. The higher the air temperature, the greater the amount of refrigerant turns into gas.
  • The refrigerant gas travels through a compressor, which further raises its temperature and pressure.
  • After which, the refrigerant passes into another heat exchanger – the condenser – where the heat is transferred into your wet heat distribution system (radiators and underfloor heating).

The article ‘how does a heat pump work’ explains this method in detail.


Air source heat pump in winter (without defrost cycle)

  • The air source heat pump extracts heat from cold moist air (e.g. 0oC) as per usual. However, by extracting this heat, the moisture in the air freezes to the heat exchanger coils.
  • The difference between refrigerant and air temperature is smaller, so less heat is absorbed and less refrigerant evaporates.
  • If the cold weather persists for several days and the heat pump is working constantly to meet the increased heat demand, there isn’t enough time for the warm ambient air to thaw the ice. More and more ice forms over the heat exchanger coils. Like an insulator, the ice blocks the air from hitting the coils, reducing the total surface area for heat transfer.
  • Any warm air that does reach the heat exchanger coils is already cooled by the ice block, further reducing the heat differential. As a result, less refrigerant evaporates into gas, and the compressor must do more work to meet the heat demand. The heat pump’s efficiency plummets, and eventually the system stops working.

…Or it would without the heat pump’s defrost function.


What is a defrost cycle?

In short, a defrost cycle does runs the heat pump backwards. Rather than using ambient air as the source of heat, a small amount of hot water from inside your home is used to melt the ice on the coils.

  • Once the system has recognised ice building up on the heat exchanger coils (usually with temperature sensors outside), the defrost cycle will automatically activate.
  • The system switches a reverser valve, essentially turning the process on its head. High temperature liquid from the buffer tank passes back through the compressor, creating a large amount of high temperature, high pressure gas. (In the meantime, either the immersion heater or some other secondary heating system will kick in to provide heating for your house.)
  • This high temperature gas passes back through the external heat exchanger coils, quickly thawing the ice.
  • The ice melts, and water droplets start filling up the soak way beneath the heat pump unit. Once the outdoor thermostat reaches a certain temperature (or the built-in timer finishes depending on your heat pump), the system returns to heating mode.

The whole process only lasts a couple of minutes, but it’s a crucial step to maintain your air source heat pump’s high efficiency.


How to defrost a heat pump in winter

This depends on the model of heat pump, the manufacturer, and the demand for heat. Generally speaking, the whole process is fully automated, but for some heat pump models, you may need to run the fan to thaw the ice, or select an exhaust setting. You should be able to tell that your heat pump is in defrost mode from the user module in your home. You can also check the unit outside – the fan won’t be rotating, but the compressor should still be whirring.

Should the cold weather persist, an air source heat pump may trigger a defrost cycle every 30 – 90 minutes.

How much ice builds up ultimately depends on the outdoor temperature and the heating demand. While we might not be able to change the weather, we can change the demand for heat. Installing underfloor insulation and cavity insulation will go a long way to reduce the demand for heat. Other measures include keeping the doors and windows closed, the curtains drawn shut and wearing more clothes to reduce the heat demand.


Heat pump in winter – defrost cycle isn’t working

If your heat pump isn’t running defrost cycles in winter, this may mean that there’s an underlying problem with the system. Be it the outdoor thermostat isn’t signalling for a defrost cycle, or there’s a flow issue, we suggest calling up your heat pump manufacturer to get an accredited professional to solve the problem. We don’t advise pouring boiling water over the coils or attempting to chip the ice away. While they may lead to a temporary fix, these measures often do more harm than good.

Other common problems associated with defrost cycles include debris causing a blockage, improper sizing of the heat pump (this may be the case for you if your heat pump frequently freezes over), insufficient refrigerant being circulated, and improper installation.


Why ground source heat pumps don’t need defrost cycles

Since the temperature a few metres beneath the earth remains a moderate 10oC all year round, there isn’t a need to have a defrost cycle built into the system. The top layer of earth serves as insulation, blocking the cold from reaching that far underground.



Cold temperatures reduce the efficiency of your heat pump in winter in 3 ways:

  • Smaller heat differential between the refrigerant and the air
  • Ice build-up on the heat exchanger coils reducing the surface area
  • Ice block cooling the ambient air temperature, further reducing the heat differential

Fortunately, modern heat pumps fix this issue with automatic defrost cycles. If you need to be put in touch with a certified HVAC engineer, we can help. Fill in the details on the contact form, and we’ll soon be in touch.

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