What Happens to An Air Source Heat Pump During Defrost Cycles?

StingFellows

February 19, 2026

Heat Pump

Households rely more on their heating systems to keep their homes warm and cosy during the cold winter. Air source heat pumps are the most well-known for how well they work as heating systems. However, the defrost cycle is one aspect of heat pumps that often raises questions.

Heat pumps are highly efficient, versatile, and reliable. They can provide year-round comfort. Because they can heat and cool your home. However, some households have concerns about heat pumps freezing up during the colder days of the year. Especially when temperatures fall enough to bring snowfall and frost. 

If you’ve experienced this problem during winter’s past, Read on to explore how to defrost your heat pump when using it in the winter.  In this detailed guide, we’ll describe everything you need to know about the heat pump defrost cycle and how it works. It impacts your heating system’s efficiency.

If you do not have a modern heating system. And you are struggling with poor heating devices. It has let you face cold in winter and you have no money to buy an air source heat pump. Thus, you must consider the government-backed Energy Company Obligation ( ECO4) scheme. The scheme aims to provide free air source heat pump grants to low-income households in the UK.

What is the Heat Pump Defrost Cycle?

The defrost cycle of an air source heat pump system is designed to maintain optimal performance even in cold weather. When outdoor temperatures drop, frost can be set on the heat pump’s outside coil. 

If this issue is not noticed, this frost can impede the heat pump’s capability to move heat, lowering efficiency and potentially causing damage. When the defrost cycle runs, the refrigerant flow is briefly switched around. This heats the outer coil and melts any frost that has built up.

How Does the Defrost Cycle Work?

A heat pump sucks air and converts into heat. It also moves the heat inside your home to keep you warm. It can get below freezing on the outside coil. That freezes the water in the air and turns it into frost. 

To keep it safe, the heat pump will sometimes switch to cooling mode.It cools down and heats the outdoor coil to melt the snow in thaw mode. But it doesn’t change the temperature inside.

When Does a Heat Pump Swap in Defrost Mode?

When an air-source heat pump shuts off the frost mode. It relies on the temperature outside, the frost on the coils, and how long it’s been since the last defrost run. 

Most systems use sensors and timers to understand the optimal moment to start a defrost cycle. It involves ensuring the process happens only as needed to maintain efficiency.

How to Defrost a Heat Pump?

Sometimes, the heat pump may take a few hours to mould a frozen coil completely, so don’t be alarmed if ice doesn’t melt right away. If the heat pump defrost cycle doesn’t work correctly or can’t melt away the frost or ice on the unit, you’ll need to know how to defrost a heat pump manually.

1.Power off the heat pump system at the circuit breaker.

2.Use a garden hose to spray water over the frozen heat pump components and melt the ice. Do not use tools or sharp objects to chip away ice from coils, as this can cause severe damage.

3.Check why your heat pump has excess ice. Check the air strain and change it if it is dirty. Clear debris has been collected on the heat pump’s exterior. Fix leaky overhead gutters.

4.To test problems with the blower fan motor, turn the fan “ON” at the thermostat. If the fan power on, the blower motor isn’t the issue. You can also leave the fan on for an hour. This will help move air through the system and warm up the coils.

Summary

The core reason for having a defrost system is to ensure the heat pump works efficiently and effectively during winter. Without it, frost buildup can significantly reduce the unit’s ability to heat your home. This can lead to higher energy use and even potential system damage.

In emergency heat mode, the heat pump shuts off and a secondary heat source is used instead. While this mode does not actively clear frost from the heat pump, it helps keep your home warm. If the heat pump stops working properly due to excessive frost, it may indicate issues with sensors, the control board, or other components.

In such cases, an air source heat pump that fails to defrost correctly should be inspected by a professional HVAC technician.

If you are researching reliable options, exploring the Best Air Source Heat Pumps can help you choose a model designed for better cold-weather performance and efficient defrost systems.