Heat pump defrost: why it happens in UK winters

Why does my heat pump defrost in winter?

If you own an air source heat pump, you’ve probably seen it pause on a cold day, blow out a burst of steam, then get back to heating as if nothing happened. That’s the defrost cycle. It can look dramatic, but it’s a normal way the system protects itself in cold humid weather.

Homeowners often search for things like why does my heat pump defrost, why is my heat pump defrosting so often in winter, or heat pump icing in cold humid weather.

This guide explains what is happening, why it happens more in the UK, and what to look for if it seems excessive.

Table of contents

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This article is part of my wider series of blog posts around heat pumps and renewables.

And if you have a Vaillant Arotherm, then check my Vaillant Arotherm guides.

Vaillant Arotherm Heat Pump Guide & User Manual

What the heat pump defrost cycle actually does

The outdoor coil (usually at the back of the unit) is where the heat pump pulls heat from the air. When the weather is cold and humid, moisture collects on the coil and freezes. Once the fins ice up, airflow drops and the system cannot pull heat efficiently.

To fix this, the heat pump briefly switches mode. For air to water units, this usually means pulling warm water out of the heating circuit and sending that warmth into the outdoor coil to melt the ice. You’ll often see a cloud of steam because the ice clears quickly.  Don’t be alarmed, this is perfectly fine and normal.

During the defrost cycle, your flow and return temperatures reverse and fall. If the closest heat emitter is a radiator or underfloor loop, that loop will supply the warmth for the defrost. This is why many systems include a volumiser on the return pipework so the heat pump draws from stored water instead of stealing it from a room circuit.

You can see in this Open Energy Monitor trace from my own system how the flow and return temperatures reverse during a defrost.

Why the coil gets cold enough to freeze

Inside the unit is a vapour compression cycle. Refrigerant evaporates at low pressure on the outdoor coil. That evaporation pulls in heat from the outside air and makes the coil run cold. In mild weather this isn’t a problem. The air has enough heat to keep the coil above freezing.

Bit things change when it gets colder. When the air is close to zero and still humid, the coil can run well below freezing while it is absorbing heat. Moisture touches the cold fins, condenses, then turns straight into ice. The defrost cycle clears this before it builds up too far.

If you want to see a defrost in action, so you know what to expect, watch this YouTube video from Tom Bray.

Why heat pump defrosts happen more often in the UK

Humidity is the big factor. The UK sits under moist Atlantic air through winter, which keeps humidity high even when temperatures are near freezing. According to Climate to Travel and CurrentResults, the UK spends much of winter in the 80 to 90 percent relative humidity range.

Parts of Europe often see lower humidity in winter. Even Sweden comes in lower on broad averages when you compare countries using WorldData. More moisture means frost builds faster on the fins, which is why heat pumps in the UK tend to defrost more often that our European neighbours.

What outside temperature triggers the heat pump defrost cycle?

On my own system, I’ve seen frosting happen as high as around 3°C outside.

You might expect freezing only at zero degrees, but the coil can be several degrees colder than the outdoor air. If humidity is high, moisture will condense and freeze on the coil even when the thermometer is above zero.

Cold damp weather creates frost long before the air reaches freezing.

Here’s a direct link to my monitoring showing the defrost, along with an image from my Open Energy Monitor setup.

How often should a heat pump defrost in winter?

A defrost every 45 to 120 minutes in cold humid weather is common. As long as the ice clears fully and the system gets back to heating normally, there’s no cause for concern.

Here is my system, when between -0.9 and 1.9C outside defrosting around every 90 to 120 minutes.

What causes a heat pump to defrost too often?

Poor airflow around the outdoor unit

Restricted airflow makes the coil colder and speeds up frost build. Leaves, fences, snow, tight alcoves and dirty fins can all cause problems.

Fault with sensors or the defrost routine

If the coil sensor is inaccurate or the routine does not clear ice properly, the heat pump can fall straight back into another cycle. If ice remains after defrosting or the unit spends more time defrosting than heating, it needs checking.

Meltwater from defrost not draining away

If the water from the defrost can’t drain properly and it sits inside the unit, it can make the next defrost happen sooner than it should.

Undersized heat pump

If the unit is too small for the home, it runs flat out and the coil runs colder. Frost builds faster and cycles repeat more often.

Noise reduction / low compressor mode

I simulated the undersized heat pump the other winter by running noise reduction mode for the whole 24 hours on a very cold day when testing Sweet Spot Hot Water mode.  This limited the output of the heat pump, so in essence turned my heat pump into an undersized one.

You can see on the monitoring trace (and in the image below) that I was defrosting close to every 40 minutes and the heat pump could not put out enough heat to reach target room temperature because of the capped compressor.

Where does the defrost water go?

When the heat pump melts the frost on the outdoor coil, all that water needs a clear route to drain away. Most units drip from a small outlet underneath the outdoor casing, so it’s important that this path is kept clear and directed into a proper drain or a small soakaway beneath the heat pump.

If the water can’t drain properly, it may sit inside the base of the unit, which can make the next defrost happen sooner than it should. And if the water spills out onto the ground instead of into a drain, it will pool around the base of the heat pump. In winter that pooled water will freeze, which creates a slip hazard and can make access for maintenance difficult.

A quick check before winter helps avoid this. Clear any leaves, debris, or blockages under the unit so meltwater can drain safely.

Some heat pumps also include a small heat trace wire on the drain outlet. This keeps the meltwater warm enough to flow into the pipe or soakaway instead of freezing on the way out. If your unit has one, make sure the cable is intact, positioned correctly and activated so the drain stays clear during cold spells.

What can homeowners do about heat pump defrosts?

• Keep the outdoor unit clear of leaves, plants, and snow

• Maintain good space for airflow

• Ensure the condensate drain hole is clear

• Avoid excessive use of low compressor / noise reduction mode

• Review sizing if the unit never reaches stable operation (check Vaillant Performance Tables if you have an Arotherm)

• Take a few winter photos if you want to track icing patterns or time length between defrosts to track frequency

• If things get really bad, consider contacting an installer to fit a volumiser so the heat pump draws stored water during defrost from the volumiser rather than radiators and/or underfloor circuits

Summary

Heat pumps defrost because the outdoor coil gets cold enough for moisture to freeze on the fins. The coil often runs colder than the outdoor air due to the vapour compression process, so frost can form at temperatures above freezing.

The UK sees more frequent defrosts because our winter air is chilly and humid. Most of the time this is normal. If the cycles feel too frequent, the usual causes are restricted airflow, an undersized unit, or a system issue.