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Explaining the defrost cycle

A common question asked by many Ecodan owners is: ’What happens to my air source heat pump in cold weather?’

The answer is, essentially, 'business as usual’- The Ecodan will carry on heating your house and providing all the hot water you need.

What does happen, periodically when the outside temperature gets to near zero, is that the outdoor unit automatically defrosts itself.

This means that the system temporarily diverts power to the Ecodan outside to stop ice building up on the back of the unit.

This is known as the defrost cycle or mode and occasionally, in cold, damp weather, this can give the appearance of ‘steam’ coming off the unit.

Heat pumps are used in Scandinavia where temperatures get much, much colder than in the UK

PeteMcNaughton Peter McNaughton Renewable Heating Sales Manager

What happens during defrost mode?

As the outdoor unit of an air source heat pump operates, frost can build up on the back of the unit when the temperature drops.

This is perfectly normal and will even happen when the outside temperature is in low single figures - it doesn’t even have to be below zero.

A white frosty coating appears on the back of the unit and as it builds up, it restricts the airflow through the heat pump.

To melt this away, the unit needs to carry out a defrost cycle from time to time.

This puts heat through the fins on the back of the unit, causing the frost to quickly and easily melt, which then runs off as water.

This is usually cleared within a couple of minutes and the Ecodan heat pump is then back in action, providing heat and hot water for the home.

How often does it defrost?

At the start of the process you may hear the sound of air being released (much quieter but similar to brakes on a lorry), often followed by a plume of ‘steam’. This is perfectly normal and should not cause alarm.

How often the unit defrosts is difficult to predict as there are several factors that contribute to the build-up of the frost.

This includes how cold the air is, how humid it is (i.e. how much moisture is in the air), how much air the heat pump is drawing through itself, and how quickly the fan is rotating – all of these are determining factors.

When the outside temperature is hovering around freezing, the combination of humid air and cold temperatures can mean the unit will defrost several times per hour.

Interestingly, as the temperature continues to drop, the air tends to be drier therefore defrost doesn’t happen as frequently.

What is the freeze stat function?

The freeze stat function is often confused with defrost but is totally different.

When a property is heated to its desired temperature, the Ecodan heat pump will turn itself off.

The unit itself and the pipework to the outdoor unit contains a quantity of water. During very cold weather, there is a risk this water could freeze and, with that, the potential for burst pipes or damage to the heat pump itself.

To stop the risk of this happening, the heat pump has a freeze stat function which is activated automatically and circulates water through the pipes and the outdoor unit to prevent the things freezing.

During the freeze stat function, no heat is delivered to the heating system or hot water tank, it is merely moving the water to prevent it freezing.

What if the water is mixed with Glycol?

If the water within in the heat pump system is mixed with Glycol, the freeze stat function can be turned off as the chemical will stop the water freezing and therefore there is no need to keep it moving. Glycol is used widely as an antifreeze chemical; a variant is often used in car radiators to stop coolant water in the engine from freezing.

However, this is something that only a qualified heating engineer should attempt.

So, ahead of any upcoming cold snaps, rest assured your heat pump will continue to heat your home and provide you with hot water as it should.

It’s worth remembering that heat pumps have been widely used for decades in Scandinavia where temperatures get much, much colder than in the UK and yet they continue to work effectively.

Check out this video I took at my home so you can familiarise yourself with what the defrost process looks like.

Remember, you can find more helpful videos on our Home Portal FAQs section. If you have any questions, you can always contact the Ecodan Home Helpline for help and advice on 0161 866 6064.

Peter McNaughton, Renewable Heating Sales Manager