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Get the best out of your air source heat pump in sub-zero temperatures

Posted by John Lightfoot on 4 January 2011 at 9:46 am

With temperatures more associated with Scandinavia than the UK, 2010 tested heating systems of all sorts in the UK, gas boilers and air source heat pumps included!

Here are some of the problems have people reported over the past year and how can we avoid them being repeated as we go into 2011.

Freezing Pipes

During period of very low night time temperatures some people have discovered that their pipes are freezing when the time clock has switched off the heat pump.

This could have happened for a number of reasons:

  • in the worst case, the problem has been cause by units being installed by inexperienced installers who fail to put any anti- freeze (glycol) in the system
  • Installers who have added glycol to the system, but not a adequate amount for temperatures that have dropped, in some cases to below -10 degrees C
  • Home owners when decorating have drained down their heating system and simply filled it up again with water only

The solution to this is to get your heating engineer to check the concentration of the anti freeze in your system. You may also want to consider making sure your night set back is enabled or setting your thermostat to say 10 degrees C at night time to reduce the amount of work your heat pump has to do when it’s normal start time commences. This also, of course, has the advantage of causing the heat pump to activate a couple of times during the night, thus helping to keep the heating water temperature above zero.

Insufficient heat

The conditions at which heating installers design systems to provide adequate heating output range from ambient (outside) conditions of zero to -5 degrees C depending on where you are in the UK. These conditions normally account for 99% of the year. When temperatures fall below these levels, your heating emitters (radiators or underfloor heating) and heat producing equipment (e.g. boiler, heat pump, stoves, storage heaters etc) may not be sized to cope with the greater heat output required to compensate for the higher heat loss from your property due to the lower outside conditions. In these circumstances you need to add some supplementary heating from another heat source.

Some systems already have this designed into their control strategy such that, in exceptionally cold conditions, direct electric heating will automatically cut in to help supplement your heat pump. If your heating system does not have this automatic feature it is probably best to turn off the radiators (or other heat emitters) in rooms where you can use local heaters such as, for example, wood stoves or electric heating, thus allowing the heat pump to cope with other rooms in the property.

Where you have rooms that are not being used or are not essential to be kept at normal room temperatures,set the thermostat for those rooms down to low.


In some cases, heavy snow fall has blocked either the air intake or outlet of a heat pump. Simply keep the outlet and inlet air paths to the heat pump clear to ensure it works at optimum efficiency.


When a heat pump defrosts, water will drain from it. If adequate arrangements for drainage are not made there is the possibility, when the temperature remains below zero for days on end, that ice may build up and start “backing up” to the heat pump’s coil. It can then start to block the air way through the coil. In these exceptional conditions one would normally melt the ice, with hot water for example, making sure the heat pumps coil is clear of ice.

More information about air source heat pumps from YouGen

Heat pumps information page

How (and where) to position your air source heat pump

Common problems with air source heat pumps

When to consider using an air source heat pump

What domestic RHI means for air source heat pumps

About the author: John Lightfoot is director at Thermal Energy Ventures Ltd.

If you have a question about anything in the above blog, please ask it in the comments section below.

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1 comments - read them below or add one


BARRIECATSComment left on: 16 February 2012 at 1:01 pm

Have been turning off the ASHP from 11pm to around 7:30 am this year Jan/Feb . House seems OK temp wise and fairly soon warms up when Pump restarts although sounds like plane taking off for a time. Just had full double glazing throughout and insulation in roof space but walls are not cavity so nothing done to them which is a pity. 

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