One of the most common questions homeowners ask me when considering an air source heat pump is where to put it. A common train of thought suggests they should be placed on the south side of a property to benefit from the heat of the sun. However, an air source heat pump moves a lot of air – up to six cubic metres per second – and the compressor only actually starts when the fan has been running for a couple of minutes, by which time the air will be cooler, and thus there is little advantage to be gained from the unit being in the sun. Direct sunlight only actually benefits a heat pump when it is covered in snow, and even then a better solution would have been to protect it with a small roof and keep air flow unrestricted.
In fact, older heat pumps may have sensors on their casings that actually get ‘confused’ by sunlight. This can cause them to run erratically and prevent heat from being produced according to the settings on room thermostats, or limit the supply of hot water on hot days.