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There is much conflicting information on this, e.g.

Nu-heat:

Bear in mind that your heat pump will need to work harder if it is situated in a cooler, north-facing, shady position, than if it is located in a southern-facing, sunny spot.

Yougen:

any benefit in positioning the unit against a south facing wall rather than a north facing wall? Not normally ... whilst it is possible that the shade air temperature may be a degree or so warmer on the south side of the property than the north, this is only likely to be in daylight summer hours when the unit is unlikely to be operating.

Mitsubishi:

Avoid locations where the unit is exposed to direct sunlight or other sources of heat.

Has this been systematically looked at? What do they do in countries that have a long history of heat pump use (unlike the UK where most installers seem unsure themselves)?

I am in the northern hemisphere, UK climate, semi detached family home.

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Nu-heat's advice:

Bear in mind that your heat pump will need to work harder if it is situated in a cooler, north-facing, shady position, than if it is located in a southern-facing, sunny spot.

True, in heating season. As the delta-T (difference between outdoor temperature and desired indoor temperature) increases, the heat pump will be less efficient (lower COP), meaning it will have to work harder to heat your home:

COP vs temperature curve (source)

You-gen's advice:

any benefit in positioning the unit against a south facing wall rather than a north facing wall? Not normally ... whilst it is possible that the shade air temperature may be a degree or so warmer on the south side of the property than the north, this is only likely to be in daylight summer hours when the unit is unlikely to be operating.

Maybe true, depending on your location and schedule. If you're on the crest of a hill, the north side might be windy and snowy compared to the south side, and thus a lot colder. If you're in a row of houses or a valley, it probably doesn't make much difference. Also, you know if you're likely to be heating during sunlight hours, so keep this in mind.

The author seems to be saying not to put too much weight on aspect when deciding where to place the outdoor unit. First think about cost (to run piping and electric), aesthetics, and noise concerns.

Mitsubishi's advice:

Avoid locations where the unit is exposed to direct sunlight or other sources of heat.

True, to prevent damage to the unit over time. This directive is from the manufacturer, so it has more to do with preventing damage to the unit, as direct sunlight will break down metals and plastics over time. The rubber coating on tubing will be particularly vulnerable.

TL;DR:

Don't think about aspect when deciding where to put your heat pump outdoor unit. If there are a few possible locations, some on the north side and some on the south, you could consider some of what these sources have said.

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I don't know much about heat pumps but I think they work by utilizing the temperature differential between the top soil and the subsoil. For this purpose I would think they need to be buried at least an inch deep, possibly more. They do not utilize sunlight to any great degree, it is more about ambient temperature at two levels. In the summer the top soil in most places will be warmer than the subsoil. However in a cold winter this reverses, and a well buried heat mat should pump at its most efficient. Hmm. well I think logically this is what should happen, but its early days. I hope they do prove efficient because they are arguably the greenest form of heating.

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