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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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  • Place it in a location that is closest to the electric power and the inside unit /air handler. Sep 22, 2021 at 15:57

3 Answers 3

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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 thought to expand a bit on user the answer provided by lshaver above. The following discussion concerns the use of an air heat pump though the discussion of some of the mechanics also applies to a geothermal heat pump. I would expect that a geothermal heat pump does not depend to any great extent on the placement of the outside unit since it uses geothermal source/sink for the heat rather than air.

The bottom line

The outside condenser unit should be in an environment that provides an air temperature as close to the desired air temperature of the inside evaporator unit as possible.

For warmth, a Southern exposure with provisions to create a warm micro climate in the area of the outside unit is preferable.

For cooling, a Northern exposure with provisions to create a cool micro climate in the area of the outside unit is preferable.

In both cases due to the amount of airflow through the condenser, the area of the micro climate will need to be large enough to handle the volume of air that flows through the unit.

However also see this article, Heat Pump Efficiency | At What Temperature Is a Heat Pump NOT effective?, about two types of heat pump units, the split heat pump and the central/whole-house heat pump which do have different characteristics.

The details

Looking at the link at Engineering Toolbox he provided there is an additional graphic that is important.

graph of COP versus difference in temperature of heat pump condenser and evaporator

What this is saying is that as the temperature difference between the inside air temperature and the outside air temperature gets larger, the ability of the heat pump to move heat between the inside and the outside gets smaller.

Using a heat pump in the winter to warm a home, the heat pump is more efficient when the outside temperature is warmer and closer to the setpoint for the internal air temperature. This is why heat pumps are not desirable in cold climates and why there is a radiant heat device built into the heat pump to handle when the temperature difference is large enough that the heat pump isn't able to pump sufficient heat from the outside into the inside.

The reverse is the case for a heat pump in the summer to warm a home. As the outside temperature becomes greater than the internal temperature, a point is reached at which the heat transfer from inside to outside is too small to be useful.

I'm in a warm climate, the southern part of the state of Georgia in the Southeast United States, where summertime temperatures (Fahrenheit) are routinely in the 80s and 90s. Winters are mild with the temperature seldom dropping into the 20s.

I found this article, Does Your Air Conditioner Need Shade? – Energy Efficiency Myths, which has links to a couple of research articles. The research question was whether shading the outside condenser unit of a heat pump improve efficiency by cooling the unit.

Research by the Florida Solar Energy Center, Measured Impacts of Air Conditioner Condenser Shading, indicates that shading the outside unit provides negligible benefit. They also found that plantings or surrounding walls/roofs to provide shade must not interfere with airflow through the unit nor redirect airflow so that air coming out of the outside (condenser) unit mixes with air entering the outside unit since that affects the temperature of air entering the unit.

The probable explanation for the lower than expected performance in the experiments is that outdoor AC units draw in a volume of air that greatly exceeds that of the nearby shaded air volume. Air-cooled condensers move a quantity of 600 - 1200 cfm of air per ton (80-160 L/s/kWt) of cooling capacity (ASHRAE, 1992). For instance, a typical 3-ton (10.6 kWt) air conditioner's 300-W condenser fan would draw 2,800 cfm (1321 L/s) of air at a very low static pressure across the coil (Proctor et al., 1994). Thus, the unit would process 168,000 cubic feet (4.76 x 106L) of air per hour. Assuming no mixing, this would represent a volumetric equivalent to a cube of air with sides measuring 55 feet (16.8 m).

The article doesn't mention the effect of turbulence on airflows but I would expect any installation that may generate turbulence within the airflows would reduce efficiency as well.

The article has a final conclusion for their results on testing cooling.

We conclude that any savings produced by localized AC condenser shading are quite modest (<3%) and that the risk of interrupting air flow to the condenser may outweigh shading considerations. The preferred strategy may be a long-term one: locating AC condensers in an unobstructed location on the shaded north side of buildings and depending on extensive site and neighborhood-level landscaping to lower localized air temperatures.

Looking at the average temperature in the UK, the range seems to be from around 40F (5C) to around 70F (20C) though it seems I've read of some above average days in in the 80s Fahrenheit.

Monthly average daily temperatures in the United Kingdom (UK) from 2015 to 2021 monthly average daily temperatures in the UK

Looking at the above chart, it appears that average temperatures are well within the efficient operating range of most recent air type heat pumps and unobstructed airflow is the main consideration for the placement of the unit,

See Air-Source Heat Pump or Air-to-Air Heat Pump

Heat pump balance point graph outside air temperature vs. BTU/Hour

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  • This is why heat pumps are not desirable in cold climates: This isn't as true as it once was. See recent developments from NREL, RMI, and the Dept of Energy.
    – LShaver
    Sep 22, 2021 at 15:37
  • @LShaver I took a look at some of these resources which had links to some research which appears to have been done around 2017. The DOE research seemed to involve duct-less splits which do have a lower outside temperature tolerance according to what I found for Fijitsu and Mitsubishi product offerings. Some of these materials also reference backup heating systems. The reported COP was low as in less than 5 and typically less than 2 in some of the test results at low temperatures. What specific product offerings are being used in cold climates by which I mean lower than 0F? Sep 22, 2021 at 20:12
  • For specifying HVAC equipment industry standard in the U.S. is to use heating degree days, and "cold climate" is any region with over 5,400 HDD per year -- zones 6 and 7 in this map. Utilities in these regions currently run cold climate heat pump programs such as: Minnesota and Maine.
    – LShaver
    Sep 22, 2021 at 21:06
  • Does it make a difference if it's air-air or air-water? For air-water central heating, I think often heating rods such as this one are used, but it seems your source considers only air-air heat pumps.
    – gerrit
    Jul 12, 2023 at 7:40
  • It's not enough to look at average temperatures. People want to stay warm on the coldest days of the year too.
    – gerrit
    Jul 12, 2023 at 7:46
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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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  • 1
    You're talking about ground source heat pumps. The question specifically refers to air source heat pumps
    – John M
    Sep 22, 2021 at 11:36

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