Part 1: How to select the right air source heat pump and why?

When selecting a heat pump, there are approximately 9 factors which we believe makes up a well justified, and as far as there ever can be, correct, selection. In the first part of this six-part series, we take a closer look at the aesthetics and noise levels of ASHP.  


Air source heat pumps come in a variety of shapes and sizes, and as they need to be installed outside (there are exceptions to this rule, but we’ll stick to the most common units in the UK for this article) the aesthetics of the equipment can be important. The two main variables when comparing the looks of heat pumps are the shape, the size and the colour.

Typically, a heat pump will be coloured a shade of white, ranging from ivory to pure white, but there are exceptions to this. It’s worth noting that there are companies out there who will provide a heat pump colour change service, whether that’s through the application of vinyl or paint. Personally, I would urge caution against changing the colour of the heat pump to “blend it in” to the wall. A red brick wall looks very different to a vinyl wrapped or painted metal heat pump. Different materials painted the same colour will look different against one another, hence why paint companies will always advise that the colour on their booklet will look different when painted on the wall. It’s two different materials, but the same colour. So, what colour should you go for? I for one have always found real success with tying the heat pump colour into doors or window frames. If you take a red brick property with anthracite window frames and door, a heat pump painted the same colour, from a short distance, will just look like another door / window. Colour is personal preference, and your customer may decide on pink or purple for their heat pump, more power to them in that case.

In the UK now, heat pumps are similar sizes. To comply with permitted development rights, they must be below 0.6m³ in volume, so they are, albeit that volume may be shared out in different dimensions. Currently, there are two main trains of thought when it comes to size and shape: low and wide, or tall and slim. I go into the difference between compact and “true” monobloc heat pumps in another part, but classically, the low and wide heat pumps are your “true” monobloc where they contain primary circulation pump and expansion vessel at the minimum, whereas the tall and slim heat pumps are the compact type of heat pump, with less equipment built in the outdoor unit. The idea here is that many “true” monobloc heat pumps are split type outdoor units with a hydronic module bolted on the side hence the low and wide construction makes the most sense. A good example of a low and wide monobloc air source heat pump would be the Midea MHC range or the LG Therma V monobloc, whereas an example of a tall and slim heat pump would those from Samsung EHS or the Mitsubishi Ecodan.


Noise level, in this context, is sound pressure level i.e., the pressure disturbance that is felt by our ears. This is based on sound power, but with variables taken in to

account such as number of acoustically reflective surfaces surrounding the equipment and the distance from the measurement point. Now we know what noise, at least in this context, is, let’s just outline a couple of reference points, so we have something to compare against.

140dBA = Threshold of Pain
130dBA = Jet Taking Off (200ft. Away)
90dBA = Boiler Room
40dBA = Soft Whisper
20dBA = Silent Study room
Courtesy OSHA (US)

So, now we have some reference points, how do air source heat pumps stack up? I’ve tried to select heat pumps below with similar kW output to give a good comparison. As you can appreciate, in most cases, the larger the heat pump output, the higher the noise level.

Mitsubishi 8.5kW Ultra Quiet Ecodan = 45dB(A) at 1m

Samsung AE080 = 48db(A) at 1m

Midea MHC-V8 = 48.5dB(A) at 1m

Hitachi RASM3 = 50dB(A) at 1m

As above, there is quite a range over a relatively small selection of heat pump units. It’s worth noting that reductions in sound level, especially when looking at the heat pumps listed above, tend to correlate with size of equipment. The larger the heat pump, the lower the noise level. Take the Mitsubishi 8.5kW Ecodan and the Samsung AE080 as good examples. Similar in terms of internal construction and output, but the Samsung is approx. 0.31m³ in volume whereas the Mitsubishi is 0.51m³. The larger cabinet for the Mitsubishi obviously helps reduce the sound pressure level somewhat. Again, also worth noting is that both the Hitachi and the Midea have circulation pumps built into their outdoor units, which will add to the noise level of each of the units.

In short, whilst sound pressure or noise levels can be used to great effect to compare different heat pumps against one another, the search for the quietest possible heat pump may yield tradeoffs, such as a larger outdoor unit, or having to have the circulation pump inside the property.

Join Chris for part 2, where he talks about arguably the most important factor in selecting an air source heat pump – kW output.