Guide

Part 2: How to select the right air source heat pump and why

In part two of this series looking at how to select the right ASHP and why, Chris Higgs, managing director of Freedom Heat Pumps talks about arguably the most important factor in selecting an air source heat pump - kW output.

Did you miss part one looking at aesthetics and noise levels of the heat pumps? Find it here.

kW Output

What we mean by kW output is the amount of thermal energy that the heat pump delivers at a specific ambient and flow temperature. Other factors such as relative humidity and viscosity of the heating water (this could be an energy transfer medium such as glycol or just plain water, depending on the application) will affect the energy available from the heat pump, but for the purpose of this guide, we will just concentrate on the information you would expect to find within manufacturers capacity table.

Assuming at this point, you have completed a heat loss calculation of some description (floor by floor, room by room etc.), you will now have a peak heat loss figure for the property. For this article, let’s assume that the heat loss of the property is 5kW, we are in London, and the intention is to serve heated water to radiators throughout, so we would most likely require a 50°C flow temperature. If we refer to the below table, I hope you can see fairly quickly that at -2°C (as we are in London, we need cover just to -1.8°C as per guidance figures for 99% of the year) with a 50°C flow temperature (our design flow temperature for the radiators we have chosen), the output from the heat pump will be 5.73kW, which is more than enough to cover the 5kW heat loss we have calculated (or been provided).

It’s always handy to allow for at least a 10% “oversize” when selecting a heat pump, to allow the heat pump some capacity should there be an extended below -1.8°C cold spell. In this scenario, 5kW would become 5.5kW, and as such, the MIDEA6MONO would still suit the project.

Fig. 1 MIDEA6MONO Output Table

But what if you have underfloor heating? Everything as discussed above would still apply, with the big difference being that we may look to select a much lower flow temperature of perhaps 35°C to serve the underfloor heating. Again, in this case, the MIDEAA6MONO will give us 5.5kW which is on the money for covering our heat load, and a little extra.

If you have a mix of radiators and underfloor heating, you will always select the heat pump based on the highest design flow temperature, which in this case would be 50°C. Please note that this guide isn’t intended to cover every possible set up, but hopefully it gives you enough information to confidently specify a heat pump for most heat pump installations you encounter. For anything outside of the “norm” there are specialist distributors out there who will happily assist J

Why doesn’t an air source heat pump output stay the same all the time, regardless of the ambient temperature?

In short, the further away the ambient temperature is from the flow temperature, the harder the heat pump will have to work to generate thermal energy. As with any mechanical process, there is a tradeoff between performance and efficiency, and the harder the heat pump must work to maintain the required kW’s the lower the efficiency will be as more electricity will be required. It’s at this point that a heat pump has two options; it can maintain the same kW output at every ambient temperature (but it will sacrifice efficiency), or it can choose to sacrifice kW output to maintain efficiency. What I’ve mentioned here is the general rule that applies to heat pumps and is roughly illustrated by the MIDEA6MONO table above. It is also worth noting that in many heat pumps, between ambient temperatures around 4°C and -2°C, when water density is highest, there will be a small reduction in kW output, to reduce the requirement for defrosting the air source heat pump, and as such provide continuous operation. For a reason outside of the scope of this article, this phenomenon is most obvious when the flow temperatures are the lowest.

As an aside from the previous paragraph, you may indeed find heat pumps which maintain the same kW output from +15°C all the way down to -10°C ambient temperature. If you find such a beast in the wild, please ensure you dig a little deeper in to how it performs with no reduction in output. It could be because it uses multiple compressors within its construction, which would then naturally increase the price of the equipment. It could also be specific laboratory conditions produce something that may be difficult to replicate in the real world. Alternatively, it could just be marketing which promotes a 15kW heat pump, which has a 21kW compressor.

Join us for part three where Chris explores the issues of electrics and efficiency.