Part 3: How to select the right air source heat pump and why?

Part 3 of our ‘How to select the right air source heat pump and why’ series from Chris Higgs, managing director of Freedom heat pumps, looks at electrical loading and efficiency.

From kW output in part two of this series to electrical loading – we hear once more from Chris Higgs on why electrical loading is important when selecting a heat pump and how efficiency is key in the choice that your customer makes.

Electrical Loading

Electrical loading in the context of a heat pump, is referred to as the fuse / breaker size that needs to be in place to ensure safe operation of the heat pump. Most heat pumps are split in to 3 breaker sizes: 16amp, 20amp and 32amp, with the breaker size determining the kW output / size of the heat pump. The larger the circuit breaker, the larger the heat pump. Please note that in almost all instances, this circuit breaker size will not take into account the domestic hot water cylinder immersion (if one is fitted), as it would be expected that the immersion would be fed off its own electrical circuit and so would have its own circuit protection.

Now we know what we mean when we refer to electrical loading, why is it important in selecting a heat pump? If you were to survey an older property, you may calculate that it has a 12kW heat load, and so requires a heat pump that requires at least a 32amp supply. If this older property has a 60amp main fuse, you may find that you struggle to use such a large heat pump on this property. Taking the 60amp fuse, we need to subtract 32amps for the supply to the heat pump, then perhaps 16amps for the supply to the 3kW immersion heater for the domestic hot water cylinder, and power for circulation pumps and 2 port valves etc. Quite quickly, we have reduced the 60amp main fuse down to 12amps of current draw left. If the owner has a 3kW kettle, which will require a 13amp fuse, when heat pump, immersion heater and kettle are all running at once, then the property will technically be in a negative amp’s scenario. In this case, either an increase in property insulation (to allow a smaller heat pump), an upgrade to the homes main fuse (to perhaps 100A), or a hybrid heat pump and fossil fuel boiler (to allow a smaller heat pump to tackle most of the heating demand) may all be relevant solutions.

If we contrast this against a new, well insulated property, we can see that this isn’t so much of a problem. Assuming a 5kW load (16amp supply), plus immersion heater in domestic hot water cylinder (16A) plus circulation pumps etc. (3A) = 35Amps. New homes tend to have 100A main fuses, so the 35A required for the heat pump system leaves lots of additional capacity in the remainder of the 100A main fuse (65A in this case).


When we talk about efficiency in the context of a heat pump, we are talking about how much thermal energy can be delivered from the heat pump, for a given electrical input. Divide the former by the latter, and you get the efficiency i.e., 5kW of thermal energy out for 1.5kW of electrical energy in equals an efficiency of 330%.

How we refer to efficiency has evolved over the years. Many moons ago in the early days of heat pumps, we had COP (Coefficient of Performance). Effectively, this was a snapshot of the efficiency of a heat pump at a specific ambient temperature and flow temperature i.e., 7°C Outside Temperature with a 35°C Flow Temperature would give you an efficiency of 500% or 5. While this was useful in the early days to compare different heat pumps with a known baseline figure, it wasn’t particularly useful when attempting to understand the efficiency of a heat pump over a year.

Next came SPF (Seasonal Performance Factors). As they weren’t outside / ambient temperature specific, they were good for giving an indication as to the efficiency of a heat pump at a specific flow temperature i.e., 45°C Heating circuit flow temperature would equate to a likely space heating SPF of 3 for an air source heat pump. Unfortunately, as they weren’t manufacturer specific, their usefulness extended only so far.

Nowadays, we have SCOP (Seasonal Coefficient of Performance). The SCOP is probably best described as an annualised COP (Coefficient of Performance). While the COP is at a specific outdoor/ambient and flow temperature, SCOP takes in to account more factors, in line with the European standard EN14511. This gives arguably the most accurate way to compare the different efficiencies of air source heat pumps. You can find SCOP data for every MCS accredited heat pump on the MCS accredited website.

To truly understand the difference between COP and SCOP, you could describe them in terms of car mpg. You could very broadly look at COP as either motorway driving, or alternatively around town driving. Neither type of driving will necessary be reflective of the efficiency you may see over a year, as you may have a mix of both types of driving. SCOP on the other hand takes a bit from motorway driving and a bit from around town driving, so you get a much closer reflection as to the efficiency or MPG you may expect to see.

It may surprise you to learn that what was held up as the superstar for a very high COP 10 years ago, is now a figure we are starting to see increasingly as a SCOP / annualised efficiency. A very high COP 10 years ago for air source heat pumps may have been 500% or 5. Within the past 12 months, we have seen equipment such as the M-Thermal range from Midea deliver SCOPs of 5.03 or 503% at a 35°C flow temperature. SCOPs such as this are a massive leap forward regarding closing the gap between air source and ground source heat pumps.

We hope this helps you to help your customers choose the right heat pump for them. Join us for part four of this series as Chris considers refrigerant type and flow temperature, and how you can educate customers when specifying the ASHP for them.