Guide

Part 4: How to select the right air source heat pump and why?

Part four 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 environmentally friendly refrigerant types and heat pump flow temperature.

Here, Chris Higgs brings us part four of our series, taking a closer look at refrigerant types for heat pumps and how there has been a shift to more environmentally friendly refrigerants in recent years, as well as the 3 designations of heat pump flow temperature.

Did you miss part three? Find it here.

Refrigerant Type

10 years ago, when Freedom Heat Pumps was first founded, most air source heat pumps in the UK market, used R410A or R407C refrigerant. R407C as a refrigerant was good at temperatures up to around 65°C but hasn’t been seen in the mainstream for a good few years. For many years, R410A was the de facto standard refrigerant for domestic heat pumps installed in the UK. Over the past couple of years though, we have seen a shift towards more environmentally friendly refrigerants such as R32 and R290. So, what factors determine which refrigerant you will use? We would argue that there are 2 main factors: environmental impact and application / flow temperature.

When we talk about environmental impact, we are referring to the Global Warming Potential (GWP) of the refrigerant. In basic terms, the GWP refers to the increased harm 1kg of a refrigerant would do in comparison to 1kg of carbon. So, looking at the refrigerants mentioned above, from worst to best, here is how they compare

R410A – GWP 2,088
R407C – GWP 1,732
R134a – GWP 1,300
R32 – GWP 675
R290 (Propane) – GWP 3
R744 (CO2) – GWP 1

So, from a purely environmental standpoint, a move towards R744 or R290 would make the most sense, but this is only one of the 3 selection criteria.

What do we mean by application / flow temperature? We mean is, what is the maximum temperature the chosen system is designed to? For a domestic retrofit using cast iron radiators, R744 or R134a would make sense as those are both capable of delivering very high temperatures (up to 80°C). R744 has the downside of needing a wide temperature difference between flow and return (between 35 and 60°C temperature difference) which limit its viability in domestic installs, and R134a needs to operate within a cascade refrigerant system and so will be a split heat pump system. If very high temperature is a must have, then we would tend to err towards R134a. For a “standard” domestic retrofit or new build, R32 (and perhaps less mainstream R290) has become the go to refrigerant of choice with its 60-65°C maximum flow temperature and 5°C Delta T. It slots quite neatly into the place left by R410A by providing higher temperatures, better efficiencies and a lower GWP.

Flow Temperature

As far as our industry is concerned, there are 3 designations of heat pump flow temperature: medium temperature, high temperature and very high temperature. Medium temperature classically has a flow temperature which doesn’t exceed 55°C.

High temperature tops out between 60 & 65°C and very high temperature sits around the 80°C mark. As far as what determines the flow temperature, this is all dependent on the application. Commercial hot water systems may have a specific requirement for a stored temperature of 60-65°C, so while on the face of it a high temperature heat pump sounds like it may tick the box, I’d be inclined to specify a very high temperature heat pump to ensure that any losses in the cylinder heat exchanger / coil don’t end up causing the stored temperature to be less than designed.

Commercial hot water systems aside, 99% of UK heat pumps will provide domestic hot water with no problem (provided they are coupled to a heat pump specific hot water cylinder or similar), so the key determining factor regarding flow temperature will be the emitter (heating) circuit. Chances are, if your project has heat pump specified underfloor throughout, the maximum designed flow temperature will be no more than 40°C and so a medium temperature heat pump will do. If the project is the typical new build radiators upstairs and underfloor down, then your flow temperature is determined by the radiators, so this should be no more than 50°C mean water temp, again meaning a medium temperature heat pump will suffice. An example of a medium temperature heat pump would be the Hitachi Yutaki RASM4.

So, when would you need higher temperatures? Retrofit. Most high temperature heat pumps in the UK are monoblock and operate with either R290 or R32 refrigerant. High temperature heat pumps potentially allow you to reuse some if not all the radiators on your customers property. Please approach this with caution though, and make sure you speak to your radiator supplier to understand what effect a 60°C mean water temp would have on the output of the radiator.

An example of a high temperature heat pump would be the Midea MHC-V16.

In the case of cast iron rads, single panel rads, or properties where increasing the size of or number of radiators in a particular room would be unfeasible, then very high temperature heat pumps are a brilliant choice, with their 80°C flow temperature. Traditionally all very high temperature heat pumps were split type systems with an R410A refrigerant outdoor unit linked with refrigerant pipework through to an R134A refrigerant indoor unit. An example of a very high temperature heat pump would be the Hitachi Yutaki-S80. Nowadays, we also have CO2 refrigerant units which can provide the same very high temperatures but in a monoblock construction. CO2 (R744) air source heat pumps have their own specific requirements for efficient operation, so careful design is required when looking to use the current range of small CO2 units on domestic properties.

Join us for part five of the series as Chris looks at split vs mono heat pumps – and how you can specify the right choice for your customer.