What you need to think about when installing an air source heat pump

Global Energy Systems

Those of you who have followed our journey at My Home Farm will know that we installed our air source heat pump in 2018 when information about heat pump systems was relatively hard to come by. Despite doing our utmost to screen installers (we had six companies that quoted us) we ultimately had a system installed by Global Energy Systems that we’d grade at about 70%. The heat pump itself is fine, but it’s all the supporting bits that turned out to be contentious.

Four years on, if we were lucky enough to have a do-over, there are a lot of things we’d have done very differently and we would have been insistent on certain things being done.

There is a large and growing installer base in the UK and many of them continue to install heat pumps as they would any other boiler, and this is what leads to complications.

The problem is compounded by the fact that it’s salespeople that visit your property and quote you for your ‘requirements’, and in many instances they’re not qualified or technically equipped to make the necessary suggestions for you to have a highly efficient system; especially in retrofit projects.They just want to book the sale.

So if you don’t know what you’re looking for and don’t have the requirements mapped out in your head, you could get sold anything and it’s an expensive game of Russian roulette on whether you get a system that’s excellent, good, average or not fit for purpose.

Once you’ve been sold a system and signed on the dotted line, the installation team will arrive and fit your new heat pump based purely on the salesperson’s recommendations and paperwork.

This is precisely what happened to us, and if you don’t request certain things to happen you’re likely to end up with a system that is below par, which is unacceptable when you’re paying in excess of £12,000 (the UK average) for a new air source heat pump.

This post is not meant to be a definitive guide on what every system should entail. Each property is unique with its own requirements, and we just want to steer you in the right general direction so that some important bases are covered.

System design, heat loss calculations & heat pump size

There’s a lot that goes into system design like your pipework and heat emitters. Everything in a well-designed heat pump system should be balanced to work efficiently. You need a heat loss calculation to show you how much heat your home loses and what size heat pump will be able to generate enough heat to keep your house warm when temperatures outside drop to 0C or below.

This all needs to be considered in line with your heat emitters and pipework. Pipework in retrofit installs like ours is a problem because you may not know what the bore size is and this can cause problems, but there are workarounds.

Most importably, you need to pay attention to your heat emitters. These are your radiators or underfloor heating. If you have existing underfloor heating that was designed for an oil or gas boiler, it typically isn’t a problem. Our underfloor heat was designed and spaced for an oil boiler and the air source heat pump delivers the perfect amount of heat.

Radiators are different, and our biggest suggestion is that if radiators are touch and go whether they should be replaced as part of your heat loss calculations we suggest upsizing them because it’s not a massive cost in the greater scheme of things, and the delivery of heat is noticeable.

We had double fin rads throughout the property and there were some rooms that struggled to come to temperature (we think as a result of pipework issues) and this was resolved by simply replacing those rads with Stelrad K3s. The difference was massive, and we’re now fully utilising the heat generated by the heat pump in those rooms.

Get bigger rads if they fit your rooms. You won’t regret it because if a heat pump is to cope with icy outdoor temperatures, the emitter temperatures will probably have to exceed what was outlined in your calculations, and the bigger the rads, the easier it’ll be for your heat pump to cope.

Weather compensation

In my opinion, most air source heat pumps should be set up to run on weather compensation. Due to a lazy install, our system cannot run on weather compensation properly because of shortcuts undertaken by the installers.

You should insist that the system is set up to run on weather compensation from autumn to spring. This should be a non-negotiable.

Many installers don’t want to enable weather compensation because in months like October and November rads can feel cool and homeowners complain that they’re not hot. A system correctly set up on weather compensation will ensure that all rooms are warm, hitting their target temperatures, without killing you financially on your electricity bill. Just be prepared to adopt a shift in heating mentality because rads do not have to be hot to heat rooms and keep them warm.

There may also be a period of trial and error, so get your installer to educate you on how you can change heat curves for weather compensation on your system so that you can get it just right for your property. This will empower you in the long term. It can be a bit daunting at the outset, and if you’re struggling, you can always post a question on our forums.

Buffer tank debate

This is a tricky one. Buffer tanks or cylinders provide an additional volume of water to assist heating systems, and heat pump manufacturers can’t decide whether you need one or not. Installers are also divided.

Heat pumps need adequate flow to be efficient. Our heat pump alarms or stops when there’s insufficient flow. Heat pumps should also not cycle (coming on for brief bursts) so installers like to fit a buffer tank to work around this.

Conversely, when there’s no buffer, the heat pump simply circulates to the emitter circuits (rads or underfloor heating), but the system must ensure sufficient water flow at all times or you’ll have heat pump dramas.

So installers like to fit buffer tanks because they can eliminate issues when retrofitting a heat pump in a system where they don’t know exactly what’s happening in a heating circuit. Buffer cylinders increase the installation price, take up additional space and can affect the efficiency of your system.

This’ll be a big decision to make because by many accounts buffer tank solutions can reduce your COP. Nevertheless, most installers continue to recommend and fit them because it makes their life easier in the long-term, but it may not necessarily be the best solution for your system.

Don’t be afraid to ask questions

Our biggest suggestion is to ask a lot of questions. And if you’re not getting definitive answers, this should set off alarms in your head about your installers.

If an installer can’t address the subjects we’ve listed above, you’re probably better off looking for another installer because heat pumps are expensive to fit. Sadly, there are a lot of cowboys out there, so be sure you’re hiring the right outfit.

If you’re uncertain about anything, post your reservations or thoughts on our forums, and we’ll try to point you in the right direction.

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Paul Lewis
Paul Lewis
1 year ago

Hi Mars
Just watched a couple of YouTube vids and jumped on to this site. I got a question I have just had air source installed we have a 7 bedroom stone converted barn 329 square meters very rural very similar to you I guess. We have had internal insulation on all external walls and 24 sloar panels on the roof feeding to 2 invertors. Unfortunately no buy back at the moment due to. signal issue where we live.
The air source is a grant aerona 3 17kw with the minus temps outside currently we are only hitting temps inside of around 18 degrees with the air source running for average 13 hours per day. We are calling for the default temp of 22 never reaches it. We are new to airsource installed end Nov 22.
Do you have any tips to run it more efficiently as on average it is costing us £30 a day just on air source alone to keep it running which is unsustainable in today’s climate.
What is the weather compensation setting you talk about is that something I can change on the controller?
You have had airsource for alot lo ger and I’m sure have ironed out all the issues it comes with.


Paul Lewis
Paul Lewis
1 year ago

Hi Mars

Sorry forgot to add in previous comment we have a dual air source system feeding 2 tanks because of size of house and that feeds 27 radiators that like you say only become warm not hot


Paul Lewis

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