What I’d do differently now

21 Feb 2024

I’m very happy with my heat pump, but I blundered my way through the installation - this is what I’d do differently now

I’ve been up and running with my heat-pump for about a year now. I’m going to write separately on how it’s performed and its costs relative to my previous gas heating. (In summary: pretty good.) In this article I’m going to reflect on my experience of installing a heat pump and, I hope, make the journey smoother for others motivated to embark on it.

Here I’m going to write about retrofitting an air source heat pump and linking it to an existing hot water central heating system with radiators or under-floor heating. There are of course many other approaches including ground source, air-to-air, new build and so on, but I’m going to focus on what remains the most common configuration for existing homeowners wanting to move to a renewable energy system for their home heating.

The reality is that the UK market remains immature. The retrofitting homeowner has to contend with planning rules that can seem designed to discourage rather than encourage heat pumps; a patchy market for installers; and the potential need to upsize radiators and get a larger hot water tank. All of these are likely to get easier. The Government has announced a review of planning requirements for heat pumps; upskilling in the installer industry is underway; and improvements in heat pump technology will likely get us closer to the point where they can simply be plugged into an existing system in place of a gas boiler. But we’re not there yet and so the retrofit market remains dominated by those with the resources and interest to try to encourage the market for this important component of decarbonisation.

 I blundered my way through the installation of my system and would’ve had a smoother (and significantly cheaper) ride if I’d planned better rather than just bulldozing through.

 Here are the four key lessons I’ve learned.  

1.     Build your knowledge

If you are planning to retrofit a heat pump, you’d be well-advised to be a knowledgeable customer. Heat pumps can work every bit as well as a gas boiler at similar or lower ongoing cost but, given the unusually high difference (a factor four) we have in the UK between the price of a kwh of gas and electricity, the system needs to be well-installed to do so. The current quality of the market for installers is variable and good ones often have too much work and are stretched. How the heat pump is installed also affects the upfront cost.

As I was going through the installation process I came across Heat Geek. This organisation is trying to raise the quality of installers across the country, but their YouTube videos are also great viewing for a potential purchaser. The knowledge I’ve gained has been very helpful to me in fine-tuning our system to improve its efficiency and would’ve helped me think through the installation in a more structured way rather than changing my mind at various points of the job, which added to my installation costs.

Essential areas in which to build knowledge are the basics of heat pump efficiency, low-temperature heating, and weather compensation. A full description of these concepts is beyond the scope of this blog. But, in brief, the magic of a heat pump is that, rather than generating heat, it gathers it from outside your home, concentrates it, and moves it inside by heating up water to go through your radiators. Because a heat pump gathers rather than generates heat, it takes only one kilowatt hour (kwh) of electrical energy to put as much as four or five kwh of heat energy into your house. It is this fantastic efficiency that enables a heat pump to have comparable, or even lower, ongoing costs compared with a gas boiler, despite UK electricity being around 4x as expensive as gas per kwh.

But to get this level of efficiency, you need a well-installed system that pushes water through your radiators at somewhere between 30C and 50C rather than the 70C commonly used in our central heating systems. It’s possible for a heat pump to heat water to 70C, but the efficiency at these temperatures can drop by half, which would make the ongoing energy bills much higher. So, rather than occasionally blasting the house with high amounts of heat to bring it up to temperature, with a heat pump you need to drip feed heat in the whole time.

Therefore, an efficiently installed heat pump will be operating most of the time during the heating season, drip feeding exactly the amount of heat that your house is losing through its walls, floors, windows, and roof at any time. This required amount of heat is lower when it is warmer outside (as your house will cool less quickly) and so the water temperature in the system should reduce (increase) as the outside temperature increases (reduces). This is called weather compensation. You will often hear that you need larger radiators for a heat pump – that may be true, so you can run the system at lower water temperatures while still giving the required amount of heat for the house. This is probably true, but may not require you to change all your radiators.

At some point technology will probably develop so that heat pumps can essentially be plugged in to replace a gas boiler with no other changes, but that’s not the case today. Building your knowledge before seeking quotes can help you test the knowledge and experience of potential installers as well as help you to optimise the performance of your system once installed.

 I built my knowledge as I was going along. In the end it didn’t cost me too badly. I chose what turns out to be an excellent brand of heat pump and their authorised installer turned out to be very competent. But it could easily have turned out differently if I’d been unlucky. And by chopping and changing along the way I certainly added a lot of cost.

2.     Plan ahead

Think through the installation and all the consequences. There are a number of aspects to this.

Do you need planning permission?

At the moment it is quite common for a heat pump installation to trigger planning permission requirements. Two common triggers are:

• The location of the heat-pump: is it going to be within 1m of the boundary with your neighbour’s property? (Quite possibly if you want to put it down the side of your house.)

• The size of the heat-pump: is it more than 0.6 cubic metres? (Quite possibly if you have a detached house with four or more bedrooms as you may well need a double rather than single fan compressor unit.)

The requirement for planning permission will normally trigger the need for a commercial noise survey (cost around £1,000) to check whether the noise from your heat pump will be below the background noise level (in the middle of the night) at the closest window of an inhabited room in your neighbour’s house. It quite likely won’t be unless that distance is 5 metres or more depending on the noise rating of the heat pump .

I got caught out by these rules and was insufficiently prepared (I’ve written about my travails elsewhere). The noise rules are a bit of a nonsense, because modern heat pumps are very quiet and are only rarely operate at maximum power (and hence noise). The times they are operating at maximum power is when it is below freezing and our neighbour has their (double-glazed) windows shut, cutting out any noise.

The good news is that the Government has announced a very pragmatic review of planning rules which would remove both of the requirements set out above, but the rules will still, for most heat pumps, require them to be situated at least 3 to 4 metres from the neighbour’s nearest window.

Fully understanding the planning implications is an important part of any installation. As it turns out, through being much more proactive rather than passive on the planning process, I have now got retrospective planning permission for my heat pump by committing to operate it on low power mode at night-time and moving it two metres down the side of my house. The costs of not doing this upfront were significant: I installed a hybrid system, which is more complex (and costly) to install, operates less efficiently than a simple heat pump system, and which did not qualify for the £5,000 Boiler Upgrade Scheme subsidy that was available. Ouch.

If possible, combine the heat pump installation with other renovations

A great time to install a heat pump is when you are having other work done. This can create the opportunity to change some pipework, move or add a radiator, put in underfloor heating, or add some insulation - all of which can make the system work better. When we started off on the heat pump adventure, we were vaguely considering having work done, but hadn’t decided. I charged ahead with our heat-pump implementation regardless. Our central heating pipework was a bit bizarre and involved water travelling from the boiler downstairs all the way up to the top of the house before returning back down through the radiators from top to bottom. To avoid taking floors up to explore pipework, we needed to run some broader gauge pipework upstairs to connect into the start of this system in order to enable the higher flow rates needed for a heat pump. This made balancing the system up complicated as the radiators in the parts of the house we most wanted heating were at the end of the radiator circuit It also added cost.

We have subsequently had some quite substantial work done to our downstairs, and the opportunity to lift a few floorboards (adding underfloor insulation) and change the heating circuit has made the system work much better, and would have been cheaper than running pipework upstairs. If we’d synchronised the work we would have saved ourselves time and money.

Combining with other work also helps with radiator placement. Because heat pumps operate at lower water temperatures, in order to give the same heat output into a room, the radiators may need to have greater capacity. You can increase radiator capacity in three ways: increase the thickness of your radiators, e.g. moving from a one-panel to a two- or three-panel radiator; increase the dimensions of the radiator; or add another radiator. We upgraded most of our radiators through the house. Because we initially did this without having further work done, we chose to keep radiators in their existing positions and with the same dimensions but just move from one to two or two to three panel radiators. We probably didn’t need to do that for a number of the upstairs rooms, which we tend to have quite cool anyway. And downstairs, now that we’ve had the work done, we’ve changed the radiator configuration a bit to remove one of the thicker radiators and replace it by two thinner and less disruptively positioned ones.

So overall, if we’d thought ahead more and waited to install the heat pump until we were having other work done, we’d have had a cheaper installation.  

Really think through what you want from your system

When I got the bad news that planning permission would only be granted from 6am to 10pm for my heat pump I had no choice (I thought) but to keep the gas boiler. I therefore initially asked the team to link the heat-pump to the heating circuit only, keeping the gas boiler for hot water. This would have enabled me to keep the existing hot water tank, and would also probably be cost (although not carbon) optimising, as heating hot water is the least efficient aspect of a heat pump’s operation. Then at the last minute I decided I wanted a system where I could take out the gas boiler at a later stage if I wanted to and also one where it was fully interoperable, enabling me to use either boiler or heat pump for either heating or hot water. Changing my mind at the last minute required reconfiguration of the existing system and pipework, including a new hot water tank, and significant additional expense than if I’d taken the time to really sort my thinking out properly.

I’m now glad I did this as with my revised planning status I now don’t use the gas boiler for anything, other than a comfort-blanket as a back-up. And I may now even remove it, if I can benefit from the boiler upgrade scheme in retrospect. But changing my mind inevitably added cost.

3.     Try before you buy

Gas boilers are normally put in with a power rating well above what is needed for the house to enable them to push water round radiators at high temperature in order to heat a house quickly. Indeed, it’s estimated that most condensing gas boilers operate at too high a temperature for them to condense (a process that captures excess heat from flue gasses to heat the water and improve efficiency) and so operate below their potential efficiency. In the past that hasn’t mattered too much. The potential efficiency gains from a gas boiler operating in condensing mode are only around 10% and gas was cheap. However, gas boilers also operate more efficiently at low temperatures. This is why money-saving advice during the energy cost crisis has highlighted the benefits of turning down the water temperature on your boiler.

There is no reason why you can’t operate a central heating system with a gas boiler at much lower water temperatures to start getting a sense of what life will be like with a heat pump. If you have a gas boiler and are thinking of moving to a heat pump, I think there would be some benefit in trying this out. So pick a spell when it is going to be less than 5C and reduce your boiler flow temperature to, say 45-50C, but leave the heating on all the time. Or if you want to be even cleverer, try running it at 50C if it’s less than 5C outside, 40C if it’s 5-10C outside, and 30C if it’s above 10C.  

This has a couple of benefits. First, it will help you build an understanding of how the heating will work once you have a heat-pump. Second, it will tell you whether you really need to upgrade your radiators or not. If your house is comfortable when it’s 0C outside with a flow temperature at 50C you can probably operate a heat pump on your existing radiators. In retrospect I think we changed some radiators unnecessarily, especially upstairs where we like the house cooler anyway. If your existing radiators work ok when it’s 5C outside, you may even decide to operate a heat pump without changing any of them and have a couple of auxiliary heaters to cope with the small number of very cold days in the year, rather than designing your whole system around the few worst case days.

If you can do this during a cold snap, you can also, by monitoring your gas usage over that period, get a really quite accurate measure for how much heat you have to put into the house to keep it at a nice steady temperature when it’s cold outside. This will help with making sure you don’t get a heat pump that is bigger than necessary.

4.     Be prepared to optimise

The one aspect of my installation that I felt left something to be desired was the final commissioning of the system. Heat pump manufacturers and installers are paranoid about customers being cold after installation. They therefore seem to prioritise comfort over efficiency. As an example, because of my hybrid system, Vaillant had apparently insisted that the heat pump operate at a fixed water temperature of 50C with the control operating like a conventional on-off thermostat.

However, this is a very inefficient way to run a heat pump. When I noticed the early efficiency stats weren’t what I expected, I had to take a crash course in heat pump optimisation, courtesy of Heat Geek. Hopefully your installer will do this for you, but it’s worth having some knowledge. To operate at maximum efficiency you want your heat pump to be operating at the lowest possible water temperature consistent with your house being warm enough. You can play around with this by adjusting down the heat curve (the relationship between the outside temperature and the temperature of the water in your system) until your house is just perfectly warm. By doing this you make sure the heat pump works almost constantly rather than constantly cycling on and off as the house hits target temperature, which is a disaster for efficiency.

Fail to prepare, prepare to fail

Undertaking a heat pump retrofit is, today, far from a plug-and-play operation. I’m confident that this will improve over the next five years, but we’re not there yet. I blundered through my installation process, making a number of mistakes along the way and veering around like a wonky shopping trolley. I largely got away with it in terms of the final quality of my installation, but it made the installation more expensive than it should have been and has probably left me with a more complex hybrid system than I really needed.

I hope you can learn from my mistakes and do better. But if you’re able, I’d encourage you to go for it. It will heat your home very nicely, ongoing costs, based on my experience, will be lower and you’ll be contributing to getting the market to operate at scale. In that way you can make a real contribution to decarbonising home heating.

---