I was on a panel discussion just before Christmas and at a certain point, this exchange happened:

Me:  There are maybe 950,000 on heat networks in the UK…

Host (interjecting): Hold on a second, I thought there were something like 450,000 domestic connections!

Me: Nope, it’s a lot more than that. I’d be happy to show my working if you like.

This moment struck me because it highlights a widespread and significant misunderstanding about the size of the UK heat network market. Even among trade associations and government departments responsible for heat network policy, people routinely rely on outdated or inaccurate figures. This matters, because the way we perceive the scale of the heat network market shapes investment decisions, policy development, and public awareness.

So how many domestic connections on UK heat networks are there really? The answer is, nobody knows exactly. But we can get to a good estimate easily enough. Let’s walk through the details.

A CMA report on the heat network market in 2018 said there are 446,517 domestic customers on heat networks. This number is sourced from a market report from the ADE, which, in turn, used notifications submitted under the Heat Network (Metering and Billing) Regulations 2014 (HNMBR). These regulations required all heat suppliers to notify the government of their heat networks by the end of 2015.

BEIS published the 2015 HNMBR figures for the first time in 2017, at which point they were already two years out of date. Also, because they represent self-reported data, the figures are almost certainly an underestimate: they exclude heat network operators that weren’t aware of (or didn’t take action on) the requirement to report the existence of their heat networks to the government in 2015.

As it happens, the same underlying dataset from 2015 got cleaned up and published a second time in 2018. This time they estimated 439,549 domestic connections.

Let’s split the difference and call it 440,000 domestic connections at the end of 2015, ignoring that it’s probably an underestimate.

Under the HNMBR, heat networks have to submit another notification every four years and new networks have to provide a notification when they go live. So can’t we just use newer HNMBR data?

Unfortunately, no.

DESNZ released new HNMBR data covering a period ending in December 2022. 

This new data implied that between 2015 and 2022 there had been a net addition of only 37k domestic connections on heat networks, or around 5k a year. That’s obviously inaccurate; as we’ll see in a moment, the market was probably growing at ten times that rate. It’s far more likely that heat network operators had already worked out that HNMBR wasn’t being enforced and had simply stopped providing notifications under the regs. In any case, newer HNBMR data isn’t reliable enough for us to use.

How can we estimate the rate at which the market grew from 2015 until today? One way is to use sales figures for heat interface units (HIUs) as a proxy for the rate of growth of the market overall. Luckily the folks at BSRIA compile and publish a report on the UK HIU market every year or two.  

Looking at the BSRIA reports that Guru purchased, the figures for HIU sales volumes by year look like this:

2019: 60,600 (actual)

2020: 57,400 (actual)

2021: 67,700 (actual)

2022: 74,500 (estimate)

Let’s walk through the arithmetic, starting with the 440,000 domestic connections in 2015 and adding the annual growth from the HIU data. In the interests of being conservative, let’s assume just 50k new connections for 2016-2018. Let’s also assume that BSRIA’s estimate for 2022 was overly optimistic and use the 2021 figure for 2022-2024. Where does that put us?

440,000 (2015) + 50,000 (2016) + 50,000 (2017) + 50,000 (2018) + 60,600 (2019) + 57,400 (2020) + 67,700 (2021) + 67,700 (2022) + 67,700 (2023) + 67,700 (2024)

This brings us to an estimated 978,800 domestic connections on UK heat networks. Given that we’ve used conservative assumptions (such as 50,000 connections per year for 2016-2018 and the 2021 BSRIA estimate for 2022-2024), it’s very likely that the actual number of domestic connections is higher than this total — potentially significantly higher.

Based on the above, I think in 2025 we can say that there are about a million homes on heat networks in the UK. As this number grows, so too does the importance of accurately measuring and understanding the scale of heat networks. The mistaken reliance on outdated figures can hinder investments, slow the pace of policy development, and obscure the vital role that heat networks play in the UK’s future energy landscape.

Heat networks are not a niche or fringe technology — they are already a major part of the UK’s energy infrastructure. In 2025, the question shouldn’t be “How many homes are on heat networks?” but “What’s next for the million homes already there — and the millions more that will follow?”

¹I know this double counts the retrofit of HIUs to existing homes on heat networks (leading to overestimation), but I don’t believe that there were many. And anyway it also omits new homes on heat networks that didn’t have HIUs (leading to underestimation). I’m also aware that the BSRIA figures are self-reported by HIU suppliers (and so may perhaps lead to overestimation) but this is the best data we have in the interest of getting to an approximate figure.

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