If I remember correctly, there were around 100,000 heat pumps installed across the UK in 2024. But if that’s meant to be progress, it’s being built on sand. Behind the glossy rollout figures lies a truth the industry refuses to confront: most systems are still being sized, specced and installed based on a cocktail of guesswork and misinformation. The result? Poor performance and too many underwhelmed homeowners.
Let’s start at the root: heat loss calculations. This is supposed to be the essential first step in any competent heat pump design. But the current process is so riddled with variation that it’s become a running joke among homeowners. It’s not unusual for someone to commission three separate calculations for the same house, and get three different results: 5kW, 8kW, 12kW. How is that possible? How has the industry managed to make this so imprecise?
This is compounded by the next failure: sizing the heat pump. Even if the heat loss is accurate, most homeowners are being misled into thinking they just need a heat pump with the same number on the box. House needs 8kW? Buy an 8kW heat pump. Simple, right? Completely wrong.
Because that “8kW” is not 8kW. It’s 8kW under very specific, very mild conditions, typically 7C outside, with a 35C flow temperature. At -2C, which is exactly when you need your heating system to deliver, that same unit might barely scrape 5kW. That’s a ridiculous shortfall. And yet somehow this is industry standard practice.
It’s not just misleading, it’s unacceptable, and borders on outright deception. You wouldn’t be allowed to sell a car based on its top speed hurtling downhill with a tailwind. So why is it still permissible to badge a heat pump with a nominal output that collapses the moment it’s actually needed? Manufacturers know exactly how their units perform at different outdoor temperatures and flow rates. Yet they choose to market them using the most flattering figures, not the ones that reflect real-world performance in British winters. And regulators continue to let it happen.
Take the Caernarfon ASHP from Global Energy Systems as a real-world example. This unit was installed at our property six years ago. At the time, we weren’t provided a heat loss calculation by Global Energy Systems because we didn’t know any better. We were simply told that, given the size of our home, we needed a big unit. Global Energy Systems’ own paperwork estimated a heat loss of 20.2kW at -3C and installed what they claimed was their largest available model: an 18kW unit.
Yet the manufacturer’s own technical documentation tells a very different story. According to the spec sheet, this machine delivers:
- 12.13kW at -7°C
- 10.85kW at -10°C
Based on these figures and my sketchy maths, the unit likely delivers around 13.8-14kW at -3C, around 6kW short of the heat required to maintain 21C indoors. In our case, that gap is bridged by an existing oil boiler we chose to keep, despite Global Energy Systems recommending its removal. On paper, the system appears substantial. In practice, it’s underpowered, and we feel it every time the ambient temperature drops towards 0C. Other households without a secondary heat source could be forced to rely on electric boosters, an extremely costly safety net when temperatures drop.
“Any heat pump being sold as a 16 or 17 kilowatt unit is, frankly, a lie, unless it’s 30C outside and you’re barely asking it to do anything,” said Patrick Wheeler on a recent episode of the Homeowners Q&A podcast. It’s a complete misrepresentation of what these systems can actually deliver when it matters, in cold weather, when homes genuinely need heat.
This is not a one-off. Across the sector, heat pump outputs are routinely marketed based on mild outdoor temperatures (typically 7C) and low flow temperatures (usually 35C). But that’s not when people need heating. Peak demand occurs when it’s freezing, during those bitter, short-lived cold snaps that punctuate British winters. We accept that sub-zero temperatures aren’t sustained for weeks on end, and that oversizing a system for those extremes can introduce challenges such as cycling in milder conditions. But it’s precisely in those coldest moments that homes need reliable heat. That’s when performance matters, and it’s exactly when most systems begin to fall apart. Yet this real-world data is often buried in technical manuals, poorly communicated by installers or completely ignored.
The situation is so bad that it borders on fraudulent. We are selling systems under the pretence that they can do something they often can’t. And when it fails, it’s always the heat pump that gets the blame, never the flawed methodology that put it there in the first place.
What’s even more galling is that the industry seems to be actively moving away from transparency. The MCS has recently changed how seasonal performance is estimated, now assigning standardised COPs based solely on output temperature, regardless of the make or model of heat pump. In other words, everyone gets the same assumed performance. This is not just wrong, it’s outright lazy. It papers over real differences between machines and makes it even harder for installers or homeowners to understand what’s being specified.
James Clark, another friend of the Homeowners’ Q&A podcast, has repeatedly asked a fair question: why are manufacturers allowed to badge products using misleading nominal outputs? Why are they not legally required to declare what the system delivers at design temperature: for example, -3C at 45C flow? That’s when it actually matters.
His suggestion? Strip out the kW nonsense and rename the models with consistent, temperature-verified labels. Call it a “Bob” if it can reliably deliver 5kW in freezing conditions. A “Peter” for 6kW. A “Luke” for 7. Whatever. Just make the name reflect the reality, because as it stands, we’re selling dreams and delivering disappointment. And then we have the audacity to wonder why so many homeowners end up frustrated, cynical and cold. We’re a classic case. Cosy and snug at 7C, but we struggle when we hit 2-3C for all the reasons cited above.
It doesn’t have to be this way though. We have the data and insights. We have the tools. What the industry and the MCS don’t seem to have is the will.
This is not how we should build a serious, trustworthy low-carbon heating market. It’s time to drag the industry out of the Wild West era of heat pump sizing, and start demanding proper design, honest labelling and real accountability.
Stop badging. Start verifying. Stop bluffing. Start delivering!
I love the definitely-not-a-Vaillant heat pump you have in the picture for this article.
I’ve seen some Urban Plumbers videos talking about the same thing. Szymon specifically calls out the 7 kW aroTherm+ unit, and mentions past installs where he’s put something in that was apparently the correct size, but defrosts meant it just didn’t perform.
I appreciate your need to avoid alienating manufacturers, but if there’s any way for you to work with installers to assemble some tables showing _actual_ achieveable output at typical design temperature for the popular brands and models on the market – perhaps accompanied by some price-range info – it would surely be a huge service to the industry and for consumers.