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Here is a question we get asked a lot: why is the heat pump you’ve recommended to me smaller than the heat pumps other installers have recommended?
When it comes to heat pumps, size really does matter, and not in the way you might think. Unlike boilers, where oversizing was often considered a safe bet, heat pumps need to be precisely matched to your home’s specific heating needs to perform at their best. Get the sizing wrong, especially by going too big, and you could end up with a system that’s less efficient, more expensive to run, and prone to problems.
In this blog, we’ll explore why choosing the right size heat pump is essential, and what can go wrong if you don’t.
What does 'sizing' a heat pump actually mean?
Heat pumps come in different capacities, usually ranging from around 3kW to 16kW or more for domestic use. The correct size depends on how much heat your home loses and how much energy is required to keep it warm on the coldest days of the year.
This is determined through a room-by-room heat loss calculation, which takes into account:
The goal is to match the heat pump’s output as closely as possible to your home’s actual heating demand.
What happens if your heat pump is too big?
It’s easy to assume that going bigger just gives you a bit of extra power — but in reality, oversizing can seriously impact performance, efficiency, and comfort.
Here’s how:
1. Short cycling and system stress
Oversized heat pumps deliver heat too quickly. This causes the system to short cycle, turning on and off repeatedly, rather than running steadily. It’s like revving your car engine in traffic all day: inefficient, noisy and bad for long-term wear.
The result?
Higher energy bills, more wear on components, and a shorter system lifespan.
2. Poor efficiency (Lower COP)
Heat pumps are most efficient when running continuously at a low speed. Oversized units rarely get the chance. They cycle up to high output, overshoot the set temperature, and then shut down, wasting energy in the process.
This lowers the system’s COP (Coefficient of Performance), the very thing that makes heat pumps such a low-cost heating solution in the first place.
3. Higher upfront costs
Larger heat pumps cost more to buy, more to install, and may even require larger buffer tanks, cylinders or electrical upgrades, none of which you actually need if the system is sized properly.
So you're paying for capacity you’ll never use, and reducing your return on investment.
4. Uncomfortable indoor temperatures
It might seem odd, but an oversized heat pump can actually create temperature swings in the home. Because it delivers heat in short, intense bursts, you get a less consistent and comfortable indoor climate, especially if your controls aren’t properly calibrated.
Getting it right, what to look for
When we design a heat pump system, we don’t size it for some once-in-a-decade Arctic blast. Instead, we base our design on the 35 coldest hours in a typical year, and let’s be honest, those usually happen in the middle of the night when you’re tucked up in bed under a duvet. No one’s checking the thermostat at 3am. This approach means your system is designed to run efficiently and comfortably when it really matters, without overspending on unnecessary capacity you’ll probably never need.
Why does that matter? Because if you size a heat pump for the absolute coldest day imaginable, rather than the real-world climate you actually live in, you end up with a system that’s oversized, more expensive, and less efficient. It's like buying a 4x4 just to drive through the occasional puddle.
This is where Warmur technology gives us a real edge. It uses 10 years of historical MET Office climate data, allowing us to design your heating system based on what the weather is actually like where you live based on our postcode and altitude, not outdated assumptions on the nearest weather stations (there is only 2 for the central belt) or one-size-fits-all estimates.
The result? A system that’s right-sized, cost-effective, and energy-efficient, giving you reliable comfort all year round, without overpaying for capacity you’ll never use. That’s smart design for a smarter, low-carbon home.
To get the right heat pump for your home, you’ll want to work with a qualified, experienced installer who will:
✅ Carry out a full room-by-room heat loss calculation
✅ Design the system based on your home, not guesswork
✅ Recommend the best heat pump type and size for your needs
✅ Assess whether your current radiators and pipework are suitable
✅ Commission the system correctly, including flow temperatures and controls
✅ Explain how to use it efficiently once it’s installed
Done well, your heat pump will quietly keep you warm, year after year, all while cutting your bills and your carbon footprint.
A heat pump is one of the best home upgrades you can make, but only when it’s the right system for your home. That means asking questions, working with a trusted installer, and never settling for a generic or rushed approach. Because the cost of getting it wrong isn’t just financial, it’s the comfort of your home, your confidence in renewables, and your ability to meet your low-carbon goals.
We’ve installed over 1,000 heat pumps across the UK, each one carefully designed to create warm, comfortable, energy-efficient homes. From cosy cottages to large castles, our team has the experience to get it right, every time.
We’re now proud to offer a new range of high-performance heat pumps that combine excellent value with impressive efficiency. These systems are specifically designed for the UK market, with sizes and specifications tailored to match the heat loss profiles of British homes, not oversized for colder climates abroad.
Our range includes:
We also work closely with UK-based manufacturers who understand our climate and housing stock, ensuring every system we fit is properly sized, expertly commissioned, and built to perform.
If you’re looking for a reliable, efficient and well-priced heat pump solution for your home, we’re here to help.
Here is another article we’ve written about the hidden cost of getting the wrong heat pump - click here.
