Air source heat pumps (ASHPs) are a fantastic way to warm your home while cutting carbon emissions, but picking the right size is key to getting the most out of your investment.
Whether you’re upgrading your heating system or exploring eco-friendly options, understanding ASHP sizes can feel like a bit of a maze. Don’t worry, we’ve got you covered with this straightforward guide to help you find the perfect heat pump for your UK home.
From what makes a “standard” size to why getting it right matters, we’ll break it all down with insights from our team here at Heatable.
Let’s dive in and explore everything you need to know about air source heat pump sizes!
Ready to make your home more energy-efficient and cut down on your heating bills? Get a heat pump quote today from our trusted partners at Heat Geek here.
Quick Summary:
Most UK heat pumps range from 4kW to 16kW, with physical dimensions similar to a washing machine.
A heat loss calculation considers your home’s size, insulation, and hot water needs to find the perfect fit.
The right size ensures efficiency, comfort, and lower running costs.
Insulation, bigger radiators, and outdoor space are key to heat pump success.
Expect an average annual running cost of £900-£1,200 for a typical home, cheaper than gas with the right setup.
Vaillant aroTHERM Plus 12kW for large homes; Mitsubishi Ecodan 5kW for smaller spaces.
What Size Is a Standard Air Source Heat Pump?
There’s no one-size-fits-all when it comes to air source heat pumps, but most domestic models in the UK range from 4kW to 16kW in power output.
Physically, they’re compact, think roughly the size of a washing machine, standing about 1 to 1.5 metres tall and 0.5 to 1 metre wide, weighing around 60-100kg.
Power output varies; a “standard” heat pump for an average 3-bedroom home is typically around 5- 8kW, but larger homes may need up to 16kW.
Physical dimensions: Most units are designed to fit neatly outside your home, requiring a small footprint (about 1m x 1m) with good airflow.
Also, consider your hot water requirements - if your heat pump also supplies hot water, you might need a slightly larger unit to handle the extra demand.
“We often get asked what a ‘standard’ heat pump looks like, but it really depends on your home’s needs. A 5kW unit might be spot-on for a terraced house, while a sprawling detached property could need double that.” - Patrick Garner, Gas Safe Engineer.
How to Work Out What Size Heat Pump You Need
Sizing a heat pump isn’t a guessing game - it’s about matching the unit’s output to your home’s heating demands.
If you have a new build property, feel free to use the table below to get an idea of the ASHP size you will likely need:
House Size | Number of bedrooms | Air Source Heat Pump Size | Cost |
100m² | 3 | 4-5kW | £10,000 |
200m² | 4 | 8-10kW | £10,000 |
300m² | 5-6 | 14-16kW | £10,000 |
Whether you live in a new or old property, it’s a wise idea to get a professional heat pump installer to carry out a heat loss calculation before you choose a particular-sized model.
The heat loss calculation will take into account the following:
Assess your home’s size: Depending on insulation, a rough guide is 1kW per 10-25 square metres. A 100m² home might need a 4- 5kW pump, while a 200m² one could require 8- 10kW.
Check insulation levels: Well-insulated homes need smaller pumps as they lose less heat.
Consider climate: Colder UK regions, like Scotland, may require a slightly larger unit to cope with lower outdoor temperatures.
Account for hot water: Larger households with high hot water use (e.g., multiple bathrooms) will need a pump with extra capacity.
An MCS-certified installer will measure heat loss room by room, factoring in windows, walls, and existing radiators.
If you live in a property with poor insulation and no underfloor heating, it may not be possible to have a regular ASHP, and you may be advised to opt for a High Temperature Heat Pump instead.
Why Does Heat Pump Size Matter?
When it comes to heat pumps, size matters - a lot. Getting it right is the key to unlocking peak efficiency, cozy comfort, and manageable costs.
A heat pump that’s off the mark in terms of size can turn into a real headache, throwing everything from your energy bills to your home’s warmth out of whack.
Let’s break it down:
First, there’s efficiency. A heat pump that’s sized just right hums along like a well-oiled machine, sipping electricity rather than guzzling it to keep your space toasty.
It’s all about finding that sweet spot where the system runs smoothly without overworking itself - saving you money and keeping things green.
Then there’s comfort, which is where things get personal. An undersized pump is like a little engine that could, but can’t. It’ll leave you shivering as it fights to heat your home.
On the flip side, an oversized unit is no hero either; it short-cycles, flipping on and off like an impatient light switch, which wastes energy and leaves your comfort inconsistent.
Longevity ties into this, too. A heat pump that’s the wrong size is under constant stress, whether it’s straining to keep up or overcompensating for no reason.
That kind of wear and tear chips away at its lifespan, which should ideally stretch 15-20 years. The right fit means less strain and more years of reliable warmth.
And finally, let’s talk cost. An oversized pump hits you twice - once with a heftier price tag upfront, and again with higher running costs as it chews through power. Meanwhile, an undersized one sneaks up on you, racking up bills as it works overtime to compensate.
Either way, your wallet feels the pinch. Nail the size, though, and you’re looking at a system that’s as cost-effective as it is comfortable.
Prefer video? Watch our ASHP Case Study in our YouTube video below:
How Can You Prepare Your Home for a Heat Pump?
Heat pumps work best in homes that are ready for them.
A few tweaks can make a big difference to performance and efficiency:
Boost insulation: Upgrade loft, wall, or floor insulation to minimise heat loss. This can reduce the size of the pump you need.
Upgrade radiators: Heat pumps use lower water temperatures (45-55°C vs. boilers at 70-80°C), so larger radiators (often 2-2.5 times bigger) help spread heat effectively.
Consider underfloor heating: It’s ideal for heat pumps as it works well with lower temperatures, though it’s a bigger investment.
Check outdoor space: Ensure you have a 1m x 1m flat, solid surface outside for the unit, with enough clearance for airflow.
Plan for a hot water cylinder: Most heat pumps need a cylinder (about 80cm x 80cm) for hot water storage - check you have space indoors.
Learn more about heat pumps:
Heat Pump Running Costs
Heat pumps are a savvy choice when it comes to running costs, but how much you’ll spend hinges on a few key factors: the size of your home, how well it’s insulated, and the price of electricity.
What makes them stand out is their efficiency - typically delivering 3-4 units of heat for every unit of electricity they use, a metric known as the Coefficient of Performance (COP). That’s a serious bang for your buck compared to traditional heating systems.
For a typical 3-bedroom home in the UK, the numbers paint a promising picture. Annual running costs for a heat pump generally fall between £900 and £1,200, based on projected 2025 energy prices (Ofgem, 2025).
Stack that up against a gas boiler, which could easily top £1,500 a year, and the savings start to shine through. The exact figure depends on your setup, but the efficiency edge is clear: heat pumps work smarter, not harder, to keep you warm.
If you’re swapping out older systems like electric or oil heating, the savings can really stack up. You might pocket £300 to £500 annually, especially with electricity-intensive setups giving way to a heat pump’s leaner operation (Energy Saving Trust, 2025).
Plus, there’s a sweetener: the UK’s Boiler Upgrade Scheme offers a £7,500 grant to cut those upfront costs, making the switch even more tempting. It’s a lifeline for anyone looking to ditch fossil fuels without breaking the bank.
Want to squeeze out even more savings? Pair your heat pump with a smart tariff - think cheaper off-peak rates - or hook it up to solar panels if you’ve got them. These tweaks can shave your bills down further, letting you harness the system’s efficiency like a pro. It’s all about working with the tech to fit your lifestyle.
Location plays a role too. If you’re in a colder corner of the UK, your heat pump might need to flex a bit harder in winter, nudging costs up slightly.
The unit’s still efficient, but sub-zero temps can mean more effort to pull heat from the air. Even so, with the right setup, a heat pump keeps your home cozy and your expenses in check - proof that smart heating pays off.
The Best Large Heat Pump on the Market
For bigger homes (4+ bedrooms or 200m²+), you’ll need a powerful heat pump to keep things toasty. Our top pick is the Vaillant aroTHERM Plus 12kW.
Why it shines: Delivers robust heating and hot water for large households, even in temperatures as low as -25°C.
Efficiency: Boasts a SCOP (Seasonal Coefficient of Performance) of up to 4.8, meaning excellent energy savings.
Quiet operation: Designed to run quietly, so it won’t disturb your peace.
Eco-friendly: Uses R290 refrigerant, which has a lower environmental impact.
The Best Small Heat Pump on the Market
For smaller homes or flats (1-2 bedrooms or under 100m²), a compact unit does the job perfectly. We recommend the Mitsubishi Ecodan Ultra Quiet 5kW.
Why it’s great: Ideal for smaller spaces, delivering consistent heat and hot water with a tiny footprint.
Super quiet: One of the quietest models around, perfect for urban homes or tight spaces.
Efficiency: Achieves a SCOP of up to 4.5, keeping running costs low.
Easy install: Compact design makes it a breeze to fit in most properties.
Learn more about heat pumps:
Looking for the best deal on a new heat pump?
Get a heat pump quote today from our trusted partners at Heat Geek here.
