What size boiler do I need? A Sussex homeowner's guide
The short answer: for most three-bedroom homes in Sussex with one or two bathrooms, a 24–30 kW combi boiler is the right fit. If you have more bathrooms or higher hot water demand, a system boiler is likely the better choice. Get the sizing wrong and you will either run out of hot water at the worst possible moment or pay more than you need to for a boiler that is simply too powerful for your home. Here is how we help homeowners across Brighton, Worthing, Eastbourne and beyond get it right first time.
Why boiler size matters more than most people realise
Boiler size is not about how big the box on your wall is. It is about kilowatt (kW) output, which determines how much heat and hot water your boiler can produce. Getting it right matters for two reasons: comfort and cost.
If your boiler is undersized, it will struggle to heat your home on a cold January morning or keep up with the shower demand of a busy household. If it is oversized, you will pay more upfront for a boiler you simply do not need, and it can lead to a problem called short cycling, where the boiler fires up and cuts out repeatedly, which reduces efficiency over time.
According to the Energy Saving Trust, heating and hot water account for over half of what most households spend on energy each year, so getting the right boiler matched to your home really does make a long-term financial difference.
The good news is that choosing the right size is straightforward once you know what to look for.
The three things that determine what size boiler you need
When we visit a property in Sussex to advise on a new boiler, there are three factors we always assess:
First, the number of bedrooms and radiators. More rooms mean more radiators, and more radiators mean a higher heating demand. As a rough guide, adding approximately 1.5 kW per radiator gives a working estimate of the heating output you need.
Second, the number of bathrooms. This is arguably the most important factor, particularly for combi boilers. A combi heats water on demand directly from the mains, so if two showers are running at the same time, it can struggle. The more bathrooms you have, the more output you need, or the stronger the case for switching to a system boiler.
Third, your home's insulation. Older Sussex properties, particularly Victorian terraces in Hove or Edwardian semis in Worthing, tend to lose heat more quickly than a modern new-build. A poorly insulated home needs a boiler that can work a little harder. If your property has solid walls, single glazing or no loft insulation, your heating engineer should factor this into the sizing calculation.
What size boiler for a one or two-bedroom home?
For a flat or small house with one bathroom and up to around ten radiators, a 24–27 kW combi boiler is usually the sweet spot. These properties have modest heating demands and rarely run multiple hot water outlets at once, so a compact combi does the job efficiently and without unnecessary expenditure.
Two-bedroom homes follow a similar logic. A 28–30 kW combi is the most common recommendation. If there are two bathrooms and the household regularly uses both at the same time, it is worth having a conversation about whether a system boiler with a hot water cylinder would serve you better long term.
What size boiler for a three-bedroom house?
This is the question we get asked most often, and it is easy to see why. The three-bedroom semi is probably the most common home type across Sussex, from Crawley down to Lewes.
For a three-bedroom home with one or two bathrooms and around ten to fourteen radiators, a combi boiler with a central heating output of approximately 25 kW and a domestic hot water output of around 30–35 kW is typically the right recommendation. Combi boilers are the most popular choice in the UK for good reason. According to the Energy Saving Trust, over 70% of UK households use one.
If your three-bedroom home has more than two bathrooms, or you regularly have multiple family members showering at similar times, a system boiler becomes a more sensible option. System boilers store hot water in a cylinder, meaning they can meet higher simultaneous demand without the pressure drop you might experience with a combi.
For a three-bedroom home with two bathrooms, an 18–24 kW system boiler paired with a suitable hot water cylinder is a common and reliable solution.
What size boiler for a four or five-bedroom home?
Larger properties, particularly the kind of detached and extended homes you find in areas like Haywards Heath or the villages around the South Downs, typically have more radiators and multiple bathrooms. Here, a system or regular boiler almost always makes more practical sense than a combi.
A larger house with more radiators and multiple bathrooms will generally need a boiler in the 30–35 kW range. For properties with fifteen radiators or more, or with significant hot water demand, your installer should carry out a full heat loss survey to make sure the specification is accurate rather than estimated.
One thing worth saying clearly: bigger is not always better. An oversized boiler in a well-insulated four-bedroom home is a waste of money. Modern boilers modulate their output to match demand, but there are limits, and oversizing still increases short cycling risk.
Combi, system or regular boiler: which type should you choose?
The boiler type and the boiler size are two separate decisions, and both matter.
A combi boiler is the simplest and most compact option. It heats water on demand with no need for a separate cylinder or loft tank, making it ideal for homes where space is tight. It works best in homes with one bathroom and modest hot water demands.
A system boiler uses a separate hot water cylinder but draws water directly from the mains, giving you stored hot water that is ready whenever you need it. It is the most popular choice for family homes with more than one bathroom across Sussex.
A regular (or conventional) boiler works alongside both a hot water cylinder and a cold water tank in the loft. These are less common in new installations but are sometimes the right choice when replacing a like-for-like system in an older property, as converting to a combi can involve significant additional pipework costs.
How much does a new boiler cost in Sussex in 2025?
Once you know the right boiler type and size for your home, cost naturally becomes the next question.
For a straightforward combi-to-combi replacement, most homeowners in Sussex can expect to pay in the region of £2,000 to £3,000 all in, including the unit and installation. For a system boiler installation with a new cylinder, costs typically run from £2,500 to £4,000. If you are converting from one boiler type to another, say from an older regular boiler to a modern combi, additional pipework and labour means the cost can rise to £3,000–£5,000 depending on the complexity of the job.
It is worth keeping the long-term picture in mind. Replacing an old, inefficient boiler with a new A-rated model can save households up to £580 per year on energy bills, according to the Energy Saving Trust, which means a new boiler can pay for itself over time through lower running costs.
Don't just guess: get a proper assessment
One mistake we see fairly regularly is homeowners buying a boiler based purely on what their neighbour has, or what appeared cheapest online, without accounting for their own home's specific needs. A three-bedroom terraced house in Hastings with one bathroom and eight radiators has completely different requirements from a three-bedroom detached home in Horsham with two bathrooms and fifteen radiators.
All gas boiler installations in the UK must be carried out by a Gas Safe registered engineer. Not just for legal compliance, but because a properly conducted installation includes checking that the boiler is correctly sized and commissioned for your specific property. A good installer will always assess your home before recommending a model.
At Platinum Boilers, every installation starts with a proper survey. We never just quote on the cheapest unit and push it through. Our job is to make sure you get the right boiler for your home, at a price that is fair and transparent.
FAQ
What kW boiler do I need for a 3-bedroom house?
For most three-bedroom homes in Sussex with one or two bathrooms, a 24–30 kW combi boiler is the standard recommendation. If you have more than two bathrooms or high simultaneous hot water demand, an 18–24 kW system boiler with a hot water cylinder is often the better choice. The exact output depends on your number of radiators, insulation levels and how much hot water your household uses.
Is a bigger boiler always better?
No. An oversized boiler will short cycle, firing up and cutting out repeatedly because it produces more heat than the system needs, which reduces efficiency and puts unnecessary wear on the components. Modern boilers do modulate their output, but a properly sized boiler will always perform better and last longer than one that is too large for the home.
Can I keep the same size boiler when I replace it?
If your current boiler has kept you warm and you have had no issues with hot water pressure or recovery time, a like-for-like replacement is usually a perfectly sensible route. However, if your household has grown, you have added bathrooms, or you have had ongoing performance issues, your installer should reassess sizing before recommending a replacement.
How do I know if my boiler is the wrong size?
The most common signs are running out of hot water during busy periods, radiators that take a long time to heat up, or an unusually high gas bill for the size of your home. These can all point to an undersized or ageing boiler that is struggling to keep up with demand.
Does a Gas Safe engineer need to install my boiler?
Yes. In the UK, all gas boiler installations are legally required to be carried out by a Gas Safe registered engineer. This is not optional. An unregistered installation will void your warranty, breach building regulations and could pose a serious safety risk.
If you are in Sussex and thinking about a new boiler installation or replacement, Platinum Boilers would be happy to help. We provide fixed-price quotes, honest advice on sizing and type, and professional installation across East and West Sussex — from Brighton and Hove to Horsham and Haywards Heath. Get in touch with our team today for a no-obligation quote, or take a look at our boiler installation page to find out more about how we work.
