It’s the question we get asked most often when a customer’s thinking about a new boiler. And almost every time, the answer they’ve been given by another company is wrong.
The honest truth is that “what size boiler do I need” isn’t a simple question with a simple answer. It depends on the type of boiler you’re fitting, the size and condition of your property, how many hot taps and showers are likely to run at once, and a proper calculation of how much heat your home actually loses. The “two bedrooms equals 24kW, three bedrooms equals 30kW” rule that gets thrown around isn’t sizing, it’s guesswork.
Why “Size” Means Two Different Things
The first thing worth understanding is that boilers do two jobs, and the size you need depends on which one matters most for your property.
A boiler heats your home (central heating) and your hot water (showers, taps, baths). Some boilers do both on demand, whereas others heat hot water in a separate cylinder for later use. The numbers you see quoted in kW (kilowatts) refer to the boiler’s heat output, but how that output is split between heating and hot water depends entirely on the type of boiler.
This matters because sizing a combi boiler is mostly about hot water demand, while sizing a system or regular boiler is mostly about heating demand. Confuse the two, and you’ll end up with a boiler that’s either underpowered for the way you actually use it, or wastefully oversized for what your home actually needs.
The Rule of Thumb (& Why It’s Misleading)
The shortcut most installers use is something like this:
- 1-2 bedrooms: 24-28 kW combi
- 3 bedrooms: 28-32 kW combi
- 4+ bedrooms: 32-40 kW combi or system boiler
This works often enough that most plumbers use it as their default, but it ignores almost everything that actually matters.
For example, a 4-bedroom modern new build with proper insulation and a single bathroom might genuinely need a smaller boiler than a 2-bedroom Victorian terrace with single-glazed windows and a poorly insulated loft. A retired couple in a 3-bedroom house with one bathroom uses water completely differently from a family of five with two bathrooms and a teenager who likes long showers.
Therefore, sizing by bedroom count is sizing for the average, and the average isn’t your home.
Combi Boilers: Sized For Hot Water
A combi boiler heats hot water on demand, so when you turn the tap, the boiler fires and water flows. The kW rating of a combi indicates how quickly it can heat water, which in turn determines the flow rate you get from your taps and showers.
In terms of kW numbers, a 24kW combi will struggle to run two hot taps simultaneously at decent pressure. A 30kW combi handles a bath and a kitchen tap at the same time without much drop in flow. A 35kW or larger combi gives you flow rates that approach mains pressure, even with multiple outlets running.
But here’s where it gets interesting. The central heating demand for almost any UK home is between 7kW and 15kW, even on the coldest day of the year. So when you fit a 30kW combi, around 15kW of that capacity is sitting unused for heating duty, available only for hot water.
This is why combi sizing is really about how many showers and taps you want to run at once, not how many bedrooms you have.
If you’ve only got one bathroom and you’re never running two showers simultaneously, a 24-28kW combi is genuinely enough for most Cardiff homes. The “bigger is better” approach is how customers end up paying for capacity they’ll never use.
System & Regular Boilers: Sized For Heating
System and regular boilers operate differently because system boilers heat water indirectly, storing it in a hot water cylinder for later use. The boiler itself only needs to be powerful enough to heat the home and gradually heat the cylinder, not to deliver hot water on demand.
Because the cylinder does the work of holding hot water ready, the boiler itself can be much smaller than you’d expect. A typical 4-bedroom home with a properly sized cylinder might only need a 15-18kW system boiler. This is because the hot water capacity comes from the cylinder, not the boiler’s burner output.
This is why upgrading from a regular boiler with a cylinder to a combi often means buying a much larger boiler. You’re shifting from a system where the cylinder buffers hot water demand to one where the boiler has to meet it instantly.
Cardiff Property Types: Why Local Knowledge Matters
As we’ve been working as plumbers in Cardiff for over 15 years now, we’ve seen every type of property Cardiff has to offer, and the boiler sizing logic shifts considerably depending on what you live in.
Victorian and Edwardian terraces (common across Pontcanna, Roath, Cathays, Splott, and similar areas) typically have higher heat loss due to an older brick construction, single-skin walls in some cases, and older windows. This means that they often need slightly larger boilers than the floor area would suggest, but rarely as large as a builder might fit by default.
1960s-1980s semi-detached and detached homes vary wildly. Some have been properly insulated over the years and need surprisingly small boilers, whereas others still have the original loft insulation and need larger boilers to compensate for the heat loss.
Modern new builds and apartments (across Cardiff Bay, the city centre, and newer estates) are typically very well insulated, with low heat loss. These properties almost always require much smaller boilers than the number of bedrooms would suggest. Fitting a 30kW combi in a well-insulated 2-bed flat is one of the most common oversizing mistakes we see.
A recent example of a mismatched boiler size for the property occurred the other day (and is why we are creating this article).
A customer came to us recently asking us to quote for the same boiler another company had recommended (a 35kW Worcester combi for a 3-bedroom Victorian terrace). We looked at how many bathrooms they actually had and how they used hot water, and fitted a 24kW boiler instead. This saved them several hundred pounds on the install, and the boiler runs more efficiently because it’s properly sized for the actual demand. The previous quote would have been a boiler that spent most of its life short-cycling and burning gas wastefully.
Why Oversizing Is The Bigger Risk
Most customers worry about getting a boiler that’s too small. In reality, oversized boilers are a far more common problem, and they cause real issues.
A properly sized boiler runs in its most efficient mode most of the time, modulating its output to match demand, whereas an oversized boiler short-cycles instead. It fires up, heats the system quickly, and shuts off. Then fires up again a few minutes later. Then shuts off. Then fires up again. Each ignition cycle is the least efficient part of a boiler’s operation, and short-cycling means you’re paying for inefficiency continuously.
The consequences of oversizing add up over time:
- Higher gas bills because the boiler runs inefficiently
- Shorter boiler lifespan because components see more on-off cycles
- More breakdowns for the same reason
- Higher purchase cost for the capacity you’ll never use
Properly sized boilers cost less to buy, cost less to run, and last longer. The myth that “bigger is safer” only benefits the people selling the bigger boilers.
How We Size Boilers For NJ Harkus Customers
When we install a new boiler, we don’t guess at the sizing.
We carry out a proper heat loss calculation for the property, assess hot water demand based on the number of bathrooms and how the household actually uses water, and recommend a boiler size that genuinely fits the home.
If you’ve been quoted by another company, we’re happy to give you a second opinion. We’ve often recommended smaller boilers than the original quote, which usually means lower installation costs and lower running costs for years to come.
For boiler repair or boiler servicing on your existing system, see our dedicated pages.
Where an existing boiler is oversized and reaching the end of its life, we’ll discuss whether right-sizing the replacement is worth doing as part of the upgrade.
Thinking about a new boiler? Contact our team, and we’ll come out, run the proper calculations, and tell you honestly what size you actually need.
