Pick a boiler that’s too small and you’ll be standing in a lukewarm shower. Too big and you’ve paid extra for a boiler that switches on and off all day and wastes gas. Getting the size and type right matters more than the brand. Most homes in Bristol and Bath need one of three types, a combi, a system or a regular boiler, and the right size comes from how many bathrooms you’ve got and how much heat your house loses. If you’re replacing yours, our boiler installations page shows how we work it out at the survey.
Start with the type
The type decides how you get your hot water, and it narrows down the size before you even look at the numbers.
Combi boilers
A combi heats water straight from the mains as you turn on a tap. No hot water cylinder, no tanks in the loft.
They’re ideal for flats and homes with one bathroom. You get hot water whenever you want it and you free up the airing cupboard. Most of the boilers we fit are combis, and you can read more on our combi boiler installation page.
The weak spot is two taps at once. Run the shower and someone turns on the kitchen tap, and the flow drops. Fine for most families with one bathroom. Not great for a busy house with two.
System boilers
A system boiler heats a hot water cylinder, usually in the airing cupboard. Because you’ve got a store of hot water ready to go, two or three showers can run at once.
They suit bigger households and homes with more than one bathroom. Paired with an unvented cylinder, you get mains-pressure hot water at every tap.
Regular boilers
Also called conventional or heat-only boilers. They work with a cylinder and a cold water tank in the loft. You’ll find them in a lot of older houses, and if your pipework was designed around one, sticking with a regular boiler can avoid a lot of disruption.
Now the size
Boiler size is measured in kilowatts (kW). But combis and system or regular boilers are sized in different ways.
Sizing a combi
With a combi, the hot water decides it. The bigger the kW, the faster it can heat water and the better the flow at the shower.
As a rough guide, smaller combis in the mid-20s kW suit flats and smaller homes with one bathroom. Bigger homes, or one bathroom plus an en suite, usually need something in the 30s.
Your incoming mains water matters just as much. A big combi can’t give you a powerful shower if the water coming into the house is weak. We check the flow rate and pressure at the survey.
Sizing a system or regular boiler
These are sized on how much heat your house needs. That depends on the number of rooms and radiators, how well the house is insulated, the windows and how many outside walls there are.
A well-insulated three-bed semi needs a lot less than a draughty detached Victorian house of the same size. That’s why a proper heat loss check beats a rule of thumb.
Why bigger isn’t better
People often ask for the biggest boiler going, just to be safe. It’s a mistake.
An oversized boiler heats the house quickly, hits temperature, switches off, cools down and fires up again. That stop-start cycling wastes gas and wears the boiler out faster. We’ve written about short cycling if you think yours might be doing it.
Sizing boilers in Bristol and Bath homes
The housing round here is a real mix. Bristol has thousands of two-up two-down terraces where a mid-sized combi is the obvious choice.
Then you’ve got larger Victorian villas in Clifton, Redland and Cotham, and Georgian townhouses in Bath, spread over three or four floors, often with more than one bathroom. A combi can struggle there. A system boiler with a decent cylinder usually makes more sense.
Thick stone walls, single glazing in listed buildings and high ceilings all push the heat loss up as well.
Frequently asked questions
Can I switch from a regular boiler to a combi?
Usually, yes. We take out the cylinder and the loft tanks and rework the pipework, which frees up space. It’s a bigger job than a straight swap. Check your mains water first, though, because a combi relies on it entirely. If the pressure’s poor, you’ll notice it at every tap.
What size combi do I need for two bathrooms?
It depends how you use them. If you rarely run two showers at once, a larger combi in the 30s kW can cope. If you do, a system boiler with a cylinder is the better answer, because no combi can run two showers properly from a weak water supply.
Do I need a survey to get the size right?
Yes. Online calculators give you a ballpark, but they can’t see your mains water pressure, your radiators or how draughty your house is. A boiler survey takes the guesswork out and makes sure the price you’re quoted is for the right boiler.
Not sure what you need?
Ring us on 01275 400 687 or send us a message. We’ll come out, check your water, your radiators and your house, and tell you straight which type and size will suit you.