Combi, system or regular boiler: which do you need?

Checked November 2026

A combi heats water straight from the mains when you turn a tap on, so there’s no tank or cylinder. A system boiler heats a hot water cylinder but has no tanks in the loft. A regular boiler, sometimes called heat only, works with a cylinder and with tanks in the loft.

Which one you need comes down to how many bathrooms you use at once, your water pressure and whether a heat pump might be on the cards later. We fit all three, and heat pumps too, so here’s how we’d think it through.

What is a combi boiler?

A combi does heating and hot water from one box on the wall. Cold water comes in from the mains and gets heated as it flows through. No cylinder, no loft tanks, no waiting for a tank to warm up.

That’s why they’re so common in flats and smaller houses. You get the airing cupboard back and there’s less to go wrong. The catch is that a combi can only heat so much water at once. One shower runs well. Run a second tap at the same time and both get weaker.

How well a combi works also depends on what comes into the house. If the mains flow is poor, a bigger combi won’t fix it. Our guide to what size boiler you need explains why the flow at your tap matters as much as the number on the box.

In England a combi has to meet the Boiler Plus rules. Every new gas boiler must be at least 92% efficient (ErP) and have a timer and room thermostat. A combi also needs one extra measure on top. That’s weather compensation, load compensation, flue gas heat recovery or a smart thermostat with automation and optimisation.

What is a system boiler?

A system boiler heats your radiators directly and heats a hot water cylinder, usually in the airing cupboard. The pump and expansion vessel are built into the boiler, so there are no tanks in the loft.

Because the hot water is stored, two or three people can use it at once without the shower dropping off. Most modern cylinders are unvented, which means they run at mains pressure, so the showers are strong too.

The downsides are space and waiting. The cylinder takes up a cupboard. And once the stored hot water is used, it takes time to heat up again. For a family with two bathrooms that’s normally a fair trade.

What is a regular (heat only) boiler?

A regular boiler is the older setup. There’s a boiler, a hot water cylinder, and usually two tanks in the loft. One feeds cold water to the cylinder. The other is a small feed and expansion tank for the heating.

You’ll find these in a lot of older houses round here. They’re not bad systems. If yours works and you’ve got low mains pressure, a new regular boiler on the same pipework can be the sensible choice. It keeps disruption down and suits homes where the water coming in from the street is weak. Our page on boiler installation in Leamington Spa covers boilers for Regency and Victorian homes.

What they don’t give you is pressure. Hot water from a loft tank comes down by gravity, so showers can be gentle unless there’s a pump. And the tanks take up loft space and can freeze or leak if they’re old.

Is a combi or system boiler better for two bathrooms?

This one comes up a lot. Our honest answer: if two showers often run at the same time, a system boiler with a cylinder usually suits you better.

A combi splits its hot water between every tap that’s open. In a house where one person showers in the morning and someone else does later, a good combi copes fine. In a house where two teenagers are in two bathrooms at 7.30, it’ll struggle, whatever size you buy.

A system boiler gives stored hot water to both bathrooms at once. The trade off is the cylinder cupboard and the reheat time. If you’ve got the space we’d normally lean that way for a busy two-bathroom house.

There’s a middle ground in some homes. A one-bathroom house with an en suite that’s barely used can be fine on a combi. That’s why we’d rather look at how you actually use the water than go by the number of bathrooms alone.

Can I switch to a combi boiler?

In most homes, yes. You take out the cylinder and the loft tanks, re-pipe to the mains and fit the combi. A lot of people like getting the cupboard and the loft back.

It’s a bigger job than a straight gas boiler replacement. There’s work in the loft and the airing cupboard as well as at the boiler, and there may be some making good afterwards where pipes used to run. It’s worth planning for a bit more disruption than a like-for-like change.

Which? says switching from a system or heat only boiler to a combi adds £400 to £800 for the extra labour and re-piping, on top of the boiler itself (Which? boiler cost guide, updated September 2025). Our guide to how much a new boiler costs covers the rest of the price, including a new cylinder if you’re going the other way.

Before you do, think about two things. Will a combi give you enough hot water for how you live? And might you want a heat pump later? We cover that second point below, because it changes the answer for some people.

Is your water pressure good enough for a combi?

A combi only works as well as the mains supply into your house. If the flow is weak, you’ll notice it at the shower straight away. A loft tank system can hide poor mains pressure. A combi can’t.

So we test it before we recommend a combi. If the flow isn’t there, we’ll tell you, and keeping a cylinder may be the better route.

Should I keep my cylinder if I might get a heat pump?

This is the bit most boiler guides leave out. A heat pump needs a hot water cylinder. It doesn’t heat water on demand the way a combi does.

So if you rip out your cylinder now to fit a combi, and then want a heat pump in a few years, you’ll be paying to put a cylinder back. Plus the pipework to go with it. That can be money you didn’t need to spend.

If there’s a real chance you’ll go for a heat pump, keeping a system or regular setup with a cylinder now can make the switch easier later. The cylinder itself may still need changing for a heat pump hot water cylinder. But the space and much of the pipework will already be there.

We fit both boilers and heat pumps, so we’ve got no reason to push you either way. If you’re torn, our guide on heat pump or gas boiler is a good place to start.

Frequently asked questions

Which type of boiler is best?

There isn’t a best one. A combi suits smaller homes with one bathroom and decent mains flow. A system boiler suits busier homes where two people want hot water at once. A regular boiler can still be the right fit for older houses with low mains pressure. The best one is the one that fits how your house uses water.

Do I lose my hot water tank with a combi?

Yes. A combi heats water as you use it, so the cylinder and the loft tanks come out. That frees up space. It also means you lose stored hot water, which matters if you’d ever want a heat pump.

Do combi boilers work with low water pressure?

Not well. A combi can only deliver what the mains gives it. If your pressure or flow is low, a system or regular boiler with a cylinder is often the better option. We check this on the survey before recommending anything.

Not sure which type suits your home? We’ll look at your system and water supply and tell you straight.

Sources

• Which?, “Boiler prices: how much does a new boiler cost?”, updated 17 September 2025, checked 3 November 2026, www.which.co.uk
• BEIS, “Boiler Plus: New standards for domestic boiler installations from April 2018” (FAQs), checked 3 November 2026, assets.publishing.service.gov.uk
• GOV.UK, “Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments” (Table 6.2 and paragraph 6.2), checked 3 November 2026, assets.publishing.service.gov.uk