System boilers — the answer for busier homes
Stored hot water at mains pressure — several taps at once, without the drop-off.
Not sure which type you need? The quote tool asks about your home and works it out with you.
What is a system boiler?
A system boiler heats your radiators and heats a separate hot-water cylinder, which stores the hot water until you need it. Unlike an old conventional setup, the pump and expansion vessel are built into the boiler itself, and the cylinder is sealed and fed straight off the mains — so there are no tanks in the loft.
Because the water is already hot and stored, several taps or showers can run at once without the flow collapsing. That is the whole point of it.
It is the setup we most often recommend for four-bedroom houses, homes with two bathrooms in daily use, and anywhere a family is all trying to get ready at eight in the morning.
The honest pros and cons
We fit all three types. We have no reason to talk you into the wrong one.
Where it wins
- Several showers and taps at once without the flow dropping away
- Strong mains-pressure hot water throughout the house
- No cold tanks in the loft — the loft stays usable
- An immersion heater in the cylinder means you still have hot water if the boiler fails
- Works well alongside solar thermal, if you have it or want it later
Where it falls down
- You need somewhere for the cylinder — usually the airing cupboard
- The stored water can run out if you all take long showers back to back
- A tank of hot water loses some heat standing there, even well insulated
- More components than a combi, so slightly more to go wrong and to service
- Unvented cylinders need an annual check by a qualified engineer
Is a system boiler right for your home?
Find the row that sounds most like your house.
| Your home | Verdict | Why |
|---|---|---|
| 4+ bed house, two or more bathrooms | ✓ Good fit | This is what a system boiler is for. |
| Family all showering in the same hour | ✓ Good fit | Stored water handles simultaneous demand; a combi will not. |
| Home with an existing cylinder you want to keep | ✓ Good fit | Often the cheapest sensible upgrade — swap the boiler, keep the cylinder. |
| Older house with loft tanks you want rid of | ✓ Good fit | A system boiler removes the tanks while keeping the stored-water benefit. |
| 2 bed flat, one bathroom | ✕ Not ideal | You would be giving up a cupboard for capacity you will never use. Go combi. |
| No airing cupboard or suitable cylinder space | ✕ Not ideal | Without somewhere for the cylinder, it is a non-starter. |
What size system boiler do I need?
System boiler sizing is driven mostly by the radiators and the cylinder, not by tap flow — the cylinder handles peak hot-water demand for you. Broadly:
| Your home | Typical size |
|---|---|
| 3 bed house, 10–15 radiators | 15–18 kW |
| 4 bed house, 15–20 radiators, 2 bathrooms | 18–24 kW |
| Large or poorly insulated house, 20+ radiators | 24–30 kW |
Cylinder size matters as much as boiler size — around 150 litres suits a typical family of four, 210 litres or more if you have two bathrooms in heavy use. We size both together on the survey.
What does a system boiler cost to fit?
A like-for-like system boiler replacement in a typical East Kent home lands between £2,650 and £3,850 fitted, including VAT. Where you sit in that range comes down to the size of boiler, the brand and warranty you choose, and how much pipework needs reworking.
That is a guide, not a quote. We are not going to pretend a number off a web page is your price — the only way to know is to look at your home. The quote tool asks six questions and gives you a real fixed price for your house, with the boiler, the fitting and everything in the quote included. It takes about two minutes.
0% finance over 12 or 24 months, or spread it over up to 10 years at 11.9% APR.
Coming from an old conventional system
If you currently have a conventional boiler with tanks in the loft, moving to a system boiler is a well-trodden upgrade. The loft tanks come out, the cylinder is usually replaced with a sealed unvented one, and the pump and controls that were scattered around the airing cupboard get absorbed into the boiler.
You keep the thing that was working — stored hot water for a busy house — and lose the things that were not: weak gravity pressure upstairs, tanks that freeze, and an airing cupboard full of separate components.
It is a bigger job than a straight boiler swap because of the cylinder work, but nothing like the upheaval of converting to a combi.
Common questions
What is the difference between a system boiler and a combi?
A combi heats water on demand as it flows, with nothing stored. A system boiler heats a cylinder full of water in advance and stores it. The practical difference is simultaneous use: a combi splits its hot water between outlets, a system boiler can supply several at full strength.
Do I still need a tank in the loft?
No. That is the difference between a system boiler and an old conventional one. A system boiler runs sealed and fed off the mains, so the loft tanks come out. You keep the hot-water cylinder, usually in the airing cupboard.
Can I run out of hot water with a system boiler?
Yes, if you use more than the cylinder holds — four long showers back to back will do it. It reheats on the timer or on demand, so it is a wait rather than a problem, and sizing the cylinder properly at the survey largely avoids it.
What size hot water cylinder do I need?
Roughly 150 litres for a family of four with one bathroom, 180 to 210 litres with two bathrooms, more again for a large household. Undersizing the cylinder is the most common reason people are disappointed with an otherwise good installation.
Does an unvented cylinder need servicing?
Yes — annually, by a qualified engineer. There are safety components on an unvented cylinder that need checking, and skipping it can invalidate the warranty. We cover it as part of a normal service visit.
Is a system boiler more expensive to run?
Slightly, because stored hot water loses a little heat standing in the cylinder. A modern insulated cylinder keeps that small. In a busy household the trade-off is well worth it — a combi that cannot keep up costs you in showers, not pounds.
Still weighing it up?
The other two types, explained the same way.
System boiler installation across East Kent
We fit system boilers from Deal and Dover across to Canterbury, Folkestone and Thanet — Gas Safe registered (№ 2836), 25 years at it, and next-day installation wherever we can.