Who a system boiler suits
A system boiler heats your radiators directly and heats a hot water cylinder, usually in an airing cupboard. The pump and expansion vessel are built into the boiler, so there are no tanks in the loft. Water is stored at mains pressure in an unvented cylinder, which means strong showers on more than one floor at the same time.
That is why system boilers are common in family homes with two bathrooms, or a bathroom plus an en suite. If that is your house and the current boiler is old or unreliable, replacing it with another system boiler is usually the right call. A typical system boiler replacement costs £2,800 to £4,000 fitted (checked October 2026).
Keep the cylinder or replace it?
This is the biggest decision on a system boiler job, and quotes often differ here.
| Cylinder condition | Usual advice |
|---|---|
| Unvented, under about 15 years old, serviced, no leaks | Keep it and fit the new boiler |
| Unvented but never serviced, or the safety valves drip | Get it checked; repair or replace |
| Copper vented cylinder with a tank in the loft | Consider replacing with unvented, or stay conventional |
| Leaking, corroded or slow to heat | Replace at the same time |
An unvented cylinder holds hot water under pressure, so it has safety valves that need checking every year. The engineer working on it needs a G3 qualification for unvented hot water systems, on top of Gas Safe registration. Ask about this if your quote includes cylinder work.
What changes the price
- Boiler output. System boilers are sized to your home’s heat loss, not to hot water flow. A three bed semi often needs somewhere between 15kW and 24kW. Oversizing wastes money and can make the boiler switch on and off too often.
- Cylinder work. Replacing the cylinder is the largest optional extra.
- Flue. Reusing the same flue position keeps cost down; a new route adds £150 to £500.
- System cleanliness. Sludge in old radiators shortens the life of a new boiler. A full power flush typically adds £300 to £500.
- Controls. System boilers work well with zoned controls, so upstairs and downstairs heat separately. Extra zone valves and thermostats add to the bill.
What happens on the day
- The installer isolates gas, power and water, then drains the heating system.
- The old boiler comes off and the new one is mounted, normally in the same position.
- Any cylinder work happens next, with the pressure and temperature relief valves tested.
- Pipework, filter and controls are connected, the system is refilled and checked for leaks.
- The boiler is commissioned, the gas pipe tested for tightness, and the controls explained.
- The installer notifies Building Control through Gas Safe and registers the manufacturer warranty.
Before you sign
Ask every installer the same three questions: which boiler model and output, is the cylinder being replaced or kept, and is a system flush included. Without those answers two quotes are not comparable.
Check the engineer’s Gas Safe ID card and confirm it on the Gas Safe Register. Every installer City Boiler connects you with is Gas Safe registered, and we check before they quote.
When a system boiler is the wrong choice
If you have one bathroom and good mains pressure, a combi may free up the airing cupboard and cost less to fit like for like. See combi boiler replacement for that route, or combi conversion if you are thinking of switching. If your mains pressure is weak, a conventional boiler with tanks may serve you better.
A system boiler already has a cylinder, which also makes it one of the easier setups to move to a heat pump later. If your home is well insulated, compare the numbers on our heat pump installation page before you commit.
Common questions
How much does a system boiler replacement cost?
A system boiler replacement typically costs £2,800 to £4,000 fitted, checked October 2026. Replacing the hot water cylinder at the same time adds to that, and so does a full system flush if the water is dirty.
Do I need a new cylinder when I replace a system boiler?
Not always. If the cylinder is unvented, in good condition and has been serviced, it can usually stay. If it is old, poorly insulated or leaking, replacing both together saves a second visit and a second drain down.
Can I swap my system boiler for a combi?
You can, but think about how many people need hot water at once. A system boiler can run two showers at the same time from the cylinder. A combi shares one flow between taps, so it suits one bathroom homes best.
How long does a system boiler replacement take?
Usually one to two days. A boiler only swap in the same position is often done in a day, while a boiler and cylinder together tend to take two.