When a combi swap is the right call
A combi heats water on demand and runs your radiators from one box on the wall. There is no hot water cylinder and no tank in the loft. If you already have a combi and it is over 12 years old, breaking down more often or needing parts that are hard to find, a like-for-like replacement is usually the simplest and cheapest upgrade.
It suits most homes with one bathroom and good mains water pressure. It is less suited to homes where two showers run at once, or where the mains pressure is weak, because a combi can only share its flow between taps.
What you pay for
| What changes the price | Typical effect |
|---|---|
| Boiler output (24kW vs 35kW) | £150 to £400 more for the larger model |
| Warranty length (5 vs 10 or 12 years) | Longer warranties usually mean a pricier boiler |
| New flue or flue in a new position | £150 to £500 |
| Gas pipe upgrade to 22mm | £150 to £400 |
| Magnetic filter and system flush | Often included; a full power flush adds £300 to £500 |
| Smart thermostat | £150 to £300 fitted |
Your installer gives you one fixed price covering all of this before any work starts.
What happens on the day
- The installer protects the area, isolates gas and water, and drains the system.
- The old boiler comes off the wall and the new one goes up, usually in the same place.
- Pipework, flue, filter and controls are connected, then the system is refilled and checked for leaks.
- The boiler is commissioned, gas tightness tested and the settings explained to you.
- The installer registers the boiler with Gas Safe (Building Regulations compliance) and the manufacturer warranty.
Before you sign
Ask for the boiler model, its output in kW, the warranty length and whether a system flush and magnetic filter are included. If a quote does not name the model, you cannot compare it.
Check the engineer’s Gas Safe ID card and look up the number on the Gas Safe Register. Every boiler installer City Boiler works with is Gas Safe registered, and we check before they quote.
Should you consider a heat pump instead?
If your home is well insulated and you plan to stay for a decade or more, it is worth comparing. A heat pump costs more to install, but grants cut the gap and running costs can be lower. If your house is draughty, has small radiators or you need a quick fix this week, a new combi is usually the smarter buy.
Common questions
How much does a combi boiler replacement cost?
A like-for-like combi swap usually costs £2,200 to £3,400 fitted, including VAT. The price moves with the boiler's output, the brand, the warranty length and whether the flue or gas pipe needs changing.
How long does it take to replace a combi boiler?
Most like-for-like swaps are done in one day. If the boiler moves to a new position or the gas pipe needs upsizing, allow up to two days.
Do I need a bigger combi for a bigger house?
Not exactly. Combi size is about hot water flow, not house size. A one-bathroom home usually suits 24 to 30kW. Two bathrooms used at the same time point to a system boiler rather than a bigger combi.
Will I lose heating and hot water during the install?
Yes, for the working day. Your installer drains the system, fits the new boiler and recommissions it before leaving, so you should have heating and hot water back the same evening on a straight swap.