Key Takeaways
- A new boiler fitted in London typically costs £1,800 to £3,000, with the full range £500 to £4,500 by type and complexity (PriceYourJob and MyJobQuote, 2025 to 2026).
- A combi gives roughly 0.4 litres per minute per kW, so a 30kW model manages about 12 litres per minute. Two showers want closer to 20.
- Your water company is only measured against 1 bar at 9 litres per minute at the stop tap, less than one good shower (Ofwat DG2 indicator, 2026).
- With two or more bathrooms in daily use, a system boiler and an unvented cylinder is the right answer, at about £2,500 to £3,500 fitted in London for a straightforward job.
Most people choose a boiler on price and regret it in the shower. The real decision is how much hot water you need at once, and whether your mains can deliver it. This guide compares combi, system and regular boilers for London homes in 2026.
The three types compared
The difference is where the hot water comes from: a combi makes it on demand, a system boiler stores it in a pressurised cylinder, and a regular boiler stores it in a cylinder fed from a loft tank.
| Type | London cost, 2026 | Hot water | Best for |
|---|---|---|---|
| Combi | Unit £500 to £2,800; like-for-like swap £1,800 to £2,500 fitted | On demand, one outlet | Flats, one-bathroom houses |
| System | Unit £1,000 to £2,500; with a new cylinder £2,500 to £3,500 fitted | Stored, several outlets at once | Two or more bathrooms |
| Regular (heat-only) | Unit £1,300 to £2,700, plus fitting | Stored, loft tank fed | Homes already set up that way |
Those are straight-replacement prices. Changing type is where the money goes: a conventional to combi conversion costs £3,000 to £4,500 in London, because the cylinder and loft tanks come out and the pipework changes.
Why does a combi struggle with two bathrooms?
Because a combi heats water as it passes through, its output is capped by flow rate rather than by a tank. Flow is quoted at a 35°C temperature rise, which lifts roughly 10°C mains water to a usable 45°C, and works out at about 0.4 litres per minute per kW.
| Combi output | Flow at 35°C rise | What it realistically runs |
|---|---|---|
| 24 kW | about 9.5 litres per minute | One shower, nothing else |
| 30 kW | about 12 litres per minute | One good shower, or a shower and a basin |
| 35 kW | about 14 litres per minute | One strong shower, still not two |
| 42 kW | about 17 litres per minute | Two disappointing showers |
A thermostatic shower that feels good uses 9.5 to 10.6 litres per minute, a power shower 12 to 15. Two at once want around 20. The Heating Hub notes that when several outlets run together, the flow rate of even the largest combi is roughly halved. A combi shares one stream between every open tap; a cylinder serves them all at mains pressure.
Can your London mains actually feed a combi?
Often not, and it is checked far too rarely. A combi needs at least 10 litres per minute of incoming mains flow, yet the benchmark your water company is measured against, Ofwat's DG2 indicator, is only 1 bar at the stop tap at 9 litres per minute. Plenty of Victorian and Edwardian terraces still run on a 15mm or lead supply pipe rather than the modern 22mm, and London's hard water narrows the bore further. Test it first: time a kitchen cold tap filling a one litre jug with every other outlet closed. Under six seconds is workable; over ten and a combi will disappoint whatever its kW rating.
Source: Ofwat, Thames Water Customer Guarantee Scheme and The Heating Hub, 2025 to 2026.Combi boilers
A combi suits a flat or a house with one bathroom and no second one coming. It needs no cylinder and no loft tank, which frees an airing cupboard, and it is the cheapest type to install like for like. Our new boiler cost guide breaks the prices down further.
System boilers
A system boiler with an unvented cylinder is the right answer for two or more bathrooms. The cylinder stores hot water at mains pressure, so two showers run at full flow together. Size it at 180 to 210 litres for three or four people and 210 to 250 litres for a family bathroom plus an en-suite. A system boiler and new cylinder fitted together run about £2,500 to £3,500 in London on a straightforward job, more where the pipework changes, and any unvented cylinder over 15 litres falls under Building Regulations Part G3, so it needs an installer holding the unvented qualification and an annual service.
Source: PriceYourJob and MyJobQuote, 2025 to 2026, Best Builders on cylinder sizing, and Approved Document G.Regular boilers
A regular, or heat-only, boiler still earns its place in a large older home with a sound cylinder and loft tank, or where mains pressure rules out anything mains-fed. Otherwise it is the type most often replaced. See our central heating cost guide.
What the regulations require in 2026
Two rules shape every replacement in England. Boiler Plus, in force since April 2018, requires 92% ErP efficiency plus time and temperature controls on every gas boiler, and every new combi must also have one of: flue gas heat recovery, weather compensation, load compensation, or smart controls with automation and optimisation.
Approved Document L (2021 edition with 2023 amendments) then says a heating system newly installed or fully replaced in an existing dwelling should be sized for a maximum flow temperature of 55°C, or the lowest achievable. A radiator rated at the standard 80/60 test condition gives only about half its output at 55°C flow, so a compliant job often means larger radiators too. The Boiler Upgrade Scheme's £7,500, meanwhile, goes to heat pumps only, never to a gas boiler. See building regulations explained.
Source: Boiler Plus factsheet, Approved Document L Volume 1 and Ofgem, 2018 to 2026.How to choose
Start with bathrooms, not budget. One bathroom, one shower at a time: fit a combi, sized 24kW to 30kW, and put the savings into controls. Two or more bathrooms in simultaneous use: fit a system boiler and an unvented cylinder, and accept the extra cost and cupboard space, because no combi on the UK market solves that problem. Keep a regular boiler only where the cylinder and tank already work and poor mains flow rules out the alternatives. Whichever you pick, measure the mains flow first. GS Renovation coordinates heating upgrades as part of full renovations across London, with the boiler and every gas connection fitted by a Gas Safe registered engineer. If a second bathroom is what is pushing you towards a system boiler, plan the two together as part of a bathroom renovation. For a free quote, contact us or call 07472 424 226.