Combi vs system vs regular boiler: which fits your London home?
The honest answer to which boiler type works best for flats, terraces, and larger London homes. Pros, cons, and real costs for each, by London plumber Ilir Nuredini.
The wrong boiler type is one of the most expensive mistakes you can make in a London home. A combi in the wrong property gives you weak hot water for a decade. A system boiler in a small flat takes up a cupboard you needed. Here is the honest comparison, with real London context.
The three types in 30 seconds
Combi boiler: heats water on demand. No hot water tank, no cold water storage tank in the loft. One unit on the wall, mains water in, hot water and heating out. Most popular in modern UK installs.
System boiler: heats water that is stored in a hot water cylinder (usually a tall white tank in an airing cupboard). No cold storage tank in the loft. Suitable for homes with high hot water demand or multiple bathrooms.
Regular (or heat-only or conventional) boiler: like a system boiler but also needs a cold water storage tank in the loft. Older design, still suitable for some homes (especially with low mains pressure or shared water supplies in mansion blocks).
That is the short version. The real decision depends on three things about your house.
The three questions that decide
1. How many bathrooms?
The most important factor. A combi boiler can run one hot tap or shower at a time well. Two at the same time, the flow rate halves and both run lukewarm. Three, you lose the system.
| Bathrooms | Best fit |
|---|---|
| 1 bathroom | Combi (almost always) |
| 1 bathroom + ensuite (occasional simultaneous use) | Large combi (32kW or 35kW) |
| 2 full bathrooms (regular simultaneous use) | System boiler with cylinder |
| 3+ bathrooms | System boiler with large cylinder |
A 35kW combi can produce about 14 litres a minute of hot water at a 35-degree temperature rise. A typical shower uses 8 to 10 litres a minute. So one shower fine, one shower plus a kitchen tap is borderline, two showers will fail.
2. What is the mains water pressure like?
A combi boiler relies entirely on mains pressure for hot water. If your incoming pressure is low (under 1 bar dynamic), a combi will give you weak showers regardless of how big it is.
Common London problems:
- Top-floor flats in older mansion blocks (W1, SW1, NW8) often have low pressure
- Houses on shared supplies (some Victorian and Edwardian terraces) where multiple properties share a single supply
- Areas at the end of a long water main run
A plumber can test your incoming pressure and flow rate in 10 minutes with a gauge. Below 1.5 bar dynamic and 12 litres a minute flow rate, a combi is going to disappoint regardless of the size.
In low-pressure properties, the right answer is usually:
- A system boiler with an unvented cylinder (which boosts hot water pressure)
- Or, a regular boiler with the existing tank-fed supply
3. How much loft and airing cupboard space do you have?
Combi boilers are wall-mounted and take no other space. Useful in flats and small houses.
System boilers need an airing cupboard for the hot water cylinder (typical cylinder is 800mm to 1,200mm tall, 450mm to 550mm wide).
Regular boilers need both an airing cupboard for the cylinder and a cold water tank in the loft. Tanks vary, typical is 50cm x 50cm x 50cm.
If you have no airing cupboard and no usable loft, you may be limited to a combi by default. If you have plenty of space, you have a free choice.
Real London scenarios
One-bed flat, one bathroom
Combi boiler. End of discussion. A 24kW or 28kW combi handles everything you need. Wall-mounted in the kitchen or hallway cupboard. No tanks anywhere.
Cost in 2026: £2,200 to £3,200 fitted depending on brand.
Two-bed flat, one bathroom
Combi boiler still. Maybe a 28kW or 30kW for slightly faster hot water recovery if there are several occupants.
Three-bed terrace, one full bathroom
Combi works well. 30kW to 32kW depending on radiator count. Good hot water for most household use patterns.
If you have two showers running back-to-back in the morning, a 35kW combi handles that better than a smaller one.
Three-bed terrace, bathroom plus ensuite (occasional simultaneous)
This is the borderline case. A 35kW combi can manage if the simultaneous use is rare. A system boiler with a 150-litre cylinder is better if both showers run together every morning.
Decision factor: how often do two showers run at once? Daily, system. Once a week, big combi.
Four-bed house, two full bathrooms (regular simultaneous use)
System boiler with a 180 to 250-litre unvented cylinder. Both showers run at full pressure together. The cylinder reheats over an hour or two and is ready for the next round.
Cost in 2026: £4,500 to £7,500 fitted depending on cylinder size and brand.
Period property, three or more bathrooms
System or regular boiler with a large cylinder. In some cases two boilers (one for heating, one for hot water) but that is rare in residential.
Premium choice: a Vaillant ecoTEC Plus system boiler with a Megaflo unvented cylinder. Top-tier setup, 8 to 10 year warranty on both, performs reliably for 15 to 20 years if serviced annually.
Cost in 2026: £6,500 to £12,000+ fitted depending on cylinder size and pipework changes.
Mansion block flat (top floor, low mains pressure)
Often a regular boiler with a cold water tank in the loft is the only thing that works. The tank provides head pressure that the mains cannot. An unvented system can be installed with a pressure pump if the flat layout allows.
Worth getting a flow and pressure test before deciding.
Cost over 15 years
People focus on the install cost, but a boiler is a 15-year investment. Lifetime cost matters more than purchase price.
Approximate 15-year cost for a typical 3-bed London house:
| Type | Install | 15 years annual service | Repairs over 15 years | Energy cost over 15 years* | Total |
|---|---|---|---|---|---|
| Mid-range combi | £2,800 | £1,650 (£110/yr) | £800 | £18,000 | £23,250 |
| Premium combi | £3,500 | £1,650 | £400 | £17,500 | £23,050 |
| System boiler + cylinder | £5,000 | £2,250 (£150/yr) | £1,200 | £18,500 | £26,950 |
*Energy cost is rough, depends on usage and energy prices. Assumes 80% efficient boiler usage at average 2026 prices.
The premium combi works out cheaper than the mid-range over 15 years because it lasts longer between repairs. The system costs more upfront and to run but gives you better hot water for multi-bathroom use.
When to switch from regular to combi
A common upgrade in older London terraces. You have a regular boiler with a tank in the loft, and you want a combi to free up the airing cupboard and the loft.
Things to check before switching:
- Can your mains supply handle a combi? Pressure and flow test first.
- How many bathrooms and is simultaneous use a thing? If yes, you want a system boiler instead, not a combi.
- Will you need pipework changes? Going from gravity-fed (tank pressure) to mains-fed (combi pressure) can mean rework on bathroom taps and showers.
A regular-to-combi conversion in 2026 London is typically £4,200 to £6,500 fitted, including removing tanks. More if extensive pipework needs replacing.
When to switch from combi to system
Less common but happens when households grow, second bathrooms get added, or the existing combi cannot keep up.
You need:
- Space for a cylinder (airing cupboard, under stairs, sometimes loft)
- A way to run the cylinder feed pipework
- A combi swap that cannot be reused (combi boilers are not interchangeable with system boilers)
System replacement of an existing combi is typically £5,500 to £9,000.
Brand differences
Not all boilers are equal. My ranking based on what I see fail and what lasts:
For combis in London:
- Vaillant ecoTEC Plus (best balance of price and reliability)
- Worcester Bosch 4000/8000 series (premium, best UK warranty if accredited installer)
- Ideal Logic Max (mid-range, decent reliability)
- Baxi 800 series (budget option, fine for short-term ownership)
For systems:
- Vaillant ecoTEC Plus System
- Worcester Bosch Greenstar System
- Heatrae Sadia (cylinder pairing)
Avoid: cheap own-brand boilers from supermarkets or budget DIY stores. The parts are not stocked by independent plumbers and warranty service is poor.
Heat pumps as the alternative
Brief mention because it comes up. Air source heat pumps now qualify for a £7,500 government grant under the Boiler Upgrade Scheme. They work well in well-insulated houses with large radiators or underfloor heating.
For most existing London homes with single-glazed sash windows and small radiators, a heat pump install is more disruptive and expensive than a like-for-like boiler swap, and may not heat the house as well in February. Worth considering for renovations or new builds, less attractive as a straight swap right now.
I cover heat pumps in a separate blog post.
How I quote
For boiler replacement in London, my standard process:
- Free home survey (30 to 60 minutes)
- Assessment of current system, hot water demand, mains pressure, radiator count
- Honest recommendation (combi, system, or stay with what you have)
- Written quote within a few days, itemised
- No commission deals with manufacturers, so the recommendation is what is right for you, not what pays best
WhatsApp with a few photos of your existing setup (boiler, any tanks, airing cupboard) and the basics about the property (number of bedrooms, bathrooms, occupants) and I can give you a rough sizing and cost before any visit.