For most one-bathroom homes and flats, a combi boiler is the right choice — it heats water on demand, needs no tank, and is the most efficient and space-saving option. If you have two or more bathrooms or a busy household where showers run at the same time, a system boiler is usually better, because it stores hot water in a mains-pressure cylinder and can feed several outlets at once. A heat-only (conventional) boiler — which uses a cylinder plus a cold-water tank in the loft — still suits many period homes with existing pipework or low mains pressure. The right answer comes down to four things: how many bathrooms you have, whether you run hot water in more than one place at once, how much space you’ve got, and your water pressure. We’re Gas Safe registered (657972) and will recommend the right type honestly during a free survey — see our boiler installation page.
Choosing the type of boiler matters more than chasing the cheapest unit — get it right and you’ll have efficient, reliable hot water for years; get it wrong and you’ll be living with weak showers or wasted gas. Here’s how the three types compare, and how to pick for your home.
Combi vs system vs heat-only at a glance
Here’s the quick comparison — the rest of the guide explains each in plain English.
| Boiler type | Best for | Hot water cylinder? | Two showers at once? | Efficiency | Typical West London home |
|---|---|---|---|---|---|
| Combi | 1 bathroom, smaller homes | No | No | Highest (no standing loss) | Flats, Victorian terraces |
| System | 2+ bathrooms, busy households | Yes (mains-pressure cylinder) | Yes | High | Larger Edwardian / family homes |
| Heat-only | Period homes, existing tanks, low mains pressure | Yes (cylinder + loft tank) | Yes | Good | Older / larger period properties |
What is a combi boiler — and who’s it best for?
A combi (combination) boiler heats your water on demand, straight from the mains, with no separate hot water cylinder or loft tank. That makes it the most efficient type for smaller homes — there are no “standing losses” from stored water cooling down — and the most space-saving, since the whole thing is one compact unit. It’s the natural fit for flats and Victorian terraces across West London where cupboard space is tight and there’s one bathroom.
The trade-off is simultaneous demand: a combi heats water for one outlet at a time, so running the kitchen tap while someone showers can knock the flow back. For a couple or a small family with one bathroom, that’s rarely a problem. In hard-water London, combis do need a yearly boiler service to keep limescale off the heat exchanger.
What is a system boiler — and who’s it best for?
A system boiler stores hot water in an unvented cylinder kept at mains pressure, ready to deliver to several taps and showers at once without the pressure dropping. That makes it the right choice for homes with two or more bathrooms, or busy households where the morning rush means two showers running together — common in the larger Edwardian family homes around Ealing, Chiswick and Kensington.
You’ll need somewhere to put the cylinder (an airing cupboard usually does it), but you lose the loft tanks of an older system, and you get strong, even hot water throughout the house. If your home regularly needs hot water in more than one place at the same time, a system boiler is almost always the better call — and a common upgrade we fit during a boiler installation.
What is a heat-only (conventional) boiler — and who’s it best for?
A heat-only boiler — also called a conventional or regular boiler — works with both a hot water cylinder and a cold-water tank in the loft. It’s the traditional setup, and it still earns its place: in older period properties that already have the pipework and tanks, or in areas with low mains pressure where a gravity-fed system copes better, replacing like-for-like with a heat-only boiler is often the most sensible, least disruptive option.
It handles multiple bathrooms well, like a system boiler, but the loft tanks take up space and add a little complexity. If you’ve got a working conventional system in a period home, there’s frequently no need to rip it all out — we’ll tell you honestly whether a like-for-like replacement or a conversion makes more sense for your situation.
Which boiler is most efficient?
For a smaller home, a combi is marginally the most efficient because it only heats water when you ask for it — nothing sits in a cylinder slowly cooling. For a larger, busier household, that advantage narrows: a system boiler can work out more efficient overall because it isn’t firing up and shutting down for every single hot-water draw. In practice, the bigger efficiency wins come from getting the right type for your usage and pairing it with good controls — a smart thermostat and proper zoning, which we cover under heating upgrades, often save more than the boiler type alone.
How to choose for your West London home
As a rule of thumb for the homes we work in:
- Flats and one-bathroom terraces (much of Fulham, Shepherd’s Bush, Willesden) → a combi is usually ideal: efficient, compact, no tanks.
- Larger family homes with two or more bathrooms (Ealing, Chiswick, Kensington) → a system boiler handles the simultaneous demand far better.
- Period homes with existing cylinders and loft tanks, or low mains pressure → a heat-only replacement, or a considered conversion, is often the smart, low-disruption choice.
The only way to be sure is to look at your actual home — your bathrooms, your water pressure, your space and how you use hot water. That’s exactly what our free survey is for: honest advice on the right type, then a fixed written quote with no surprises. For typical price ranges, see our new boiler cost guide; to book a survey, head to boiler installation. We cover West and North West London.
Frequently asked questions
What’s the difference between a combi and a system boiler?
A combi heats water on demand straight from the mains, with no cylinder or tank. A system boiler stores hot water in a mains-pressure cylinder, so it can supply several taps and showers at once. Combis suit smaller homes; systems suit busier ones.
Which boiler is best for 2 bathrooms?
A system or heat-only boiler is usually best for two or more bathrooms, because the stored cylinder can feed multiple outlets at the same time without the flow dropping. A combi can struggle to run two showers at once.
Which boiler is most efficient?
For smaller homes, a combi is marginally the most efficient, as it heats water only on demand with no cylinder losses. For larger households, a system boiler can be more efficient overall. The right type for your usage matters more than the headline figure.
What’s the best boiler for a flat?
Usually a combi. It’s compact, needs no hot water cylinder or loft tank, and is efficient for the one-bathroom layouts typical of flats — ideal where cupboard and loft space is limited.
Can I switch from a system to a combi boiler?
Often, yes — it’s a conversion rather than a straight swap, since the cylinder and tanks are removed and pipework reconfigured. It suits smaller households wanting to free up space. We’ll advise honestly whether it’s right for your home during a survey.
What is a heat-only (conventional) boiler?
A heat-only boiler works with a hot water cylinder and a cold-water tank in the loft. It’s the traditional setup, well suited to period homes with existing pipework or areas with low mains pressure, and handles multiple bathrooms well.
Are you Gas Safe registered?
Yes. The Happy Heating Co is Gas Safe registered (657972). Every installation is carried out by a qualified, fully insured domestic gas engineer.
Not sure which boiler is right for your home?
Tell us about your home and we’ll recommend the right type honestly — no upselling, just the boiler that fits how you actually live.
- WhatsApp us — Book in 60 seconds
- Call 020 8051 2124 — our line is answered 24/7 and we’ll get your details to an engineer
- Book a free survey: boiler installation
We’re Gas Safe registered (657972) and fit the right boiler for homes across West and North West London — from Acton and Chiswick to Kilburn, Ealing and beyond. Heating that just works, no drama.
About the author Adam Said is a Gas Safe registered domestic gas engineer (657972) at The Happy Heating Co, installing combi, system and heat-only boilers in homes across West and North West London. This guide reflects what we see on real jobs every week.
Related reading
- Boiler installation in West London
- How much does a new boiler cost in West London?
- Heating upgrades
- Annual boiler servicing
- Areas we cover
Boiler-type comparisons in this guide reflect general UK industry guidance (2026), including The Heating Hub and iHeat. Your best option depends on your own home — confirmed in a free survey.
