For most one-bathroom homes and flats, a combi boiler is the usual choice: it heats water on demand at mains pressure, needs no cylinder or loft tank, and is space-saving and often suitable for smaller households. If you have two or more bathrooms, or a busy household where showers run at the same time, a system boiler is usually better — Worcester Bosch describes them as suited to homes with more than one bathroom used at the same time, because stored cylinder water can feed several outlets at once. A heat-only (conventional) boiler, using 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 hot water runs in more than one place at once, the space you’ve got, and your mains flow and pressure. We’re Gas Safe registered (657972) and recommend the right type honestly during a free survey — see boiler installation.
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.
Combi
1 bathroom, smaller homes
No
Not without a drop in flow
No stored-water standing loss
Flats, Victorian terraces
System
2+ bathrooms, busy households
Yes (unvented gives mains-pressure delivery)
Yes
Fewer start-stop cycles on heavy use
Larger Edwardian / family homes
Heat-only
Period homes, existing tanks, low mains pressure
Yes (cylinder + loft tank)
Yes
Depends on cylinder and controls
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 avoids the “standing losses” of stored water cooling down, and it’s the most space-saving type, 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 can serve more than one outlet at once, but it heats water on demand at a given flow rate, and that flow is shared: run the kitchen tap while someone’s showering and performance at both can drop. For a couple or a small family with one bathroom, that’s rarely a problem. In hard-water London, a combi’s domestic-hot-water plate heat exchanger is the part limescale can gradually restrict, since that’s where fresh mains water gets heated — one more reason for a yearly boiler service and scale protection on the cold mains supply.
What is a system boiler — and who’s it best for?
A system boiler works with a separate hot-water cylinder, and the stored water is ready to serve several taps and showers at once. The cylinder is a separate choice from the boiler: an unvented cylinder delivers hot water at mains pressure, which is what gives strong showers without a pump, but that still depends on your incoming mains flow and pressure being up to it. A vented cylinder, fed from a tank, works differently. Worcester Bosch describes system boilers as suited to homes with two or more bathrooms used at the same time, 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 often the better fit — subject to your stored-water demand, mains flow and pressure, and the space available for a cylinder. It’s 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.
Does the boiler type decide how efficient my heating is?
Not on its own. Boiler type is one factor among several, and it’s the wrong thing to choose on if it doesn’t match how your household actually uses hot water. A combi avoids standing losses, because nothing sits in a cylinder slowly cooling. On heavy, sustained hot-water use, a system boiler isn’t firing up and shutting down for every single draw. Which comes out ahead depends on your home and your habits.
In practice, the bigger 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 matter more than 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 once without the flow dropping. Worcester Bosch describes system boilers as suited to homes with more than one bathroom used at the same time. A combi can struggle to run two showers together.
Does the boiler type decide efficiency?
Not by itself. A combi avoids the standing losses of stored water; a system boiler suits sustained, simultaneous hot-water use. Which is more efficient depends on your home and habits, and good controls often matter more than the type. Getting the right type for your usage is what counts.
What’s the best boiler for a flat?
Usually a combi. It’s compact, needs no hot-water cylinder or loft tank, and suits the one-bathroom layouts typical of flats, which makes it a sensible fit where cupboard and loft space is limited. If your flat has two bathrooms in regular simultaneous use, it’s worth checking whether stored hot water would serve you better.
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, also called conventional or regular, works with both a hot-water cylinder and a cold-water tank in the loft. It’s the traditional setup and still earns its place in period homes that already have the pipework and tanks, or where mains pressure is low. Like a system boiler, it 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, and gas boilers must legally be fitted by a Gas Safe registered engineer. We handle the building-regulations notification and register the manufacturer’s guarantee for you as part of the job.
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 descriptions follow manufacturer guidance from Worcester Bosch’s system boiler pages (verified July 2026). Your best option depends on your own home — confirmed in a free survey.
