The short answer
A combi heats water on demand straight from the mains, with no cylinder or tanks — ideal for flats and homes with one bathroom. A system boiler heats a stored hot water cylinder, usually unvented at mains pressure, so two bathrooms can run at once. A regular (heat-only) boiler also uses a cylinder, plus cold water and expansion tanks in the loft, and suits older homes with gravity systems or low mains pressure.
The three types side by side
| Combi | System | Regular (heat-only) | |
|---|---|---|---|
| Hot water | Heated as you use it | Stored in a cylinder | Stored in a cylinder |
| Cylinder | None | Yes, usually unvented | Yes, usually vented |
| Loft tanks | None | None | Cold water and expansion tanks |
| Water pressure | Mains | Mains (unvented cylinder) | Gravity, unless pumped |
| Two showers at once | Struggles | Yes, with a big enough cylinder | Yes, at gravity pressure |
| Space | Least | Boiler and airing cupboard | Boiler, airing cupboard and loft |
| Best for | Flats and one-bathroom homes | Homes with two or more bathrooms | Older homes with existing tanks |
Combi boilers
A combination boiler heats the central heating and, when you open a hot tap, heats mains water as it flows through. There's no stored hot water, so there's nothing to run out — but there's a limit to how much it can heat at once, set by its kW and your mains supply.
One shower at a time is fine; a shower and a kitchen tap together will drop the flow; two showers together usually disappoint. The combi size guide shows the flow each size gives. We stock 51 combis from Vaillant, Worcester Bosch, Ideal, Biasi and Navien.
System boilers
A system boiler has the pump and expansion vessel built in and heats a hot water cylinder as well as the radiators. With an unvented cylinder, the stored water is at mains pressure, so several taps and showers can run at once until the stored water is used. Cylinders are sized by how much hot water you use — larger households need bigger ones.
Unvented cylinders must be installed by a G3-qualified installer and serviced every year. A system boiler with a cylinder also leaves the door open to solar thermal or a heat pump later.
On our shelves: 24 system boilers, 31 unvented cylinder lines from Ideal, Gledhill, Heatrae Sadia and Telford, and 44 vented cylinders.
Regular (heat-only) boilers
A regular, conventional or open-vent boiler heats the radiators and a vented cylinder, fed by a cold water tank in the loft, with a small feed and expansion tank for the heating. Hot water pressure depends on the height of the tank above the taps, so showers often need a pump.
If your home already has one, replacing it like for like is usually the simplest and cheapest job. It's also a good fit where mains pressure or flow is too poor for a combi or an unvented cylinder.
Switching from one type to another
- Regular to combi frees the loft and airing cupboard, but check the mains flow and pressure first — a combi can only deliver what comes in.
- Regular to system keeps stored hot water but removes the loft tanks, if the mains can supply an unvented cylinder.
- Combi to system makes sense when a family outgrows one-shower-at-a-time hot water.
Any gas boiler must be installed by a Gas Safe registered engineer, who registers the installation for Building Regulations. Ask them to check the mains flow, the gas supply size and where the flue and condensate pipe can go.
Questions
- Which boiler is best for two bathrooms?
- If both are often used at the same time, a system boiler with an unvented cylinder. If they're rarely used together and the mains flow is good, a larger combi can cope.
- Can I swap a regular boiler for a combi?
- Usually, yes — the tanks and cylinder come out. Have the engineer check the mains flow and pressure first, as these limit what a combi can deliver.
- Do combi boilers work with low water pressure?
- Poorly. A combi needs reasonable mains pressure and flow; with weak mains a regular boiler with a stored cylinder, and a shower pump, may give better showers.
- Can I fit a boiler myself?
- No. It's illegal for anyone but a Gas Safe registered engineer to install a gas boiler. Unvented cylinders also need a G3-qualified installer.
Related guides
- Heating & boilersWhat Size Combi Boiler Do I Need? kW Explained
- Heating & boilersMagnetic Filters and Inhibitor: How to Protect Central Heating
- Bathrooms & showersLow Water Pressure: Which Taps and Showers Will Work?
- Heating & boilersWhat Size Radiator Do I Need? Radiator BTU Calculator