By Boiler Quotes Editorial Team · Updated 20 July 2026
The Three Types, Briefly
Almost every gas boiler sold in the UK falls into one of three categories. They differ in how they produce hot water, which in turn decides how much space they take up and how many taps you can run at once.
| Type | How hot water works | Best suited to |
|---|---|---|
| Combi | Heated instantly from the mains, no cylinder | Flats and houses with one bathroom |
| System | Stored in a pressurised cylinder | Homes with two or more bathrooms |
| Regular (conventional) | Cylinder plus cold tank in the loft | Older homes, low mains pressure, existing tanks |
Combi Boilers
A combi provides heating and hot water from a single unit, heating water straight from the mains when you open a tap. There is no cylinder and no loft tank, which is why combis dominate in flats and smaller houses.
The advantages: compact, no airing cupboard or loft tanks needed, no stored water going cold and being reheated, hot water is effectively unlimited in duration, and installation is generally the cheapest of the three.
The limits: flow rate is shared. Run a shower and the kitchen tap together and both slow down. Performance also depends entirely on your incoming mains pressure and flow, so a combi in a property with weak mains will disappoint no matter how good the boiler is.
As a rule of thumb, a combi is the right answer for a home with one bathroom and one or two occupants, and often still works well for a small family. Once you have two bathrooms likely to be used at the same time, look carefully at the alternatives.
System Boilers
A system boiler heats water and stores it in a pressurised hot water cylinder, usually an unvented cylinder in an airing cupboard. Most of the components are built into the boiler, so there are fewer external parts than with a regular boiler and no tank in the loft.
The advantages: strong hot water performance at several outlets at once, which is what makes it the standard choice for homes with two or more bathrooms. Mains pressure showers throughout, and compatibility with solar thermal if you ever add it.
The limits: you need space for the cylinder, hot water is finite so a very long series of baths will exhaust it until it reheats, and the cylinder needs an annual check as part of servicing.
Regular Boilers
Also called conventional, heat-only or traditional boilers, these work with both a hot water cylinder and a cold water storage tank, usually in the loft, plus a feed and expansion tank.
The advantages: they suit older heating systems and older pipework that might not cope with mains pressure, they can supply several outlets at once, and if your home already has the tanks then replacing like-for-like is the simplest and cheapest route.
The limits: they take the most space, gravity-fed hot water pressure is often weak upstairs unless a pump is fitted, and the loft tanks need frost protection and occasional inspection.
How to Choose
Work through these in order and the answer usually becomes obvious:
- Count the bathrooms. One bathroom points to a combi. Two or more usually points to a system boiler.
- Check your mains. A combi is only as good as the water coming into the house. An installer can measure flow rate at the kitchen tap in a couple of minutes, and it is the single most useful measurement before choosing a combi.
- Consider your space. If losing the airing cupboard would be genuinely useful, that is a real argument for a combi. If you need the cylinder space anyway, the argument weakens.
- Think about hot water habits. Households where several people shower in quick succession in the morning are better served by stored hot water.
- Weigh the conversion cost. Changing type costs more than replacing like-for-like. Sometimes it is money well spent, but the case needs to be more than "combis are modern".
Does Changing Type Pay for Itself?
Rarely on energy savings alone. The efficiency difference between a good modern boiler of one type and another is small compared with the difference between an old boiler and a new one. Change type when it solves a real problem, such as reclaiming a cupboard, fixing poor shower performance or removing failing loft tanks, and treat the improved efficiency as a bonus rather than the justification.
Not sure which type suits your home? Tell us about your property and compare free, no-obligation quotes from up to 3 Gas Safe registered installers using the form below.
