Combi vs Conventional vs Unvented Boilers (2026)

The three types at a glance

Every gas central heating system in the UK is one of three things, and the difference comes down to how your hot water is made and stored.

• Combination (combi) boiler — heats water straight from the mains the moment you open a tap. No cylinder, no tank in the loft; one compact unit does both heating and hot water.
• Conventional (regular or heat-only) boiler — the traditional set-up: a cold water tank in the loft, a hot water cylinder in the airing cupboard and the boiler itself, all working together. Hot water is heated in advance and stored ready for use.
• Unvented (pressurised) system — often called a “Megaflo” after the best-known make. The boiler heats a sealed cylinder that is filled straight from the mains, so stored hot water comes out at mains pressure and there is no loft tank.

Combi boilers: how they work and who they suit

A combi takes the parts a conventional system spreads around the house — cylinder, tank, valves — and folds them into one box. Cold mains water passes through a heat exchanger inside the boiler and comes out hot, so you only ever heat what you use and you never run out. Because nothing is stored, there is nothing to take up space: most combis sit in a kitchen cupboard or on a utility wall, which is why they have become the default in flats and smaller houses.

The trade-off is flow. A combi can only heat water as fast as the mains delivers it, so if two showers run at once they share the supply, and if your mains pressure is poor the shower will be too. Modern combis are a good deal more capable than the 1980s units they replaced and will happily serve a family home of up to five bedrooms — provided the mains is strong and everyone isn’t in the bathroom at once.

• Best for: flats, terraces and semis, homes with one bathroom, anyone short of cupboard or loft space
• Needs: decent mains pressure and flow — we measure both before recommending one
• Watch out for: two showers at the same time, and no immersion heater to fall back on if the boiler is off

Conventional boilers: how they work and who they suit

This is the system most older houses were built with. A cold water tank in the loft feeds a hot water cylinder by gravity; the boiler heats the cylinder, and the cylinder feeds the taps. Because the supply comes from the tank rather than straight off the mains, it doesn’t care how weak your mains pressure is — which is exactly why it suits older properties in parts of Kent where the supply is poor.

A full cylinder will run several taps and a bath at once without flinching, the immersion heater gives you hot water even if the boiler fails, and where the tank and cylinder already exist a like-for-like replacement is the simplest, least disruptive job of the three. Against that: it takes the most room, once the cylinder is empty you wait for it to reheat, and shower pressure is limited by how high the loft tank sits unless you add a pump.

• Best for: larger or busier households, homes with low mains pressure, properties that already have the tanks in place
• Needs: loft space for the tank and a cupboard for the cylinder
• Watch out for: hot water running out at peak times — a fast recovery cylinder (below) is the usual fix

Unvented systems: how they work and who they suit

An unvented system gives you the stored volume of a conventional set-up with the mains pressure of a combi. The cylinder is a sealed, pressure-rated vessel fed directly from the mains, so every outlet in the house gets strong, even pressure — run two or three showers at once and nobody notices. There is no tank in the loft, which makes it the natural choice when you are converting the loft or simply want it back.

Two things to know. First, it lives and dies by the mains: an unvented cylinder needs a good supply of both pressure and flow, so we test the incoming main before we recommend one, and if the supply is marginal we will say so. Second, it is a regulated installation — unvented cylinders must be fitted by an engineer holding the G3 qualification, with the expansion vessel, safety valves and discharge pipework done properly, and those safety devices should be checked every year. It is the most capable of the three systems and, honestly, usually the dearest.

• Best for: homes with two or more bathrooms, busy family houses, loft conversions, anyone who wants a powerful shower without a pump
• Needs: strong mains pressure and flow, and a cupboard for the cylinder
• Watch out for: the extra cost, and the need for a G3-qualified installer and an annual check of the safety devices — both of which we provide

Fast recovery cylinders: the add-on worth asking about

If your complaint with stored hot water is the wait, this is the answer. A fast recovery cylinder uses a much larger heating coil than a standard one, so when it is paired with a modern condensing boiler it reheats in around 12 minutes rather than the usual 25. Less waiting after the morning showers, shorter boiler cycles and — because the boiler runs more efficiently for less time — lower running costs.

It can be fitted with either a conventional or an unvented system, and it can often be retrofitted in place of an existing cylinder without touching the rest of the installation. For a busy household that keeps running out of hot water it is usually a cheaper fix than changing the whole system.

Smart controls

Whatever boiler you choose, the controls decide how much of its efficiency you actually see. Smart thermostats from the likes of Hive and Nest let you run the heating from your phone — turn it on from the car, set schedules room by room, check what it has been doing — and most will talk to Alexa or Google Home. They are a genuine help for anyone with limited mobility, and for landlords who want to keep an eye on an empty property. We fit and set them up alongside the boiler so everything talks to everything else.

Radiators

A new boiler is the right moment to look at the radiators, too. Undersized or sludged-up panels will hold back even the best boiler, and the old white panel is no longer the only option — there are columns, verticals, designer and towel radiators in a range of colours and finishes. We size each room properly rather than guessing, advise on what is worth keeping, and flush the system before the new boiler goes on so it starts life clean.

So which should you choose?

As a rough guide — we confirm all of this at survey:

• Flat, terrace or smaller house with one bathroom and reasonable mains pressure → combi
• Larger or busier home with two or more bathrooms and strong mains → unvented
• Older property with low mains pressure, or one where the loft tank and cylinder are already in and working → conventional, possibly with a fast recovery cylinder
• Planning a loft conversion → lose the tank: combi or unvented
• Keep running out of hot water → fast recovery cylinder, or a move to unvented

How we decide what to recommend

We don’t recommend a boiler over the phone. When we survey we measure your mains pressure and flow rate, count the bathrooms and the people using them, look at where the existing pipework and tanks are, and ask how you actually use hot water. Then we give you a written quote for the system that fits — not the one we happen to have on the van. Plumbusters Ltd are Gas Safe registered and install all three types across Kent and the South East; ring 01622 474008 or request a call back below.