Combi, system or regular: which boiler do you actually need?

A combi heats water on demand and needs no cylinder or tanks. A system boiler heats a hot water cylinder. A regular boiler heats a cylinder and needs header tanks in the loft as well. For most flats and smaller houses a combi is right. For anywhere with two bathrooms in regular use, a system boiler usually is.

Combi boilers

One unit on the wall, no cylinder, no loft tanks. Hot water is heated as it passes through, so it never runs out and never sits around losing heat.

The limitation is flow. A combi can only heat water as fast as it passes through, so running two showers at once means both are weak. It is also dependent on your mains pressure: a powerful combi on a poor mains supply will still give a disappointing shower.

• Best for: flats, one and two bathroom houses, anywhere short of space.
• Cost: usually the cheapest of the three to fit, when it is a straight swap in the same position.
• Watch out for: simultaneous hot water demand, and low mains flow rate.

System boilers

The boiler heats a hot water cylinder, usually in an airing cupboard. No loft tanks, because the system is sealed and fed from the mains.

Because the hot water is already heated and stored, several taps and showers can run at once at full pressure. The trade-offs are that the cylinder takes up space, and that once the stored water is used you wait for it to reheat.

• Best for: houses with two or more bathrooms, larger families, homes with poor mains flow.
• Cost: a little more than a combi swap, and more again if a new cylinder is needed.
• Watch out for: cupboard space, and standing heat loss from an older, poorly insulated cylinder.

Regular boilers, also called conventional or heat-only

The traditional setup: boiler, hot water cylinder, and a cold water tank plus a small feed and expansion tank in the loft.

These are mostly found in older properties. They suit homes with low mains pressure or old pipework that would not cope with the pressure of a sealed system. If you have one and it works, replacing like for like is cheaper and less disruptive than converting.

• Best for: older houses, low mains pressure, systems with vulnerable pipework.
• Cost: broadly similar to a combi swap when replaced like for like.
• Watch out for: loft space, and the fact that fewer new homes use this design.

Should you convert from conventional to combi?

It is the most common conversion and it is the most expensive of the lot, because the cylinder and tanks come out, the pipework is reworked and the loft and airing cupboard need making good.

It is worth it if you want the space back, if the household is one or two people, and if your mains flow rate is good. It is often the wrong call for a four bedroom house with two bathrooms and three teenagers, where you would be trading reliable simultaneous hot water for a cupboard.

Ask the installer to measure your mains flow rate before agreeing to a conversion. It is the single best predictor of whether you will be happy afterwards.

Can you tell which one you have?

Look in the airing cupboard and the loft. It takes about a minute.

• No cylinder anywhere and no tanks in the loft: you have a combi.
• A hot water cylinder in a cupboard, but no tanks in the loft: system boiler.
• A cylinder in a cupboard and one or two tanks in the loft: regular, also called conventional or heat-only.

What each one costs to run

The boiler type matters less to your bill than its efficiency and how you use it, but there are real differences.

A combi heats water only when a tap is opened, so there is no standing heat loss. A system or regular boiler keeps a cylinder of water hot, which loses a little heat continuously, though a modern well-insulated cylinder loses far less than an old one in a draughty cupboard.

Against that, a combi run at the wrong flow temperature can be less efficient than people expect. Condensing boilers are at their most efficient when the return water temperature is low enough for them to condense properly, which is a setup detail worth asking your installer about.

What the conversion actually involves

If you are moving from a conventional system to a combi, the work is more disruptive than a swap and worth understanding before you agree.

• The hot water cylinder comes out, which usually frees an airing cupboard.
• The loft tanks come out, and the pipework feeding them is capped or removed.
• The system becomes sealed and pressurised, fed from the mains, so the pipework has to be sound enough to take it.
• The gas supply pipe often needs upgrading to 22mm to feed the higher demand.
• Expect two to three days, and expect to do some redecorating afterwards.

A quick way to decide

One bathroom, decent mains pressure, want the space: combi. Two or more bathrooms used at the same time: system. Old property, low pressure, existing tanks that work: regular, like for like.

If you are still unsure, get quotes for two options and ask each installer to explain why they would go that way. The reasoning tells you more than the price.