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

Get this decision right and the house works properly for the next fifteen years. Get it wrong and you live with weak showers or a cylinder you never needed. The choice is driven by how much hot water you use at the same time, not by which type is newest.

The three types

Combi. One unit heats water on demand straight from the mains. No cylinder, no loft tanks. It takes up the least space and there is no stored water to run out or to reheat.

System. The boiler works with a separate hot water cylinder, usually in an airing cupboard. The expansion vessel and pump are built into the boiler, so there are no tanks in the loft.

Regular, also called heat only or conventional. A boiler plus a hot water cylinder plus a cold feed and expansion tank in the loft. This is the traditional setup in older Manchester and Salford housing.

The question that actually decides it

How many hot taps or showers might run at once?

A combi heats water as it flows, so its output is a flow rate rather than a volume. Run two showers off one combi and both get half the water. That is not a fault, it is physics.

• One bathroom, one or two people: a combi almost always makes sense
• Two bathrooms, or an en suite used at the same time as the family bathroom: a system boiler with a cylinder is usually the better answer
• Three or more bathrooms, or a large household: system boiler with a properly sized unvented cylinder

The mains pressure trap

A combi is only as good as the cold mains feeding it. If your incoming mains flow rate is poor, a combi will disappoint you no matter how powerful it is on paper.

Flow rate should be measured before anyone specifies a combi, and it is one of the checks that separates a survey from a guess. Older properties on shared or long supply runs are the ones that catch people out.

If the mains cannot support a combi, a system boiler with a cylinder gives you strong showers regardless, because the cylinder stores the volume rather than relying on instantaneous flow.

Why converting to a combi is a bigger job

Going from a regular system to a combi is not a like for like swap. It means removing the cylinder and loft tanks, capping the old feeds, reworking pipework, and frequently upsizing the gas supply from 15mm to 22mm.

It also frees up the airing cupboard and the loft space, which for some households is the whole point. Just price it as the larger job it is, and read our guide on what a new boiler costs before comparing quotes.

Things that should not decide it

Bedroom count alone. A four bedroom house with one bathroom and two occupants is a different hot water problem to a three bedroom house with two bathrooms and five occupants.

What the neighbour had fitted. Their mains pressure and layout are not yours.

Whatever the installer fits most of. A firm that only fits combis will find a reason your house needs a combi.

What a proper survey looks at

• Mains flow rate and static pressure, measured rather than assumed
• Number of bathrooms and likely simultaneous demand
• Radiator sizes and count, to size the boiler for heat loss rather than by bedroom count
• Gas pipe size back to the meter
• Flue routes and where the condensate can legitimately terminate
• Whether the existing system needs a power flush first

An oversized boiler cycles on and off, wears faster and runs less efficiently. Bigger is not safer.

Getting it specified properly

We measure rather than guess, and we will tell you if the cheaper option is the right one for your house.

See boiler installation, boiler replacement and unvented cylinders for the work itself. We cover Manchester, Salford, Stockport and the surrounding towns.

Ring 0161 515 9977 or tell us about your house and we will work out what it needs.