Choosing a Boiler: Sizing, Efficiency and Warranties

The three types

Combi boilers heat water on demand with no cylinder, saving space and suiting smaller properties with one bathroom. Flow rate limits mean performance drops when two outlets run at once.

System boilers work with a hot water cylinder, delivering better performance across multiple bathrooms simultaneously, at the cost of the cylinder's space.

Heat-only boilers, with a cylinder and a header tank, are the traditional arrangement and mostly relevant when replacing like for like in an older system.

Sizing is where most installations go wrong

Boilers are frequently specified far larger than the property needs, on the reasoning that bigger is safer. It is not.

An oversized boiler cycles on and off rapidly rather than running steadily, which reduces efficiency, increases wear and produces less comfortable heating.

Correct sizing comes from a heat loss calculation based on the property's dimensions, insulation and window area, not from the size of the old boiler or a rule of thumb. Ask whether the installer has done one. Many have not.

For combi boilers, hot water demand usually drives the size rather than heating demand, and flow rate at your incoming mains pressure is the figure that determines shower performance.

Efficiency in practice

Modern condensing boilers are all rated highly on paper. Achieving that in practice requires the system to run at low enough return temperatures for condensation to occur.

Turning the flow temperature down — often set unnecessarily high by default — improves real efficiency measurably and costs nothing. Weather compensation controls, which adjust flow temperature to outdoor conditions, do this automatically.

Radiator sizing interacts with this: lower flow temperatures need larger radiators to deliver the same heat. Worth considering if radiators are being replaced anyway.

Warranties and what voids them

Manufacturer warranties range widely, and the longest are usually conditional on installation by an accredited installer and on annual servicing being carried out and documented.

Miss a service and the warranty may lapse. Keep the records.

Warranty length is a reasonable proxy for manufacturer confidence, but the installer matters more. Most early boiler failures trace to installation faults — poor flushing of the existing system, incorrect gas pressure, bad flue installation — rather than manufacturing defects.

Getting quotes

Three quotes, each stating the specific model, output, and what is included: system flush, magnetic filter, new controls, flue work, making good, and removal of the old unit.

A power flush or chemical clean of the existing system is important on older installations and is sometimes omitted to keep a quote low. Debris from old pipework damages new boilers quickly.