Boiler cost: what decides the price of a new or replacement boiler

Type, fuel and output

A combi boiler heats the radiators and the hot water on demand, with no cylinder. A system or regular boiler works with a hot water cylinder, which adds cost but suits homes with several bathrooms. Gas boilers need a gas supply and a flue; electric boilers need neither but a strong electricity supply, and cost more to run.

Output is sized from the home’s heat loss and its hot water demand. An oversized boiler costs more to buy and runs less efficiently; an undersized one leaves rooms cold. Condensing gas boilers cost more than older non-condensing types and use less gas.

Flue, gas pipe and location

Replacing a boiler in the same place with a good flue route is the simplest job. Moving it, running a longer flue, upgrading the gas pipe to carry enough gas, or adding a condensate drain for a condensing boiler all add work. The flue must always lead safely outside.

Cleaning the system and protecting the new boiler

Old heating systems hold sludge and rust that can damage a new boiler quickly. Flushing the system, adding inhibitor and fitting a magnetic filter cost a little extra and protect the boiler for years. A quote that leaves them out may look cheaper but is not comparing like with like.

Controls, cylinder and what else is included

A room thermostat or smart control, and thermostatic valves on the radiators, add to the price but save energy. If the boiler feeds a hot water cylinder, its size and type (direct or indirect) are a separate cost. Removal of the old boiler, commissioning, and the warranty terms should all be stated.