Is it time for a new boiler, or has someone just told you it is?

The price of a new boiler is a written figure after the survey, not a number from a table, because the things that move it are only visible in the house. In rough order of importance they are: the type of boiler (combi, system or heat-only), its output, the make and the length of its guarantee, whether the new boiler goes where the old one was or moves, what the flue has to do to reach outside, whether a cylinder is added or removed, the state of the existing system and how much cleaning it needs, and the controls chosen.

The written price covers everything: boiler, flue, filter, flush, controls, commissioning, registration and the removal of the old one. Nothing appears on the invoice that was not on the quote unless something is found and agreed during the job.

Combi boilers heat water on demand with no cylinder or tank. They suit flats and houses with one bathroom and free up the airing cupboard. Their limit is flow rate: two showers at once, or a bath filling while someone washes up, and a combi struggles.

System boilers heat a hot water cylinder, usually an unvented one at mains pressure, so several outlets run at full flow together. They suit family houses with more than one bathroom. The cylinder takes space but gives stored hot water and a backup immersion heater.

Heat-only (regular) boilers work with a vented cylinder and a loft tank. They are the like-for-like replacement in older houses with that layout and where the pipework would not take mains pressure.

Which one is right depends on the house, the number of bathrooms, the mains pressure and flow, and how the household uses hot water. That is what the survey is for.

• Removal and disposal of the old boiler, and of the old cylinder and tanks where a combi replaces a stored system.
• The new boiler, hung and connected to gas, water, heating flow and return, and the condensate drain run to a proper termination.
• A new flue to the current standard, with the terminal positioned to the regulations and any inspection hatches fitted where it runs through a void.
• A magnetic system filter on the return to protect the new boiler from any sludge the flush did not reach.
• A chemical flush or a power flush of the existing system, then inhibitor added, with the water tested.
• Controls to the current regulations: a programmable room thermostat as a minimum, with weather compensation or a smart control where the boiler supports it.
• Commissioning to the manufacturer's procedure, combustion tested and recorded in the benchmark logbook.
• Notification of the installation to building control, and registration of the guarantee with the manufacturer.

The survey visit takes about an hour. The house is measured room by room for the heat loss, the mains flow and pressure are tested at the kitchen tap, the existing flue and boiler position are checked, the pipework is inspected where it can be seen, and how the family uses hot water is talked through. From that comes a written proposal with the type, the size, the position, the controls and the price.

A straightforward like-for-like combi swap is usually a one-day job. Moving the boiler, converting from a stored system to a combi or the other way, or adding a cylinder takes two. The water and heating are off for part of the day and back on before the engineer leaves; the old boiler goes with the van.

Any new boiler is a condensing boiler, and a condensing boiler only condenses, and only reaches the efficiency on its label, when the water coming back to it is cool enough. That means running it at a lower flow temperature than most people are used to, with radiators sized to give enough heat at that temperature and controls that modulate the boiler rather than switch it on and off. A boiler set up this way uses noticeably less gas than the same boiler run flat out; one set up badly is little better than the one it replaced.

Exactly how much a household saves depends on the old boiler, the house and how the heating is used, so no figure is promised. What is promised is that the new boiler is commissioned to run at the temperatures it was built for.

The mainstream boiler makers all produce reliable boilers at several price points, and the honest difference between them is often the parts availability and the length of the guarantee rather than the boiler itself. The quote names the make and model so you can look it up, and an alternative at a different price point is offered where it makes sense.

Manufacturers' guarantees have conditions: the boiler must be registered with them within a set period after installation, the system must have been flushed and protected with inhibitor and a filter, and the boiler must be serviced every year. All three are handled as part of the installation and the paperwork is left with you.