A new boiler sized for the house, not the one that was easiest to sell

A combi heats water on demand with no cylinder or tank. It suits flats and smaller houses with one bathroom, frees up the airing cupboard and gives mains-pressure hot water. Its limit is flow rate: one shower at a time, and a bath fills at the pace the boiler can heat the water.

A system boiler heats a hot water cylinder, usually an unvented one at mains pressure. It suits houses with two or more bathrooms, or anywhere several people want hot water at once. The expansion vessel and pump are inside the boiler, so no loft tanks.

A heat-only (regular or conventional) boiler works with a cylinder and loft tanks and is the natural like-for-like replacement in older houses with a gravity-fed system, where changing the whole system is not wanted. It is also the right choice where the mains pressure is too poor for an unvented cylinder.

The choice is made from the hot water demand and the property, not from what is on offer that month. You are told the reasoning either way.

A boiler is sized in two directions. For heating, a room-by-room heat loss calculation gives the output the house needs on the coldest day; most London homes need a lot less than the 30 kW boilers that get fitted by default, and an oversized boiler cycles on and off, wastes gas and wears its components. For hot water on a combi, the flow rate is sized from the number of bathrooms and the incoming mains flow, measured at the tap, because a combi can only deliver what the main brings in.

Both calculations are done at the survey and written into the quote, so the boiler model on the paperwork is there for a reason you can see.

The quote lists everything the job needs, and it is more than the boiler:

• Removal and disposal of the old boiler and, where relevant, tanks and cylinder.
• A chemical clean or power flush of the existing system, depending on its condition.
• A magnetic system filter on the return, which every manufacturer now requires for the warranty.
• New flue and terminal, positioned to the regulations, with a plume kit where a neighbour's window is close.
• A condensate pipe with a proper fall, insulated or run internally so it cannot freeze.
• Gas supply checked for the boiler's demand and upgraded where the old pipe is too small.
• Controls: a room thermostat and a programmer as a minimum, with weather compensation or load compensation where the boiler supports it, and thermostatic radiator valves where they are missing.
• Commissioning with a flue gas analyser, the benchmark checklist completed, and the boiler registered with the manufacturer and notified to building control.

Plenty of London homes still run on a back boiler behind a fireplace, a floor-standing boiler in the kitchen or, on some estates, a warm-air heating unit. Replacing these is a bigger job than a wall-hung swap: the old unit comes out, the flue route changes, the gas run is often extended and the boiler is repositioned to an outside wall or a cupboard. It is planned at the survey, with the new position agreed with you and the making-good included in the quote.

Boiler replacements in flats have their own rules. The flue terminal has to be a set distance from windows, doors and boundaries, which in a tight lightwell can rule out the existing position. Shared flue systems in mansion blocks have to be assessed as a whole before a boiler on them is changed. Freeholders and managing agents often need to approve the flue position and the condensate discharge. All of that is dealt with before the quote is finalised, so the job does not stall on the day.

Manufacturer warranties on new boilers run for several years and depend on three things: installation by a registered installer, a filter fitted and the system cleaned, and an annual service recorded in the logbook. All three are done and documented. You receive the benchmark logbook with the commissioning readings, the building regulations compliance certificate, the warranty registration and a walkthrough of the controls before the engineer leaves. Annual boiler servicing from then on keeps the warranty valid.