A central heating system designed for the house, from the heat loss up

The boiler is chosen last, once the heat loss and hot water demand are known, because those numbers decide whether it should be a combi, a system boiler with an unvented cylinder or a heat-only boiler with a vented one. In a house with two bathrooms and a family, a system boiler and a cylinder nearly always beats a combi. In a flat with one bathroom, a combi nearly always wins. Where the property is being renovated, the boiler position, the flue route and the cylinder cupboard are agreed at the design stage so the builder can plan around them.

A first-time installation into a property with no central heating, or with old storage heaters, is a full design and a full first fix. Pipework is planned to run in floor voids, up service risers and behind boxing, with the fewest visible runs. Radiators are positioned under windows where the heat loss is highest, sized to the room, and fitted with thermostatic valves. A system with more than a few radiators is split into zones with motorised valves, so upstairs and downstairs, or bedrooms and living rooms, can run on different timings. The whole system is pressure tested before floors go back down and commissioned and balanced once the boiler is live.

Most central heating jobs in London are rebuilds rather than first installations. The survey looks at what is worth keeping: pipework that is sound and correctly sized stays, radiators that are undersized or corroded go, microbore pipework that is choking the flow is replaced with a proper 15 and 22 mm system, and the controls are brought up to date. A rebuild is planned in stages where the house is occupied, so there is heating at the end of every day of the job.

Between installations, the same engineers repair the systems that exist: failed pumps and motorised valves, cold radiators, airlocks, noisy pipework, thermostats and programmers that have stopped talking to the boiler, and heating that runs when it should not. Faults on the system are diagnosed separately from faults on the boiler, because a boiler that fires perfectly into a system with a stuck zone valve is not a boiler fault, and a repair that replaces the wrong thing solves nothing.

A designed system needs looking after like any other: the boiler serviced annually, the inhibitor tested and topped up, the magnetic filter cleaned, the expansion vessel charged and the balancing checked if radiators have been changed. The annual boiler service covers the boiler, and a system check is added where the system warrants it.

Total loss of heating in winter is attended as a priority. Say what the boiler is doing, whether the hot water works, whether any radiators are warm and what the controls are showing. Many heating failures are a single component, a pump, a valve, a thermostat, that is on the van and fixed on the visit.

Controls are where the running cost is decided. A single thermostat in the hall running a whole house is a 1980s solution. A modern system has a room thermostat per zone, thermostatic valves on every radiator except the one in the thermostat's room, a programmer that understands weekends, and, on a boiler that supports it, load or weather compensation that turns the flow temperature down when the house does not need full output. Compensation is the single change that makes the largest difference to a gas bill on a condensing boiler, because it keeps the boiler in condensing mode for most of the year.

Victorian and Edwardian terraces with high ceilings and single-glazed bays that need more output than their size suggests. Interwar semis with solid walls. Post-war flats with concrete floors where pipework has to run on the surface or in boxing. New extensions and loft conversions being added to old systems. Rental properties where the landlord wants a system that will not generate calls. All of it, across London and the surrounding towns.