Combi vs System Boiler

A combi heats your central heating like any boiler, and when you turn on a hot tap it diverts its heat to a plate heat exchanger that warms mains water as it passes through. There is no stored hot water, no cylinder and no loft tank, which frees up space and means you never run out. The limit is flow: a combi can only heat as much water as the mains delivers, and it serves one outlet properly at a time. Two showers at once and each gets roughly half. The number on a combi's badge, such as 30kW or 35kW, is its hot water output; its heating output is lower.

A system boiler heats the central heating directly and also heats a pressurised (unvented) hot water cylinder, usually in an airing cupboard. Because hot water is stored, two showers can run at once at good pressure, provided the cylinder is big enough for the household. The trade-offs are the space the cylinder takes, a short wait for the cylinder to reheat after heavy use, and an annual inspection of the unvented cylinder, which most manufacturers ask for. A cylinder also works with a solar diverter or a future heat pump.

As a rule of thumb, one bathroom and a healthy mains flow points to a combi; two bathrooms, or a household all showering in the same hour, points to a system boiler. The measurement that settles it is the mains flow rate at the kitchen tap with the water running, not a static pressure reading. Below about 12 litres a minute a combi tends to disappoint whatever its badge says; 15 to 20 gives a shower most people are happy with. In Liverpool, a one-bathroom terrace is the classic combi home, while the multi-bathroom villas of Woolton, Southport and west Wirral usually do better on a system boiler. Our boiler installation page covers the survey in detail.

Plenty of older homes still have a regular boiler with tanks in the loft. Converting to a combi removes the tanks and cylinder and frees space, but it changes the pipework and puts an old system under mains pressure, so weak joints are checked first. Converting to a system boiler keeps stored hot water but removes the loft tanks. Both are quoted at survey because the pipework varies house to house, and either can be paid for monthly, subject to status. See our guide to how much a new boiler costs for what is included in a fitted price.

A heat pump cannot heat water on demand, so it always needs a cylinder. If a heat pump is likely in your future, a system boiler with a cylinder sized and positioned sensibly now makes the later switch simpler. Our air source heat pump page explains what a switch involves.