Combi or System Boiler? How to Pick the Right Smart Thermostat

Combi boiler: there is no hot water cylinder in an airing cupboard and no tank in the loft. Hot water comes on as soon as you open a tap. Most UK homes built or updated in the last 20 years have one.

System boiler: there is a hot water cylinder, usually in an airing cupboard, but no water tanks in the loft.

Regular (heat-only) boiler: there is a hot water cylinder and a cold water tank in the loft. These are common in older homes.

The model name is usually on the front or bottom of the boiler. Search it, or check the manual, if you are not sure.

A combi boiler heats water when you need it, so the thermostat only has to control heating. That means you have the most choice, including cheaper models like the Meross and Netatmo in our smart thermostat guide.

Look for OpenTherm support if your boiler has it. It lets the thermostat ask for gentler, lower heat instead of switching on and off, which can help the boiler run more efficiently.

With a hot water cylinder you also need hot water control, so you can set when the tank heats up. Pick a kit that clearly says it controls hot water, like the tado Wireless Smart Thermostat X or tado V3+ starter kits.

Thermostats sold only for combi boilers, like many Hive Mini and Meross models, will not control your hot water. Check the listing carefully.

Heat pumps work best with long, steady, lower temperature heating. Some smart thermostats, such as tado X and Honeywell T6, list heat pump support. Ask your installer before changing controls, as some heat pumps need the maker controls to keep their warranty.

Whatever your boiler, you can add smart radiator valves to heat only the rooms you use. Stick to the same brand as your thermostat. See our best smart radiator valves UK guide.

Many people fit wireless kits themselves, but it usually means working with mains wiring at the boiler or wiring centre. Turn the power off first, and use a qualified electrician or heating engineer if you are unsure. Never open the gas parts of a boiler: only a Gas Safe registered engineer can do that.