Combi vs system boiler: which suits your household

Combi and system boilers explained in plain terms

A combi draws cold mains straight through a plate heat exchanger, so hot water is instant but limited to one strong draw-off at a time. A system boiler pumps heat into a separate unvented or vented cylinder, giving several bathrooms strong simultaneous flow at the cost of an airing cupboard. Homes with more than one shower running together, or with poor incoming mains flow, generally suit a system boiler better.

Practical notes on combi and system boilers

• Combi needs no cylinder, freeing up airing cupboard space
• System boiler supports multiple simultaneous hot outlets better
• Combi output drops if two taps open at once
• System boiler recovery time depends on cylinder size and coil rating
• Low mains pressure suits a system boiler with a pump more than a combi

Questions we get asked

Can I switch from system to combi?

Yes, if mains flow and pressure are adequate — an engineer checks incoming flow rate before recommending the change.

Does a system boiler waste heat keeping water hot?

A well-insulated modern cylinder loses very little standing heat, far less than older tanks.

Extra detail on combi and system boilers

Terraced houses around Radstock with a single upstairs bathroom are usually well served by a combi. Larger four-bedroom homes with an en-suite and a family bathroom tend to perform better on a system boiler, purely because two showers rarely run together on a combi without one losing pressure.

Where we deal with this

• Glastonbury (BA6) · 13 mi
• Lansdown (BA1) · 9 mi
• Street (BA16) · 14 mi
• Weston (Bath) (BA1) · 9 mi