Heat-Only vs System Boiler: What's the Difference?

Heat-only and system boilers both work with a separate hot water cylinder — but the system design is quite different. Here's what distinguishes them.

Heat-only (regular) boilers

Also called "conventional" or "open-vented" boilers. They require:

• A cold water header tank (usually in the loft)
• A hot water cylinder
• A feed and expansion tank (for the central heating)

They're typically found in older homes built before the 1990s. The gravity-fed system means water pressure is determined by the height of the header tank above the outlets.

System boilers

System boilers work with a sealed system (no header tank) and an unvented cylinder. All major components (pump, expansion vessel, pressure relief valve) are built into the boiler. They deliver mains-pressure hot water and require no loft tanks.

Which to choose?

Replacing an existing heat-only: System boiler is usually better unless the header tank and existing cylinder are in good condition and the loft space is needed.
New installation: System boiler with unvented cylinder is now the standard specification — better performance, mains pressure, and simpler system design.

The cost difference

A system boiler costs slightly more than an equivalent heat-only model (~£100–150). The installation can be more expensive if converting from open-vented to sealed. But the performance benefits typically justify the investment.