Overview
System boilers and heat-only (regular/conventional) boilers both use a separate hot water cylinder. The key difference: a system boiler operates in a sealed, pressurised system with no cold water header tank. A heat-only boiler uses an open-vented system with a header tank in the loft.
Hot water pressure
System boiler with unvented cylinder: delivers mains-pressure hot water — powerful showers throughout the house. Heat-only boiler with vented cylinder: gravity-fed, lower pressure — weak showers unless a pump is added. System boiler wins decisively on hot water pressure.
Loft space
System boiler: no header tank needed — loft is clear. Heat-only: requires cold water header tank in the loft. System boiler wins on space.
Installation complexity
Heat-only is the simplest like-for-like replacement in an existing open-vented system. If all the existing pipework is in place, it can be quicker and cheaper to replace like-for-like.
When heat-only makes sense
When replacing an existing heat-only boiler in a property where the existing pipework layout makes conversion expensive, or where the header tank and cylinder are relatively new. A like-for-like heat-only replacement can be £300–£500 cheaper than converting to a system boiler.
Maintenance
System (sealed): pressure must be checked and topped up if low. Heat-only (open-vented): self-topping from header tank — generally lower maintenance day-to-day.
Verdict
New installation or when replacing a system boiler: system boiler every time. Replacing an existing heat-only on a budget with existing good-condition cylinder and pipework: heat-only may be the more practical choice.