Open vented systems
Open vented systems have a small feed and expansion tank in the loft that tops up the heating water and allows it to expand as it heats. They work at low pressure and are common in older homes with regular boilers.
Sealed systems
Sealed systems are pressurised and closed. Instead of a loft tank, they have an expansion vessel and a pressure gauge, and are topped up from the mains using a filling loop. Combi and system boilers normally run on sealed systems.
- Pressure gauge on or near the boiler: probably sealed
- Small tank in the loft feeding the heating: probably open vented
- No loft tanks and a combi: sealed
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Converting between them
When a regular boiler is replaced with a combi or system boiler, the system is usually converted to sealed. Older pipework and radiators may struggle with higher pressure, so engineers check for weaknesses and may recommend cleaning or replacing parts.
