The components, pressures and trade-offs that separate sealed from open-vented heating at Level 3.
Open-vented systems
An open-vented system uses a feed and expansion (F&E) cistern in the loft. The cistern keeps the system topped up, accepts expansion as the water heats, and the open vent pipe provides a safety path to atmosphere. Pressure at the boiler comes from the static head of the cistern above it. Drawbacks include loft pipework and tanks, a freezing risk, and continual oxygen ingress that encourages corrosion.
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