What is an open vented system?
An open vented (or gravity-fed) system uses a cold water header tank, typically in the loft, to supply the heating system. Water flows by gravity and the system operates at low pressure. This is the traditional setup found in UK homes built before the 1990s.
What is a pressurised system?
A sealed (pressurised) system has no header tank. Instead, it operates at mains pressure, regulated by an expansion vessel and pressure relief valve. All combi boilers and modern system boilers use sealed systems.
Advantages of sealed systems
- No loft tank required — one less component to maintain
- Better water pressure throughout the system
- Lower risk of system contamination (no open header tank)
- More efficient — no heat loss from loft tank
When open vented makes sense
If your property has an existing open vented system in good condition with a heat-only boiler, replacing like-for-like is often the most cost-effective option.
Verdict
All new boiler installations use sealed systems. When replacing an open vented system, converting to sealed (either a combi or system boiler with sealed circuit) is the modern standard and usually recommended.