Open Vented vs Sealed Heating Systems Explained

Most homes with central heating have either an open vented or sealed heating system. The difference is mainly how the system manages water and pressure. Understanding the two arrangements can be useful when replacing an older boiler, diagnosing heating problems or considering changes to the plumbing system.

How Open Vented Systems Work

An open vented heating system uses a feed and expansion cistern, usually located in the loft, to supply water to the heating circuit. It also has an expansion pipe that allows water to expand safely as it heats. These systems are commonly found in older UK properties and are typically paired with regular boilers.

One advantage is that they can work with relatively low mains water pressure because the heating circuit is supplied from the storage cistern. However, they require additional components and loft space, and low water pressure can affect the performance of some systems.

How Sealed Systems Work

A sealed heating system does not use a feed and expansion cistern in the loft. Instead, the heating circuit is filled with mains water and operates under pressure. An expansion vessel allows for the increase in water volume as the system heats, while a pressure relief valve provides protection if pressure becomes excessive.

Sealed systems are commonly used with modern system and combi boilers. They can offer good pressure and are often more straightforward to install where there is no suitable loft space for a storage cistern.

Which System Is Right?

Neither arrangement is automatically better for every property. The most suitable option depends on the existing heating system, available space, water supply and household requirements. When replacing a boiler, a qualified heating engineer can assess whether the existing arrangement should be retained or converted to a sealed system.

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