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Sealed Systems

Many homes today have a sealed system whether on a combi boiler or a boiler and cylinder type system. But how do you know if you have sealed system? And what are the advantages and disadvantages of having this type of system?

How Do I Know If I Have A Sealed System?


A sealed will have:


A pressure gauge, to indicate the fill and system pressure. - Located near the expansion vessel.


A filling loop, this is how you fill the heating back up via a temporary supply pipe that should have a double check valve or other means of preventing back-siphonage. - This can be integrated on the boiler or remotely located on the pipiework.


An expansion vessel, this takes up the expansion of the heated water (water expands when it gets heated). - Located in the boiler or remotely on the system, usually red in colour.


A pressure relief valve, this is set to open at a set pressure (usually 3bar) to allow the water to discharge to a suitable drainage point. This releases the excess pressure, stoppping any resulting damage to the system being caused. - Located near the boiler or remote expansion vessel.

Advantages vs Disadvantages




  • Less pipework is needed on installation (than open vent systems).

  • Drawing  air into the system via vent pipes or pumping over ,as fully pumped systems do sometimes, is eliminated.

  • Higher water temperatures can be achieved.

  • The boiler can be positioned anywhere, (where regulations are met, see manufacturers instruction and/or corgi for details) even in the roof space, as no header tank is required.

  • Hot water at same pressure as the cold water.

  • No chance of tank in loft freezing and bursting as there is no tank.




  • You have to manually fill the system.

  • Should you have a leak that goes unnoticed and the pressure drops too low the boiler will stop working.

  • If you have a comi boiler it will take longer to draw off big amounts of hot water - like when your running a bath.

Example of a pressure relief valve. Colours vary. Click image to get your pressure release valve.
Filling loop image,click to find a filling loop.
Boiler pressure gauge,click image to get your boiler pressure gauge.
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