In a sealed system you have to allow for this expansion. This is done by adding an expansion vessel to the system pipework and it's often inside the boiler. The expansion vessel is just a chamber with a rubber diaphragm across the middle. One side of the expansion chamber is connected to the system pipework. The other side has a car tyre type valve so it can be pumped full of air (or nitrogen when they’re manufactured).

In manufacture the air side is pumped up, typically to about 1 bar (about 15 psi). This forces the diaphragm right across to the opposite wall of the chamber. When the central heating system is filled with water and pressurised (to about 1 bar) the water pushes the diaphragm back to about the middle.

When the system is heated up the water expands a little but this expansion is accommodated by pushing the diaphragm across partway and compressing the air pocket. The pressure gauge on the boiler or on the pipework will go up by maybe a ¼ bar or ½ bar.

If the air pocket has been largely or completely lost, the pressure will rise sharply as the system is heated, maybe by more than 1½ bar.

To stop the pipework bursting, the boiler or pipework is fitted with a 3 bar pressure relief safety valve (usually with a red cap). If the pressure goes above 3 bar this valve is forced open and water passes out of the system along a safety pipe and is dumped outside the building.

Though a failed or de-pressurised expansion vessel is the most likely culprit when water passes out through the safety pipe it is not the only possible cause. If the plate heat exchanger (secondary heat exchanger) fitted to most combi boilers is leaking internally water will probably be forced out through the safety valve and the pressure gauge will read consistently around 3 bar.

So next time you see the discharge pipe or pressure relief pipe discharging or the boiler pressure goes to the read when its on.. and then the pressure gauge indicator goes below 1 bar when its cold... the problem may be faulty expansion vessel.

If you live in London within M25 and need a boiler service or other plumbing or electrical service, book us by calling 02082008580 / 07974711639, or email [email protected] . Please visit our Website www.unicuk.com to see our other services and charges.