If your heating system constantly loses pressure, it could be due to an expansion vessel problems. If the vessel is outside the boiler you can repair it yourself. An expansion vessel works in the same way whether it is fitted to a water heater, a gas boiler, or an unvented cylinder.
What is an expansion vessel?
An expansion vessel is a bubble of air sealed inside a metal tank. The water inside your radiators expands as it heats up, the expansion vessel is designed to take the expansion of the water without the pressure getting out of control.
How do I know the expansion vessel is faulty?
The simplest and least reliable technique is to tap it with something metal. If it rings and sounds hollow the vessel is probably full of air and okay. If it gives a dull thud then it’s probably full of water and requires servicing.
This method is of very unreliable and should not be used.
The correct way to test a vessel is to drop the system pressure down to 0 bar. Then attach a pump to the Schraeder valve. The air pressure should be the same as stated on the data badge.
The data badge will tell you what the air pressure inside the vessel is supposed to be set at. In this case, the pre-charge is 1.5 bar.
Why do I drain the pressure from the system first?
If you don’t remove the pressure from the system you will get an incorrect reading. There may only be a tiny volume of air in the expansion vessel that is being squeezed to the correct pressure by the water in the system.
How do I service an expansion vessel?
Turn off both filling loop valves. Gently and slowly remove the pipe from the central heating end of the filling loop. Remember there is always a chance the valves aren’t fully closed even when the handle is in the off position.
Drain any remaining pressure from the system. Attach a pump to the Schraeder valve of the expansion vessel and inflate to 1.5 bar. If the expansion vessel is full you may get a bucket full of water from it.
When the expansion vessel is re-inflated to the correct pressure refit the expansion vessel hose.
Refill the system to the correct pressure, normally somewhere between 1 and 1.5 bar.
What can go wrong with an expansion vessel?
Leaking schraeder valve
The valve used to pump up the expansion vessel can leak. It’s actually quite a common fault so it’s worth changing the Schraeder core when you service the expansion vessel. You need a removal tool which is commonly available and a replacement core.
Split diaphragm
The air bubble is held in place by a thick rubber diaphragm. Over time this can degrade and split. Unfortunately, this normally can’t be replaced and the whole tank has to be replaced.
Blocked hose
The water doesn’t move very fast up and down the hose to the expansion vessel. This can cause a build-up of dirt and sludge in the pipe. As it’s normally a small-diameter rubber hose this can cause blockages.
This fault shows itself as an expansion vessel fault when the expansion vessel isn’t faulty. The pressure will rise far too high when the central heating is used.
The pipes are normally either unblocked or replaced.
Leaks
The metal of the tank and the connections to the tank can become corroded and leak. If the tank is corroded enough to leak then it should be replaced.
Summary
The expansion vessel is an essential part of a heating system if it fails or needs to be serviced it can cause an array of pressure problems.
It’s always best to employ a fully qualified heating engineer to repair faults on your boiler, central heating system or hot water tank.
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