The expansion vessels on today’s unvented boilers is a very important part of the boiler,
but for some reason seems to get overlooked when a service is
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The expansion vessels takes up the expansion of the water within the boiler when it is running.
As the water heats up, it starts to expand and needs somewhere to go,
this is where the expansion vessels come into play.
With a normal vented boiler, the expansion disperses up the open vent pipe.
The expansion vessel also helps to heat the radiators up quicker,
the water flows around the system a lot quicker than a vented system,
because the water moves faster due to being pressurised.
When the expansion vessels stops working you will notice a couple of things
with your central heating system.
The first one being that the central heating pressure will drop,
you may have to top up the pressure a few times a week sometimes even more.
When the pressure starts to drop most people think they
may have a leak somewhere, but cannot find it.
The local gas safe engineer is called in to trace a leak on the central heating system
,but cannot find anything that’s visible.
He may then suggest the next course of action is to start lifting the floor boards to try and
trace the leak. You then pay him for his time and he moves on to the next job.
Now if the local Gas Safe Engineer is an experienced engineer with many years behind him,
one of the first things he will check
is the expansion vessel. 9 times out of 10, the vessel is the problem and there is no leak on the system.
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The expansion vessel is like a sponge, is absorbs the expansion of the heated water, and as
time goes by it starts to lose its internal pressure. Once it looses it pressure,
water then starts to enter the vessel. Once water gets into the vessel the
metal casing inside starts to rust up and eventually it start to leak water.
The expansion vessel has to be set up to the boiler and the system, too small a vessel and the vessel
will not last long. Its better to have a vessel bigger than smaller.
The vessels that are built into the boilers are normally on the small side and you may need to fit an external vessel as well.
We have seen in the past a small vessel fitted to a central heating system that had 22 radiators and 2 unvented hot water cylinders connected. When the system was running the owner had to keep topping up the system pressure twice a day to be able to have the heating working. After taking our advise that the vessel was to small for his system, as it was only designed for around 8 radiators and one hot water cylinder. He gave us the go ahead to install the correct size vessel for his system. 6 weeks later he call to say thank you as the problem had now gone.
When you have your boiler serviced you should always check with the engineer that he has checked the expansion vessel. It normally needs to be pumped up every year, only a little but should always be checked.
With the wrong size expansion vessel fitted you are running the risk of reducing the life expectancy of your boiler, changing pumps regular and even leakage from the pipe work and radiators .
Don’t throw your money down the drain always have your boiler serviced and the expansion vessel checked over.
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