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The expansion vessel has changed the way central heating systems work.
Not so long ago, most central heating systems installed in the United Kingdom were open vented.
To explain what open vented means, as a boiler starts to heat up, the water within, the system starts to expand, and the expansion needs to go somewhere,
so you have an expansion pipe, which normally runs up into the loft area,
where you will find a feed and expansion tank.
The pipe is open-ended; this is because if the expansion is too much then it will disperse into the tank.
Now with the expansion vessel, there is no need for a feed and expansion tank to be sited in the loft area.
The expansion vessel is a very clever piece of kit. It works by taking up the expansion of the central heating system as it heats up.
The central heating system now becomes, a sealed or otherwise known as a pressurized system.
Because the system is now sealed, it now works more efficiently, thus saving on the energy bills.
This valve is normally set to about 3 Bar. Once the system pressure goes above that, it will start to discharge to the outside or into a drain.
Because you now have no feed and expansion tank, you will need a fill-up system;
normally this is just flexible fill up a loop that is connected to the cold water mains and the return pipe on the boiler
With the expansion vessel, they come in many sizes, so it’s very important that you get the correct size for your system.
but it’s still very important you make sure you have enough expansion for your system,
otherwise, you will find that the pressure relief valve is always discharging.
I have been called out so many times by customers saying they have a leak on their central heating.
As soon as I start to check over the system, the first thing I look for is, does it have an expansion vessel or a feed an expansion tank.
If it has an expansion vessel, then I check to see if it has any pressure within, this can be done with a normal tire gauge.
An easy way to check an expansion vessel is to tap it with another metal object, something like a key, if it has a tinny sound, then it’s ok,
but if it has a dull sound then it’s full of water. When they are full with water, the system has no expansion, it’s then that the pressure relief valve kicks in and starts to discharge.
The main reason this happens is, one lack of maintenance, the other being undersized for the system.
but for some reason, this always seems to be overlooked.
Not so long ago we were called to a job in Notting Hill London; the problem was no central heating.
The house was very big it had a basement and 4 floors above.
We shown into the boiler room and found two system boilers, the system was sealed and pressurized,
but we could not find any more expansion vessels.
Both these boilers were less than two years old, but because they had been fitted with no extra expansion vessel, they were beyond repair.
The Boilermakers were called, as they were still under warranty, but they walked away saying incorrect installation had caused many faults inside the boilers.
The main reason being, they needed an extra expansion vessel fitted.
So to sum up these boilers were removed and new ones fitted also with the extra an expansion vessel of the correct size.
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