Expansion Vessel explained please...

[mrbriggs]

Hi all

I have a grant combi 90 with a faulty expansion vessel (going to be fixed by the weekend hopefully!) just wondered if someone could explaing the expansion vessel. I thought I understood what it does, but after leaving the heating off and just using the boiler for hot water, the boiler doesn't seem to lose pressure?

Does the hot water side of things not use the expansion vessel? Or is it because the central heating causes more pressure than heating the DHW and causes the PRV to open at 2.5 bar, thus losing pressure?

I'm learning SLOWLY!!!

 

[djheath]

As you heat water it expands.

Hot water is heated but it comes out of the tap so the expansion is minimal.

for central heating, the heating is done in a sealed system so expansion has to go somewhere. An expansion vessel is filled with air which can be compressed so this takes up the expansion. If the expansion vessel fails then there is no where for the water to go so it increases the systme pressure until the PRV pops and lets the water out to reduce the pressure.

I think thats how it all works!

 

[mrbriggs]

OK, that's great! That explains things!!! Can't wait for this part to turn up so I don't have to huddle around the fire in the front room! It would have to fail just as the UK is about to be hit by another cold spell! TYPICAL!

Thanks guys (and girls!)

 

[WDIK]

the explanation is almost right, with one minor point missed.

the expansion vessel has a hollow steel body, within that there is a bladder or diaphram seperated from the water, containing air.
that air expands or contracts to maintain an even pressure on the system.
the air pressure can be adjusted via a schrader valve just like a car tyre.

if the bladder or diaphram perrish's or splits then the charge of air is lost, then there is no where for expanding water to go hence the rise in system pressure.
when it reachs 3bar the boiler ejects the expanding water via the safety valve/prv.
so when the boiler turns off and cools down, the water contracts and now shows low pressure, so you in your innocence top it back up again, and the cycle starts all over again.

 

[mrbriggs]

Cool, thanks for that guys.

How long do these normally last?

And why is it everyone I have spoken about this, they have just said, it's either the PRV or there is a leak!