Is it possible for a Combi boiler expansion vessel to empty itself if full of water?

mrcheez

I have a Worcester Bosch 34 CDi combi boiler.. 10 years old.

For the past while the pressure was up around the 3 mark and when I had a Bord Gais engineer out to service the boiler he unscrewed something on the side of it and showed that the expansion vessel was full of water and would need to be replaced. He put the screw back on after a couple of seconds while it was still spewing out water.

So I have it on my TODO list to get the expansion vessel replaced soon.

I went out to check my boiler last week and the pressure was 1.3 or 1.5... today it's the same. So is it possible that the Bord Gais engineer did something that kicked in something else to flush out the expansion vessel?

Does it still need to be replaced if the pressure is now down to normal levels and the boiler appears to run fine?

Lenar3556

Where was the water coming out? Was it the schrader valve ordinarily used to set the expansion vessels charge pressure?

If so then it’s definitely goosed and should be replaced.

The expansion vessel basically has a rubber membrane inside, a bit like a basketball in thickness.

One side of it has the central heating water. The other side is full of air (which can be pumped up).

When the water in your system heats, it expands. So it needs somewhere to go. The expansion vessel allows it to expand against the balloon of compressed air.

That both allows the water to expand, and keeps a balanced pressure on the system.

If the air side of vessel is full of water, there’s a hole in the membrane and it’s leaked.

If the vessel is totally full of water, there’s nowhere for the system to expand. If you continue to use it like that it will be at risk of bursting a pipe.

mrcheez

How come the pressure dropped to 1.5 when it was at 3+ before?

10-10-20

https://www.boards.ie/discussion/comment/121422367#Comment_121422367

It probably vented out of the PRV (pressure release valve) as most boiler PRV's are rated to release at 3 bar.

mrcheez

so if the pressure is constantly at 1.5 what does that mean for the expansion vessel condition?

10-10-20

As explained by Retro above, the diaphragm within the vessel is known to be ruptured so the air expansion portion of the expansion vessel is now full of water, so you have no expansion capacity remaining on the system. Running at 1.5bar now is fine except when the whole system heats up, it won't have expansion space and the system will likely vent off again at the PRV. That's unsustainable in the medium term as you'll be introducing fresh water into the system to top-up, causing decay by diluting whatever corrosion protection remains and by introducing fresh water.

If the cost of the expansion vessel within the boiler is a concern then your other option here is to have a suitably sized expansion vessel fitted outside of the boiler if you have physical space. Those are typically less than 100 EUR and can be fitted in an hour or so.

But the way it is now you risk causing excessive stress on the boiler, pipework and rads when running without expansion capacity.

mrcheez

sorry to clarify, the pressure is 1.5 WHEN the heating is on, perhaps rising to 1.8

The original issue was that it was 3+ when off and on ... now it's 1.5 at rest and 1.5-1.8 when on

10-10-20

https://www.boards.ie/discussion/comment/121423117#Comment_121423117

Same applies. You hit three as the system must have done some refilling and expansion and got way too high. It's up to you at the end of the day but you're getting impartial advice here. The expansion vessel is important here, you risk further damage if the system becomes overpressurised.

mrcheez

Just so I understand, does 1.5 mean that the system is not over pressurised?

10-10-20

https://www.boards.ie/discussion/comment/121423234#Comment_121423234

1.5 is fine for a standard system. 1 to 2 is the normal guidance so that there is at least top-cover of +1 bar in the PRV.

mrcheez

ok I'll keep an eye on it to make sure it doesn't rise above while I wait for next available plumber to come around (which will probably be January at this stage)