Expansion vessel unable to pressurise

[Brotheroak]

The pressure in my CH system has been fluctuating a bit more than usual recently, so thought I'd check the pressure in the expansion vessel. Reading was zero. No water leaking from schrader valve, and "pinging" the side of the vessel sounds hollow, so I'm thinking the diaphragm may be ok.
Thought I'd try to put a bit of air into the system. Note all this was done without draining the system down - I was just trying to look for problems before I got a more accurate pressure reading.
Completely unable to put any air into the system - using two different tyre pumps (one a foot pump, one an electric one) - both just refused to put air in (e.g. electric pump reading went from 0bar to 5bar in 1 second - i.e. it pressurised the tube to the valve, but couldn't put anything into the valve)
I am able to push down on the schrader valve - no air or water comes out (as I say, reads 0bar on two different pressure gauges)
I changed the inside of the valve with a fresh valve internal - no difference.

Do you think the vessel itself needs changing? Or is this what you would expect if the system wasn't drained down? (if you hadn't guessed - this is all new to me - keen to DIY where possible, as it's all a learning experience!)

(FYI, it's an Aquasystem 18L expansion vessel, no service valve, so would need a full drain of the system to replace, I guess?)

 

[Burnerman]

Are your pumps able to depress the valve 'pip' at the centre of the valve core?
John

 

[Brotheroak]

Hi John,
They should do - they are standard tyre pumps that I use successfully to pump up the tyres on the van ok

I've just tried removing the inner core of the valve completely - same result, so I am guessing either the entire schrader valve is buggered, or there's a blockage somewhere?

 

[ScottishGasMan]

1st. You need to release the system pressure before trying to blow up the vessel.

2nd. Sometimes the Diaghram sticks to the shreader, especially if its been flat a while.

So, open drain on system, then try again at the vessel, often when the pressure gets really high, really quick you'll hear a pop and the pressure will drop off to zero again, then it will increase at normal speed. The pop is you unsticking the diaghram from the shreader port.

 

[Brotheroak]

Thanks for the responses. I "bit the bullet" and drained the system. Then I found out what an idiot I was - not for the first time. Turns out this particular expansion vessel is not attached to the heating system (in my defence, the pipework is a bit of a mess due to lots of extensions to the house before we moved in!) - it is in fact attached to the mains water. I found this out *after* I disconnected it thinking the pressure was zero.
I now have a slightly flooded airing cupboard!
Luckily I realised the issue fairly quickly and the stopcock was close by.

Once properly disconnected and emptied, the vessel was able to be inflated ok - still not sure why it wouldn't work before (maybe diaphragm stuck, as you say @ScottishGasMan) but seems ok now, and I managed to install a service valve while disconnected (though am now concerned this service valve may not be mains pressure rated?! Still, at least I can replace it easily now!)

So, I've learned a bit about my system, but no idea why my mains water inlet needs an expansion vessel?

There is another EV within the boiler (I knew this before, just thought it was buggered - it leaks water from the valve - and that a new EV had been installed as cheaper than a new boiler-based EV). I'm guessing that one will need to be replaced (as the diaphragm obviously split) to fix the original fluctuating pressure issue.

Thanks for your help

 

[ScottishGasMan]

Out of curiosity, was it quite small? like maybe it would hold a litre or two? They are sometimes on the cold in houses to take up a little expansion from the water being pre-heated by boiler or to prevent/reduce water hammer in the system. If its on the mains cold then they often are completely flat with the diaghrams pressed against the shrader.

If its part of an unvented hot water cylinder, you should be careful and not modify or add any valves or equipment to it in any way, as there is quite a few bits of safety equipment around them.

Any fittings you use on hot/cold mains water systems should be WRAS approved, so you know the materials in it are suitable for being in contact with your pottable water supply.

 

[Brotheroak]

Nah - 18 litres, so a pretty big one. Straight off the mains, with other feeds to other cold water taps and to the boiler itself, all off the same pipe.

Great info on the WRAS approval - I bet it isn't (I'll check). I'll remove it if not. At least it's now easy to remove, now I know to turn off the water supply (service valve allows easy draining of the vessel itself)