Baxi Duo-tec - Diagnosing Expansion Vessel Failure?

[colcob]

I have a baxi duo-tec that's been installed for about 2 years, since coming back on this season, the system has been losing pressure at a dramatic rate, requiring topping up daily to start with, now twice a day.

I've spent a day checking the system for leaks as best I can, didn't find anything, but I don't think it's that. When the system was running at full temp, system pressure went up to 3 bar, so I concluded that the expansion vessel was either not pressurised or failed.

So I pumped up the expansion vessel to just over 1 bar. The next time I fired it up the system pressure stayed at about 2 bar when running, so I figured it was fixed, but then the system rapidly lost pressure over the next day, needing topping up twice to stay running. I presume that the extra pressure loss was due to the expansion vessel deflating again. So on the off chance the valve was a bit dicky I WD40'd it and pushed the pin up and down a bit, then re-pressurised, but it lost pressure again just as quickly. There also seemed to be a bit of water coming out of the valve when I let it down.

It seems to me that a failed expansion vessel is the only reasonable explanation, but I'm no expert, so before I go to the trouble of replacing it, I thought I ask if there were any other plausible explanations, or things I could try.

I was going to isolate the boiler from the system and do a pressure drop test on just the pipework, but it hardly seems worth it when the 3bar running pressure seems to point strongly to the EXV. I suppose the other possibility is that the PRV is leaking, but how do I go about checking that?

Any more knowledgeable advice would be gratefully recieved.

Also, replacing the EXV looks fairly straightforward (I put the system in myself except for gas and boiler commissioning, so I have some idea what I'm doing!) but is it legal/sensible to attempt to replace it myself?

 

[John506]

Water coming out of the valve on the expansion vessel means that your diaphram has split, sorry to say but the only solution is a new expansion vessel.

 

[Baxmax]

isolate flow and return drain boiler via drain off not prv,remove xp vessel take it out side and attach a foot pump to it and blow that water out,re-presurise and refit.

 

[John506]

isolate flow and return drain boiler via drain off not prv,remove xp vessel take it out side and attach a foot pump to it and blow that water out,re-presurise and refit.

This would be pointless as the diaphram inside the expansion vessel is fooked.

You wouldn't need to take the vessel off to recharge even if the diaphram was intact, you would just have to drain the system and do it.

 

[Baxmax]

Do a lot of these Johny and i must say removing the xp and pumping out the water is an excelent method i would like to bet it is not fooked. Watch this space

 

[MANDATE]

Well if you're sure its water and not WD 40 coming out, then its a diaphragm problem.
By all means remove it and replace it cause it can't be repaired.
You could leave it in situ and install another one in a suitable position

 

[John506]

Do a lot of these Johny and i must say removing the xp and pumping out the water is an excelent method i would like to bet it is not fooked. Watch this space

Water coming out of the schrader valve, I give my life its fooked.

 

[gazthepottertonengineer]

Unless you have pumped water into the shraeder there should only be air coming out ! when they go you can normally see rust mark along the seam at the bottom of the vessel !

 

[colcob]

Well it was only a few spits of water coming out as it deflated, and it didn't carry on coming out if I held the valve down, but yeah, I cant see how it could have got on the air side of the EXV if the diaphragm was ok.
Thanks for your help everyone, new expansion vessel on order.