Expansion Vessel Problems - Advice Please!

[MikeS1]

Hi All,

My first post but I have observed for quite a while!

I have a Vokera Linea 24 which I think I have now proved has an expansion vessel problem. The same old symptom - loss of pressure in a short period of time (approx 1 - 2 days now heating is in use).

I drained the system and checked the pressure in the expansion vessel - a pig to get too right at the back of the boiler but can "just" get to the valve without removing any covers!
Found 0 psi pressure!

So pumped it up to 10 psi - approx 3/4 bar. It seemed to hold the pressure, but when I looked at the water pressure gauge it showed 1/2 bar - I assume this means the air has leaked through the vessel into the main system?

I filled it back up with water anyway to 1 bar. System performed well for about a week but has now started to degrade again, loosing its pressure slowly. I assume the air is being removed by the auto-air valve?

Sorry this post is so rambly but I wanted to fill in the background before getting to the point!

So, I think I need a new expansion vessel (to verify this is the problem I intend to drain down the system again and re-check the vessel presure, which I expect to be close to 0 psi).

I have a couple of questions:

1) I intend to fit an external expansion vessel to avoid having to touch / take apart the boiler - I assume this is just as good a fix as replacing the internal vessel?

2) From looking at previous posts and information, this should be fitted on the central heating return as close as possible to the boiler to protect the pump from air blocks - am I right?

3) I'm unsure what size of expansion vessel to fit - I have five rads - two double and three single. I see that I can get 12, 18 or 24 litre expansion vessels. I think 12 would be sufficient, but can't see any problem with fitting a vessel bigger than is required (i.e. 18) to err on the side of caution?

4) Does the size of expansion vessel I fit affect the pressure I pump it up too - I assume yes, as 10 PSI in a 12 litre vessel is much less air than 10 PSI in an 18 litre vessel - If so how do I calculate?

Thanks in advance, and sorry for such a long post!

 

[nickso]

you appear to have pressurised the vessel incorrectly if im reading all that correctly.

read the FAQ's on this site regarding vessel recharge. you didnt leave something open on the system and probably only put a 10th of the required pressure into the vessel.

 

[hi1]

Hi, i'm not a heating engineer but if you've pressurised the expansion vessel then you would get a reading on the water pressure gauge as the water is being pressurised by contact with the pressure in the expansion vessel. i believe there is a membrane separating the two otherwise the air would disappear off into the heating system.
10psi is 10 psi whatever the size of the vessel, you may have to pump more in to arrive at 10 psi with differing size vessels but when the reading reaches 10 psi stop pumping.
i'll leave the rest to greater minds than mine, regards

 

[MikeS1]

I think I missed this point from the re-pressurising instructions...

During this phase it is IMPERATIVE that water or air is allowed to escape from the heating system as it is pumped out of the PV. Otherwise the system pressure will resist your pumping and give a meaningless reading. If you aren't sure, let some more water out when you have the air pressure at the right level. If that makes the air pressure drop, pump a bit more and release water and so on, until it doesn't.

Hopefully this will solve it - when I press the valve in the Expansion Vessel, no water, moisture or anything other than air expels, so perhaps there is still hope for a simple fix!!

 

[MikeS1]

Ok,
Certainly after draining down the system and pumping the vessel to 12 PSI I can then drain more water out and repeat, and again, and again...

Finally, the system pressure gauge remained at 0 when I had 12 PSI in the vessel. I left the foot pump attached for 10 minutes and the expansion vessel stayed at 12 PSI so I assume it is good or at least a very slow leak.

Now after filling the boiler to 1.5 Bar, I have turned the boiler central heating to max, and turned my external temp stat to 30C to force the boiler to get the system nice and hot. The system pressure moved up a little - to 2 Bar, it has stayed there, and now after cooling has settled back to 1.6 Bar (probably still a little warm) so for now I think the job's a good 'un!!

Thanks all!