Pressure vessel on warmflow boiler?

[steve1]

We have a warmflow boiler, since having the UFH installed, ( approx 36 sq mtrs in floor area ), we have noticed the pressure of the boiler dropping, but when it's topped up to about 1.5 bar and the heating system switched on it goes quite high and sometimes starts venting out the relief valve pipe outside.
It only does this if the rads are put on, it seems fine when the UFH is on by itself.
Doing a bit of research it points to the pressure vessel not being big enough to cope with the newly installed UFH as well as the rads.
The vessel is 12ltrs.
Is there a sure fire way of testing the vessel, or indeed working out how big the vessel should be.

 

[PhoenixServices]

Get the guy back who installed the ufh to sort it out. Will try and find you the calculation for sizing expansion vessels though.

 

[galaxy_plumbing]

exp vessel might off lost the charge.

 

[steve1]

ive given up asking him to come back to look at it.
whats the best way of checking to see if the vessel has lost its charge, do I have to drain the system etc.
Be good if you could dig out the info on vessel sizing for me.

 

[lame plumber]

best you get someone else to look, then it will be working for Xmas.

 

[steve1]

best you get someone else to look, then it will be working for Xmas.

It works now, it just needs topping up every now and then.
i would really like to know myself what's wrong before calling someone in.

 

[lame plumber]

you know whats wrong, get an expert to repair it and make it work safely, I know if my cars bust I just get the garage to sort it that way I can get driving quicker, much the same applies to boilers, computers, everything really.

 

[steve1]

you know whats wrong, get an expert to repair it and make it work safely, I know if my cars bust I just get the garage to sort it that way I can get driving quicker, much the same applies to boilers, computers, everything really.

As per my post, I'm not sure if it's the vessel that's too small now, or simply lost its charge, there's a big difference in cost between the 2 possibilities, hence why I want to know the best way to check before going any further.
but thanx for your comments.

 

[lame plumber]

if its lost its charge as its failed it needs replacing and if it needs a bigger one the cost difference is negligible, on will pop back on the boiler the other will T on some where else.