Is expansion vessel to small.

[lofty531135]

Hi.
I have a friend who is losing pressure when the heating is on,I haven't tried repressurising as I think his ev is to small,he has a jaguar 23 kw and has 11 radiators in his house,I have read that you can calculate the vessel size by how many radiators are in the property,in his manual it states that the expansion vessel is 5 litres,does he need a 12 litre vessel.
Thanks,Lofty.

 

[Rodreguez]

sounds a bit small to me. if water pressure risngs over 3 bar wen boiler is on and prv discharges then it is defo too small!

 

[Rodreguez]

water expands 4% when heated to work out you would need to know Water Installation Volume

 

[mfgs]

I know stelrad state on their website how much water each section of their radiators hold, so that will give you a rough idea of how much a radiator holds.

 

[dickyboy]

I use my own rule of thumb method. Its not an exact science, but it seems to work for us.
For each radiator I allow 1 litre of expansion per rad and pipework to it from main runs. If its a double panel rad then I count as 2rads (2 litres expansion). Common sense prevails.
Standard 3 bed semi with upto 8 rads would equal a min of 12 litres expansion minimum. Most boiler manufacturers provide a vessel with the boiler but most are only good for upto 6 rads.
If you try to work out exactly how many litres are in the system it could be a very long day.

I was once told by an old chap who helped write plumbing literature that you cant over size a vessel but you can severely undersize one.

 

[lofty531135]

Thanks for all the replies,I will install an 18 litre vessel externally.

one other question,is it alright to leave the old expansion vessel conected even if it is faulty,I don't know if it is but it means I can fit the new ev and not have to worry about the old ev.
thanks.
lofty.

 

[oldplumber]

if its not leaking out of the shraeder valve, no issues, often easier to replace ev with ext ev as thye are cheaper and quicker to bolt on.

 

[dickyboy]

Thanks for all the replies,I will install an 18 litre vessel externally.

one other question,is it alright to leave the old expansion vessel conected even if it is faulty,I don't know if it is but it means I can fit the new ev and not have to worry about the old ev.
thanks.
lofty.

I usually deflate the expansion vessel which has failed as much as possible by pressing in the schraeder valve and expelling all the air and just allow the new one to do its job. Once its completely full of water you should be ok. I would also replace the auto air vent if any signs of leakage. This is usually the first thing to go when the vessel is low on pressure. Once the new vessel is fitted all should be ok.