250 litre cylinder

[Matt887]

If the expansion valve was still dripping at a cylinder pressure of 3.5 bar then the whole inlet valve group set must be renewed.

Where exactly from (if) the inlet group set is the EV connected, if connected (incorrectly) like the one below then effectively no EV and the expansion valve will expel the expansion volume every time the cylinder re heats but will only drip while the cylinder is re heating.

On that one they have tee'd in for the expansion on the balanced cold.

 

[Johntheo5]

I would expect you should if its screwed into the inlet group set. Once you are happy that the EV is attached downstream of the NR valve then all you have to do is to renew the exp valve and check/set the precharge pressure.

 

[Matt887]

So if 250litres of water will only have an expansion volume of 5 litres. Why are some manufacturers asking for a 25litre vessel on an 250 litre cylinder?

 

[Johntheo5]

It depends on the pressure rating of the cylinder which in turn dictates the expansion valve setting which is normally 1 to 1.5bar lower than the cylinder TPRV setting.
You said your expansion valve setting is 6bar which should mean (no harm to check it in any case) that your cylinder is rated at (least) 7bar and the TPRV should be the same. A 18L EV will result in a final pressure of 4.36bar so no danger of the expansion valve lifting.
If your cylinder had a 4.5bar expansion valve which means a cylinder rating probably of 6 bar then the 18L EV only gives a very small margin of 0.14bar before the expansion valve lifts whereas a 25L EV will result in a final pressure of 3.9bar with a margin of 0.6bar.

 

[Matt887]

Thanks I think it said on the cylinder rated to 10 bar.

 

[Matt887]

Excellent answer explains it well!

 

[Matt887]

Is this while it appears most manufacturers are supplying/specifing bigger expansion vessels now. As the cylinders are rated to a lower pressure?

 

[Johntheo5]

Quite possibly, it is/should be far cheaper (for the manufacturer anyway) to install a larger volume expansion EV with 6 bar cylinder than a 10 bar cylinder with a smaller EV especially when the maximum cold/hot water pressure required is 3 bar.

 

[Matt887]

Ah, be interesting to see what bar most unvented cylinders are these days.

 

[Johntheo5]

The Glendhill Lite has a TPRV set to 6bar & expansion valve set to 4.5 bar but there are numerous posts around re problems with these, banging noises etc, I suppose one advantage of these lighter cylinders is you can have larger cylinder where weight is important if installed in a attic or somewhere like that.

 

[Matt887]

Ah, do alot of the newer ones have 4.5bar pressure relief valves?