What size expansion vessel do I need for my C/H system?

Difficult to be sure without knowing the volume of water in your system - but an 8 or 10 litre vessel is likely to be adequate. There's no harm in having a bigger one if you've got room - and not a lot of difference in the price.

It's not actually in *parallel* with anything - it needs to be tee'd into the flow pipe. The instructions for my (non-combi) boiler say to put it close to the inlet side of the pump.

Many thanks for the help. The BES catalog offers a 12 Ltr one, so I guess that looks like the one to go for... I was amazed how inexpensive it is: £14.50!

I see... Thanks. I can locate mine about 6 feet from the boiler in that case..

Cheers,

Al

Some more info which may be useful is in the SealedCH FAQ below.

Check your boiler instructions -- some of them require it in the return pipe.

Thanks. One thing that puzzles me is that the installation instructions tell me to "Proper means must be provide for preventing the air from accumulating during the working of the plant, in the chamber of the vessel (water side) connected to the system".

I'm puzzled as to how you can remove the air in the first place, since the "wet side" is the bottom half of the cylinder, and the inlet is at the bottom, so the air in the bottom half is going to prevent it from filling up with water, and I don't see any way of removing that air. Comments appreciated.

Thanks,

Al

The boiler instructions don't have anything to say on this because the boiler had its own built-in axpansion vessel (It has failed.)

The expansion vessel instructions seem to indicate putting the vessel on the return pipe, as you suggested.

Thanks.

Al

Unless someone knows better I think I'd quietly ignore that. If you are really worried then mount the vessel so that the connection is uppermost.

However the label on most units I've seen tends to imply the connection is intended to go underneath. The majority of units seem to be made by Zilmet and have Italish instructions.

Plumber that did ours stuck it up-side down with the supplied PRV facilitating rapid air removal.

I am guessing having wet-side down might prolong the life of the diaphragm by reducing water contact, but then all that air will presumably get disolved into the CH water over the course of time requiring system top-ups?

Theory being it might prevent any vacuum on the pump inlet side and reduce chance of air ingress through dodgy joints I was told.