Adding extra C.H. expansion vessel-can I use a hot water one

[jocie]

I need to add another 5 litre expansion vessel to a new combi,as there are a lot of radiators. As I have a spare hot water one (blue) the right size, and don't need it for anything else, can I use it for the heating system. Or will the rubber diaphragm perhaps not like the C.H. inhibitor ?

 

[John506]

Are you having problems without it at the moment?

Blue expansion vessels are for drinking water, you will need a red one for central heating.

 

[bengasman]

Unlikely that you actually need an extra ev if you have a decent boiler, as every RGI knows.
You shouldn't meddle with boilers if you don't quite know what you are doing.

 

[jocie]

Unlikely that you actually need an extra ev if you have a decent boiler, as every RGI knows.
You shouldn't meddle with boilers if you don't quite know what you are doing.

Thank you bengasman for your attempt at a helpful explanation which I find totally unhelpful unfortunately. I suggest you learn to read english before suggesting I do not know what I am doing. Any large system in terms of volume of water in rads can require an larger than usual expansion vessel. Please feel free to ignore my post as I do not think any suggestion you might make would be worth reading, RGI or not. Anybody else who can actually read my specific question and answer it I would be grateful for a reply.

 

[YELLERBELLY]

depends on the precharge of vessel

 

[Slugbabydotcom]

A it works
OR
B it doesn't

Worst case scenario is B in which case the PRV may operate and you will have to replace it with a red one.

 

[jocie]

Well guys, thanks for the replies. I went ahead and fitted my blue expansion vessel, precharging it to 1.25 bar instead of the 3 bar it needs for a sealed hot water cylinder system. I attached it to the new Viessmann vitodens 100-w 35 kw combi I have just fitted. As my system has 9 double and 6 single rads, and the Viessmann expansion tank inside the boiler is only 5 litres, I feel happier with the extra 5 litre expansion capacity I get by adding this blue tank. The whole system runs fine and with all rads open the pressure rises from 1.25 when cold to just under 2 bar when hot. I think this is o.k., but I will probably reduce the "when cold" pressure to 1 bar .