Quicky on changing expansion tank

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Hi. Quick question on changing an expansion tank. First one I've done but 17 rads on an 18 litre. Pretty sure the bags failed. Going to put a 25 litre on but can I close off with the valve underneath the tank connection? Or easier done with a quick drain and under vacuum please.
 
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You are not supposed to have any valves between expansion vessel and system.
Can you not freeze the pipe?
Or if you can close off any auto vents and drop the pressure fully it might be possible to swap, perhaps by still plugging pipe temporary while you struggle to fit new vessel.
 
Thanks.
To be honest I'm still scared to freeze pipes. Seen it push compression fittings right off in the winter!
Will do the vacuum thing I think.
 
Thanks.
To be honest I'm still scared to freeze pipes. Seen it push compression fittings right off in the winter!
Will do the vacuum thing I think.

If you have the basic freeze kit of a jacket and can of freeze spray it would be easy.
Freeze spray won’t burst the pipe.
It just forms an ice plug.

As to pipes freezing and compression fittings pulling off in severe winter - the 2010 winter was so bad could burst anything including soldered fittings.
 
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Cheers.
Wonder if I can just turn a £1 air duster can from poundland upside down to freeze! :D
(do not do. Not advice. Just noticed...)
 
Personally I always fit an isolation valve and a drain point on an expansion vessel, makes life so much easier to service them.
 
If water pressure is fully dropped off a sealed system you might find no bother from water coming from the open pipe (if auto vents are all manually closed using their wee plug cap).
But the old faulty vessel will be full of water and very heavy. Can be difficult to hold while disconnecting
 

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