Hi TERRY - i have a big thank you to say to all the pros / experienced guys on here. I recently bunged my open vented system yo chang etwo rad valves . the same job that you are doing on saturday. Yes, I was nervouse also!! this is how I bunged. But I am in no position to advise!! but this is what I did and it worked, appropriately shaped potato 9a slim potato not a fat one in the expansion pipe. I then used masking tape a few wraps round to get a good seal. In the feed pipe I put the top of a silicon sealeant tube!! The lid you screw off before you use the sealant.. Now this lid was kind of "splined" on the outside, so i got a wood chisel and chiselled off the "splines" If you get a lid without the splines obviously you wont need to do this! after chiselling off the splines, I found that it fiteed into fee hole perfectly! The bung held while change the rad valves. I even left the bung in for a week to show my mate what I did. The bung did not hold absolutely water tight, but all it was was a very slow drip .I easily fitted my rad valves, even with this very slow drip. But best thing to use , to bung both expansion and feed pipe is probably a cork or rubber bungs - if you can find any round the house!! I couldn't. it you happy to spend money, its called a "drain easy kit" aat the plumbers merchants - someting like £15 at plumbase!! a lot of money for two rubber bungs!! all the best mate. Nigel - firs timer - and thanks again to the pofessional for taking their time to input into these forums.
