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I did this yesterday, but whilst researching I couldn't find any info about it so I'm posting it on here in the hope that Google will index it and it'll help someone else.
There's lots of information about how to put a bung somewhere in the header tank in the loft so you can then cut the pipes and the vacuum will stop all the water running out. Except we don't have a header tank in the loft, the system is fed from mains pressure. It's not a combi system either, it's got a hot water tank. I don't know if this is unusual or not, but it didn't seem easy to find help on it on the internet. We have two globe thingies in the airing cupboard which are the expansion vessles for when the system gets hot.
You will need two push fit stopper caps for the pipes you are going to cut.
So here's what I did:
1) Turn off the stop tap for the hot water system, which was in the airing cupboard and helpfully labelled. An old fasioned brass tap on a pipe coming out of the floor. The system will then contain water at 1 bar pressure or whatever it says on the dial. You need to drain off enough water to relieve the pressure.
2) Turn off both valves for the radiator. Open the drain tap and the air bleed valve to let all the water drain out of the rad.
3) Remove the valves from the rad, then place one in a bucket and open it gently. Water will spray out, but only about 5l or so in my case. This will slow more or less to a stop although some will still be dribbling out. This should allow you to cut the pipe and put the stop cap on without water going everywhere. Do the same for the other valve, although harldy any water will come out of that one when you open it (at least it didn't for me).
4) You then have two capped bits of pipe and you can go about replacing the rad and adding in extension pipes etc. When you come to join the new pipe to the old one, some water will have run down into the pipe again so when you take off the stop cap a bit more water will come out.
5) Tighten everything up, open up the stop tap in the airing cupboard and open the refill loop until the pressure is back up to whatever it's meant to be.
6) Open the bleed valve then the rad valves and wait for the rad to fill up. The rad I changed was on the ground floor at the bottom of the system, and no other rads seemed to need bleeding, but it's probably worth checking as there was a lot of gurgling from all over the place.
Job done, no system drained, no bungs required, no pipes frozen, no water spilled.