I have bunged central heating systems for minor repairs without a problem. I have to change a drain off cock on a hot water cylinder. It is a 1/2 inch threaded drain off screwed directly into bottom of the cylinder. I was planning to bung the feed and vent and just unscrew old one and screw in new one, obviously with aqua-vac and plenty of cloths ready. If everything goes to plan then it will be a 30 second job, however if things don't go to plan then I could be in the deep and smelly. I am now thinking to drain down and make sure it goes right. One more concern is that it is a house in my village and I do a lot of work here so do not want anything to tarnish that. Just asking what others would do?