I need to turn off the hot water supply (bog std indirect system) to change some tap washers. I've found the cold feed into the hot water tank and turned it off. I've turned all the hot taps on to drain the cylinder down. Trouble is that the taps keep running, hot water is now gone and just cold coming out. I can hear the tank in the roof filling. Is there another valve I need to turn off?

just turn off the mains stop cock.

You need to block the cold water feed from the main tank into the hot water system.

I bought a pair of conical bungs from B&Q or Homebase to do mine.

either do what stoner said and turn the mains off or just tie up the ballvalve in the big tank (bungee strap works well) the open hot taps till theyre drained

sounds like tying up the ball valve might be quicker and save water?

its usually easier than turning the mains off unless you need to work on the colds as well?

once its tied up it will drain the cold tank – roughly 25 gall

in theory you did right turning off feed to cylinder – unfortunately those gate type valves rarely shut off properly so tie up the tank and leave the gatevalve open

thanks everyone!