Only guessing
As combi boilers work off the mains I would assume that the boiler has a gatevalve(usually red )which shuts off the feed to all hot taps. Then run shower to drain pipe. I would have thought that the pipework would have a valve which would save the need shut down to all hot taps. Could be a small chrome check valve with a on/off screw in the middle slot across the water is off in line water is on.
bob, May 2013
