Turn off hot water from boiler

[vidkin]

I am about to change the taps on my bath but of the two valves fitted the one to the hot tap will not turn off as it keeps turning with no effect. I understand that it is a gate valve with a red circular handle. Clearly there is a problem with the valve and if this can be fixed I will be glad to learn how.
The alternative is to isolate the hot water supply somehow.
I have a Potterton Profile 100e fanned balanced flue gas fired boiler and if there is a way to turn off the flow of hot water I will be glad to learn this also.
I have read previously that I can turn off the gate valve on the 22mm pipe coming from the hot water cylinder and then run the hot tap until the water stops flowing. I have tried that but the water flow continues.
Solutions to either problem will be appreciated.

 

[Newboy]

If your hot water is supplied by a cylinder then you either have to stop cold water entering the cylinder - either a 22mm or 28mm pipe running into the lowest point on the cylinder with a gate or other isolation valve on it

Or

If the hot water flow from the cylinder -the highest pipe from the cylinder has a gate valve on it then close that.

Neither will stop the flow of hot water instantly - it may take a minute or two to stop.

Keep all other hot water outlets shut whilst you are working.

If neither valve works then you may have to drain off the cold water storage tank supplying the cylinder

 

[Whitespirit66]

If the gate valve to tap is just turning, make sure it is actually engaging with and turning the spindle. If it is, the spindle has probably sheared off inside valve.

If no valves are working, you will need to isolate the cold supply to float valve in large loft tank somehow. Either by using main internal of external stoptaps or poss a valve on pipe work to the float valve. Or, you could tie the arm of float valve up.

Then, run all hot taps (plus any gravity fed cold taps) until flow stops - will take a while, then change taps. Whilst you're at it, take the opportunity to change some gate valves, preferably to full flow lever valves.

http://www.screwfix.com/p/full-bore-lever-ball-valve-22mm/30584