Recommendations for plug to avoid need to drain down storage tank

[Teka]

The isolating valve on the tank outlet is sized so I'm looking for a tool or technique for plugging the tank outlet so the valve can be changed. I can feel that the bore of the fitting to be plugged is not smooth, appears to have 2 ridges!

Advice and suggestions, please

 

[Last Plumber]

Did you mean seized?
Syphon the tank out if you cannot drain it.

 

[Ben-gee]

As Last says, syphon it - you don’t want 50 gallons coming at you all in a rush when something goes wrong!

The easiest way is to place a hosepipe in the tank and then go to outside or a toilet or a bath, suck on the hose to start the syphoning and then hold the end where it can go safely to waste.

 

[steadyon]

1. If you are talking about the cold water storage cistern, as I think you are, then you have three options:
1a. Shut off the inlet valve and drain it.
1b. Use a bung such as Toolstation 31768. It should seal even against the internal splines used to secure the outlet fitting. Best to have someone in the loft to shove it back in again if it shows signs of moving. Otherwise you will have a flood.
1c. Freeze the pipe.
2. If you are talking about the hot water cylinder, then as others have suggested above. Don't forget you will have to isolate the cold water storage cistern to drain the hot water cylinder, so may have to do what is in 1. above anyway, unless there is a working isolation / gate valve on the cold inlet to the hot water cylinder.