Can’t isolate shower

[Enthusiastictrier]

Hi All, this might be a repeat because I’m not sure my first post worked….

I’ve just bought a house with a built in mira915b. The airing cupboard is right behind it so very accessible. The flow valve is playing up and I want to remove it so I can change o-rings and grease it but I can’t isolate the water. The hot is fed from the cylinder with a gate valve that only turns so far and only slows the flow, the cold is fed from the header tank with no way of isolating. I’ve tried turning mains water off and leaving the shower running to see if it empties the tank but it just seems never ending so I gave up.
Any suggestions as I really need to sort this valve out

 

[Chuck]

Replace the gate valve(s) with good quality, e.g. Pegler, quarter-turn isolating valve(s). You can either drain the system or use a freezing kit.

You can hire freezing kits but I don't recommend it; get a professional plumber to do the work. If you make a mistake a tank full of water can cause a lot of damage.

 

[king of pipes]

You can buy expanding cheap plugs that can seal off the the outlet of the tank internally you can then work on the pipe . Kop