3 way valve replacement

[Paul Plumb]

Hi I'm going to replace the 3 way vale on my open heating system. Hoping someone can advise.

In the picture, to isolate the valve I am going to turn off 1, 2 & 3. However as you can see I can't turn anything off between the valve and the hot water tank. If I use a bung in the header tank, will the water from the tank be a small amount? Are we talking just drips or will it be the conents of the top half of the tank?

Appreciate any advice, thanks Paul

 

[Paul Plumb]

I should have said, number 4 in the picture is the tank to the 3 way valve (there is no isolation on this)