unvented cylinder what is this picture attached????

[darren bergin]

what is this picture attached and what is it used for used for on unvented cylinder circuit??
picture attached

http://www.diynot.com/network/darren bergin/albums/


i want to fit a new radiator and need to drain heating sytem down any advice please thanks
sure what ive got is on a s plan!!

 

[Razor900]

What picture

 

[Goldspoon]

It's a tundish. Basically its there so you can see water passing past it. If you can then the cylinder is dumping water (safety feature).