disconnect a back boiler

[Sparkgap]

From the sound of it the cylinder is a primatic type where the primary or heating circuit is fed from the cold water tank through a device inside the cylinder which allows water to enter the heating system but maintains the separation between the two. You can get problems with heating and hot water systems mixing if the heating side gets too hot and you get boiling which forces water past the internal air gap.
If you are just capping off the primaries at the cylinder there should be no problem.

 

[lame plumber]

From the sound of it the cylinder is a primatic type where the primary or heating circuit is fed from the cold water tank through a device inside the cylinder which allows water to enter the heating system but maintains the separation between the two. You can get problems with heating and hot water systems mixing if the heating side gets too hot and you get boiling which forces water past the internal air gap.
If you are just capping off the primaries at the cylinder there should be no problem.

sounds much more like a fortic tank Fortic F1 if you ask me

 

[stevetheplumber]

sounds more like an old direct cylinder to me typical set up direct cylinder cold water storage take feeds the cylinder and the f and r to the back boiler probably in galvanised steel pipework start draing and youll see the balvale start to fill then you know its that one that needs to be turned of or tied up