which tank to drain c/h system ?

hi, on which tank would i need to "tie up" ball valve to enable me to drain down c/h system...in my loft i have 2 tanks..one is the big main water tank..but just behind that one is a much smaller tank ( i belive it could be called the header tank..having read in the wiki and i quote.......
"Vented System

As above but firstly you should tie up the ball valve to a plank of wood placed across the cistern to cut off the water supply. To refill, simply untie the ball valve. Don't forget to add inhibitor and bleed rads. "

would my system be a VENTED SYSTEM?(i have no idea )..if so...do i tie up the ball cock in the biggest tank to cut off the water c/h system rads to enable me to drain down my system, many thanks in advance

It's the small tank you want.

aha! thank you very much...now i know which tank..finally, if i ever get a leak in a radiator or c/h pipes will i just be able to tie up the ballcock in the little tank..then drain c/h system..then fix leak ? or will i have to turn off water at the mains first ? thankfully i have no leaks but i just want to be prepared...just in case many thanks once again

Hi
I'm not a plumber or one of the wonderful heating engineers but draining and refilling a heating system can be full of woes and troubles if you are not careful.
The small tank in your loft (providing you have a vented heating system) is known as the feed and expansion ( F&E) tank and is the only tank you need to tie the ballcock up to work on the heating system.
Many heating systems have a isolating valve on feed pipe from this tank and closing this valve allows you to drain the system without draining the F&E tank.