Turning heat back on from back of Hot Water Tank

[Ndiddy]

Hi,
Our main hot water immersion gave out a couple months ago and we had a plumber out to see what needed to be fixed. Turns out small immersion still working and he recommended whole copper tank be replaced as it is over 25 years old.
When he was there he showed me that you could turn a knob in the back of the tank and that way boiler would only heat water and not the rads, as this was in the middle of the heat wave. Now I can't quite remember how to turn the hot water back to heat the rads and cautious of not wanting to turn the wrong knobs. I see a red spigot...is this the one I should turn to get heating back? advice?
Thanks

 

[Leo]

Take a look at the image of a hot water cylinder here.

The valve you are looking for will be connected to one of the pipes feeding the internal coil (marked as water from boiler). Most tanks will only have two other pipes, the very lowest one for the cold water inlet, and one at the top for the hot water out.