Attic water expansion tank height above hot cylinder?

I am looking at moving the small expansion tank in the attic, at the moment it is located on top of the main water tank.
Would the heating system work ok if this small tank was moved to sit on the rafters of the attic? Is there a minimum height that the expansion tank needs to have above the hot water cylinder in the hotpress?
any advice appreciated.

The feed & expansion tank should not be above the main cw storage tank. This is against regulation, in case the small tank burst, you will have heating circuit water enter your domestic water. It should also have its own overflow pipe to the outside and not into the cw storage tank as many are.
With regard to minimum heights, yes there is. It is a calculation from the pump to the water level in the f & e tank. Depending on the head height of the pump, let's say you have a Grundfos 25/60, then the head height of the pump is 6m. The minimum distance is a third of this, so 2m. If your system is working correctly now with no air problems, then you can no the f & e tank to the same height or higher. If your heating pipes raise up into the attic, as some do, this can cause problems in this area. F & e tanks are usually located beside the cw storage tank. Do not alter where the f & e pipe enters the heating system, as this could cause "pitching".
It is not really a DIY job and only a small job for a professional.
The f & e tank will have no effect on your hw cylinder in the hot press. If you are trying to achieve a better pressure from your hot taps, raise the cw storage tank, not the f & e tank.

thanks for the detailed answer Shane007, the system is working ok with no problems, the plan was to move the f & e tank off the main storage tank to avoid any possible leak from the f & e tank. I need to check the head height of the pump...if the minimum height is, for example, 2m, what is this the distance between? i.e. between the water level in the f & e tank and ?
thanks

Hi Readysteadygo, if your pump is say a 25/50, then the head height of the pump will be 5m (the 50 being the 5m). The minimum static head height between the two will be a third of this.
However, note that if your heating pipes rise into the the attic as they do in some cases, the new head height will be from the pipe to the water level in the f & e tank and not from the pump.
The best & safest option is move the f & e tank to beside the cw storage tank but keep it at the same height. This way you are not altering the physics of the system that you have and that already works. Then run it's own overflow pipe to the outside.

Thanks Shane007