Gravity feed Radiators

[Rebel]

HI

I am wanting to link my multi burner back Boiler to radiators. I know that I need to include agravity circuit in case the pump should fail and it would be normal practice to heat the hot water by this process. I, however have a unvented electric system only which is quite new so this is out of the question so can I mount a couple of large radiators with lock shield valves on in the loft instead? and if I use 28mm copper pipe straight up to the rad is it then ok to tee off to normal 15mm for the radiator then carry on up to thevent. I believe that i need a rad at least 10% of the system is that correct?

The rest of the rads will be pumped on a seperate circuit controled by pipe thermostats in the normal way ie 22mm circuit with 15mm tails as the boiler has 4 tappings.