I have a c/h problem . only half my rads work . hot water going out of boiler warm water coming back in .I have 8 rads in my house over 3 floors only 4 are working . pipe cold under rads that don't work . the hot water seems to be circulating the system but missing some of the rads ?

c.p, January 2015

check the radiator valve are open some time thermostatic valves get stuck in close position

If the valve can not be open you may require a plumber to change the valve

either way it would be wise to have it looked at
http://www.emergency-plumber.eu/

andrew t, February 2015

On your hot radiators close off both the inlet and the outlet (lock shield) valves.

On the cold ones open both valves - I suspect the colder radiators have their outlet valves closed so water can't flow out of them and consequently can't flow into them - i.e no hot water is going to them because no water can come away from them .

But I'm not qualified to offer this opinion. A better person might come along and provide a more realistic answer.

Good luck..., January 2015