We have changed a gas central heating radiator. It is the last radiator in the circuit. It will not communicate with the boiler and only brings hot water into the pipe leading to the thermostat when on constant bleed. I have taken ten buckets of water from this radiator with the thermostat on 5. As soon as I close the purger the hot water stops coming down the pipe.
Pipes run across the attic. All the other radiators heat to really hot and I have bled all the other radiators several times.