Why would a newly installed radiator only heat when on constant bleed?

Question

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.

Answer 1

I believe that means that the drain pipe of that last radiator is blocked in some way (or simply not or incorrectly connected)

for the connection you'll need to ensure the inlet and outlets are connected to the proper pipes

if it is properly connected then this can be an airlock to fix this close all other radiators and turn on the pump this will increase the pressure before the air bubble and might clear it.

if this doesn't fix it you'll need to check the drain pipes and replace as necessary.

Answer 2

Other possibilities:

1. The balancing valve on the radiator (on the other pipe) is closed or blocked. Try opening it up and if that doesn't work open and close it a few times and see if that fixes it.
2. All the other radiators are in parallel and provide an easier return path for the water. You could test this theory by closing the hand valves on all the others and see if that forces water to the last radiator. If that does nothing then maybe see if there is a speed setting on the pump and switch this to max to shift any gunk in the pipes.
3. There's an air-lock in a high point in the attic pipework. Fitting a self-venting valve at this point may solve the problem but will require a drain-down. Does anyone know if there's a 'hot-tap' (pipe-piercing) fitting for this that would save a drain-down?