Problem with circulation, downstaires rads not heating up

Hi everyone

I know this is an often asked question but need a little help. The downstaires rads in my house will not heat up. After various attempts to sort this (balancing for example) i came to the conclusion that the pump was not working so fit a new one....it made no difference

Any ideas? It's an open vented system, ideal mexico boiler, 3 port valve. One more thing, the flow pipe going through the hot water cyclinder coil gets very hot but the return stays cool. This also applies to the primary flow pipe which gets very hot but the return stays cools?

Blockage somewhere? Thanks in advance.

Yes you have a blockage or air lock. If the flow pipe is hot yet the return
pipe is staying cool there is no circulation going on.

sorry

What's the best way of clearing an air lock? And say it is a blockage due to debris where is the most likely area it will be, primary return pipe close to the boiler?

You think the 3 port valve may be causing the flow restriction? The lack of circulation affects both the heating and hot water.