Radiator heats up when hot water is on and heating is off

[AndrewLivingstone]

I've a weird issue. I've old vented cylinder boiler. One of the radiator, nearest to the cylinder, heats up when only hot water is on, heating is off. Not sure why only one radiator is heating why not all of them heat up? I got the 3 port valve and pump replaced but problem persists. I'm not sure what else could be the issue. Any thoughts?

 

[AndrewLivingstone]

Thanks for the reply. I also think and it's obvious that this radiator is on HW circuit.