ive just changed the 2 port on the heating side as I thought it was letting by as all rads started to get hot when only water on demand. Fired up boiler on demand for water once changed 2 port, flow pipe to rads underneath 2 port stone cold but rads still warming up at a slow rate. Checked bypass and it's fine. I put 10 replacement rads a couple months ago and when heating on demand all heat up nicely. Heat only boiler, unvented cylinder. **update** I've just noticed it's the return pipes from rads heating up when hot water is on Any suggestions? Blockage somewhere?