As a test you can shut off a pump valve - and open up the upstairs rad valves. Then if the hot water only heats up ( no pump running of course ) there is your problem . And you can move pump and isert valve.OK thank you for clearing that up. Is there any reason not to fit a valve BETWEEN the boiler outlet and the pump, as that's the only bit of pipework I can access (see pic). From the pump outlet the pipe runs behind the boiler and into the wall and there's no room to even get a cutter or soldering torch to the pipe, let alone fit a valve.
If this valve was present where would it normally be situated? If it wasn't present, would fitting one be a better bet than fitting a motorised valve next to the pump?
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