Cool October nights have highlighted another fault with our Olympic Thorn boiler/Grundfos pump system... When "hot water only" is selected on the boiler, the upstairs rads get hot (and the bathroom rad is scalding! -- furthest from the pump?), while the downstairs rads stay cold. When "hot water plus central heating" is selected, the upstairs rads remain hot, while the downstairs rads cool off when the thermostat temperature setting is reached.
The pump is a Grundfos 15-60. I'd like to be able to check for a sticking impeller before I have to call out the plumber again to look at this relatively young pump (installed a couple of years back). Can anyone confirm for me that the large brass screw head in the centre of the pump can be undone, in order to check for impeller rotation? It doesn't move yet, and I'm afraid to shift it in case it's an adjustment screw of some sort.
Also, is there a built-in non-return valve? Perhaps in the box on the side of the pump? If that has failed, can it be replaced separately or is it a whole new pump?