Central heating rad problem with Grundfos pump

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?
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On Oct 5, 6:06 pm, snipped-for-privacy@gmail.com wrote: When "hot water only" is selected

Probably an installation defect . You should have two separate return pipes from upstairs, one for the upstairs heating the other for the return from the hot water cylinder. The upstairs heating return joins with the downstairs heating return before joining the HW cylinder return. Joining the two returns upstairs & having one retrun pipe will allow a flow path through the heating when the HW only is selected. Sketch out a schematic and you'll see what I mean. The water can flow backwards through the upstairs HTG return, along upstairs HTG flow, along downstairs HTG flow, up downstairs HTG return and back to the boiler (or something like that).
The bathroom rad is commonly connected to the circuit heating the HW cylinder, so you can dry towels when the heating isn't required

Yes. A little water spills. You should undo it to vent air after installation.

No. If you want a NRV you buy one.

Electric conections are in there.
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Yes that is what it is for, you will probably get a dribble of water out so stand by with the Bounty. The shaft of the impeller has a slot to rotate it manually if it has stuck.
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In an earlier contribution to this discussion,

I don't understand why you think that this indicates a problem with the pump. You say that when HW+CH is selected, the downstairs rads cool off when the stat setting is reached. As long as they get hot initially, and only cool off when the stat switches, this is exactly what is supposed to happen.
Your problem surely is that the upstairs rads get hot when they shouldn't - and you need to find out why that is - but it's unlikely to be anything to do with the pump. Do you have any motorised valves in the system? If so, how many, where are they, and what type (2 or 3 port)? If there are no motorised valves, it is probably a gravity (convection) HW system with the pump only being used for the CH. If this is the case, it sounds like you're getting gravity circulation through the upstairs rads whenever the boiler is running - even when there's no CH demand.
But tell us a bit more about the layout of your system - and the components used - and we will be better able to advise.
--
Cheers,
Roger
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Thank you for all your replies. I will be posting a proper reply shortly. Have been trying to sort things out. Cheers!
Roger Mills wrote:

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Thank you all again for your replies... Nothing at all may be wrong with the pump. It works, both for HW and CH. The perplexing thing has been that with only HW selected on the boiler, the upstairs rads were heating up. Heating up water in the summer meant that all the rooms upstairs were baking! If CH is used, and it shuts down at the appropriate temperature on the lounge thermostat, the upstairs rads remain on, and REMAIN hot -- they don't cool down until the CH is completely shut down.
I have been investigating the setup out in the garage where the pump is located and have discovered what I have now confirmed to be a non-return valve, in the run of pipe that runs parallel to the pump and it's inlet and outlet pipes. Further advice now suggests that the fault lies with the non-return valve, which I will attempt to replace myself in the near future, rather than pay a plumber 100 to replace a 3 valve.
Thanks again for your tips and advice! Regards, dennmac
snipped-for-privacy@gmail.com wrote:

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