No DHW from Combi - failed pump?

My Grant 90 oil-fired combi has stopped producing hot water. I know the heat exchanger is OK. The diverter valve is combined with a microswitch so that both operate mechanically off the same diaphragm to detect flow. The burner fires and the pump runs when you turn a tap on, so I deduce that the diverter is probably working.

However, you get a moment of luke-warm water and then only cold. After a minute, the burner turns off.

The pipes for the sealed circuit in and out of the heat-exchanger remain cold (as consequently does the hot water out pipe to the mixer). The pipes at the top of the boiler by the pump are very hot.

I'd diagnose this as a failed pump - any comments?

How can you check these water pumps? This one makes a noise (i.e. it's doing something), but it sounds too noisy for my liking. If you undo the large screw head in the middle, what should happen? On mine, water comes out... quite a lot of it. Is this right?

Thanks!

Reply to
John Carlyle-Clarke
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