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expressed precisely :
After replacing the fuse, I confirmed that the pump is running, by
powering it directly. I also suspected the two-port valve (stuck shut?)
and tried the manual open switch, but can't budge this past the central
position (so I'm not sure if the valve is open or closed). Boiler won't
start in any case.

The just might have failed due to old age.

Did it blow again?

Any suggestions what else I can try to confirm that a dead valve is
really the problem?

Remove the actuator (the head part of the valve) and see if that
operates by itself as you select HW, HW + CH, then CH alone. Then check
that you can turn the valve itself with a just a little more than bare
finger pressure on the half moon shaped shaft top of the valve.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org