"Andrew Gabriel" wrote in message
.. .
Another related issue... Many boilers have frost detection built
in, and can fire up by themselves when there's no call for heat,
and again will need a flow path.

Yep

Also, some boilers will run the
pump for a short time if it hasn't been used for 24 hours to stop
the rotor from sticking.

I think the pumps are okay running into a dead end provided there's no
actual heat involved.