"Christian McArdle" wrote in message
. net...
2. Unlimited loss of water in the
event of a leak.

Put a stopcock in the cold water feed if it bothers you. Then you're
limited to the contents of the tank.

This is only safe if the boiler has a manual reset overheat lockout (i.e.
is
suited to sealed pressurised operation). The safety "mechanism", if you
can
call it that, for a boiler without an overheat lockout is to have an
uninterrupted vent to get rid of the steam, and an uninterrupted
independent
path for quenching water, so it can boil away merrily until someone
bothers
to turn it off. With a stopcock on the feed, it will run out of water and
possibly burn the house down.

There are one pipe feed/expansion systems using a header tank. In this case
the boiler must have extra protection, such as dry fire protection. They are
pig to get the air out if no adequate auto air vents are installed.