"Andrew" wrote in message
Thanks for clearing up the furnace/boiler distinction. The ball valve
is in-line, and it is definitely closed. It always has been except
when I've re-filled the system after draining it. The cold water
from the city comes in through this ball valve to a tee that goes
*down* into the circulator pump and *up* to supply the radiators. Is
this ball valve different from the "regulator" and "fill valve"? Are
those two different things or the same thing? Here's the manual for
my unit. Figure 5 shows the plumbing configuration:

http://www.crownboiler.com/manuals/c...20ABF-EID).pdf

Thanks, Andrew

Your system is a little different than mine. I looked at page 5 of the
instructions and it looks as though the water supply line is going into a
fitting at the expansion tank, or Filtrol tank. If it is typical of most,
there is a bladder inside that will absorb the water as it expands when
heated. When they go bad they fill up and the bladder no longer moves and
you get over pressure.

This shows some of the common parts of a filling system, including the Watts
regulator often used.
http://www.ccallis.com/expansion_tanks.htm