snip

SOunds reasonable to me. Sitting here in a cold house, I'm thinking of
starting up the furnace (well it is used for hot water too). ;-)
Question: Every fall when I first turn on the heat the system is full
of air. Where does the water go? The system is pressurized to ~14lbs, so
it can't be going back into the city supply (60ish lbs.). I dont' see any
leaks and I'd expect to see water when it's pressurized.

No idea. It's one of the great mysteries of life, right up there with
where do all the missing socks go....

I mean, all such systems have backflow preventer, so even if the
boiler pressure were higher than the feed pressure the water could not
go out that way. And even if it did, how does air get in to take its
place? And leaks usually make themselves known, unless you've got
underground piping. There is dissolved air in the water that probably
separates out over time, but I find it hard to imagine that it is
enough to explain the quantity of air that seems to be in there each
fall.

Now if the expansion tank is the type that doesn't have a bladder
(most older ones don't), the air in there can dissolve slowly into the
water, but with no circulation going on, I don't see how it would end
up migrating elsewhere in the system.

So, I got nothin...maybe someone smarter than me knows the answer.

Paul