We have a hot water baseboard heater with 5 zones from 1994ish that
recently started acting weird.

About 3 months ago, I noticed that the expansion tank was leaking at
the seam...so I replaced it with new. For about 3 weeks, the system
was very loud, even after purging as much air as I possibly could out
of it. Eventually, all the air purged through the auto fill valve and
the noise is gone.

Recently, the master bedroom zone temperature wasn't varying at all as
it should with our programable thermostats. At night, the temperature
is about 70 at 10pm, and rises to between 72 and 77 by 6am...then
drops back to 70 during the day. We have it set to 58 during the day,
68 between 6 and 10, and 65 overnight. So I turned the thermostat
off, but the same behavior exists. The baseboard is always warm to
the touch.

The other zones were fine, until a few weeks ago. The dining room
zone will not move off of 70, even though we have a similar heating
patern programmed into the thermostat. I turned off the thermostat
and the temperature dropped like you'd expect it to. Turn it back on
and we're right back to 70 again.

Anyone have any ideas as to what might be happening? Our heating
bills are nearly $400 a month now and I assume that the heating issue
has something to do with it.

Thanks.