Steve,
My last house had hot water radiant heat, and I never saw the behavior you
describe. When I bled the radiators the water pressure blew out the air from
the radiator, and I knew I was done when I got a steady stream of water.
What I remember about the system was that there was a connection to the cold
water supply that added water to the system as needed. This was where the
water pressure came from. There was also an expansion tank above the boiler,
which filled up over time and needed to be drained periodically- into a
bucket in my system. You might want to see if your expansion tank just
drains somewhere instead of holding the excess water. If it does, there may
be a bad valve somewhere that's allowing your system to leak- thus the
vacuum in the radiators. I think there was also an overpressure valve that
was supposed to bleed off water if the expansion tanks were full. Again,
this could be leaking.
BTW, the systems I had (2 boilers, one for each floor) were WAY older than
me, but worked fine. In fact when I had them cleaned, the repairman told me
that the boiler manufacturer had gone bankrupt long before I was born- they
were coal-fed converted to oil... It was a gravity system and did NOT use a
circulating pump- it relied on the fact that heated water would rise to the
radiators, and cooler water would sink and return to the boilers in the
basement. I couldn't find anyone willing to move a radiator when I remodeled
the house. They were all afraid that it might ruin the balance and that the
water wouldn't flow correctly. Apparently balancing gravity hot water
systems is a disappearing art...
Hope this helps,
Larry Spector
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