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Constantly purging air in hydronic system
I keep having to purge air from one of the 2nd floor circuits of my hot water heating system in order to get adequate heat out of it: I purge the system, then several days later I hear gurgling (although the noise "seems" like its coming from a different circuit upstairs), then the heat output starts to diminish in the convector over time. I bleed and everything's fine for a week or so. Let me give you details of my system...
I have a pump circulated hydronic system (circa 1950's) in my old colonial (circa 1910's). The boiler is a gas fired (80,000 BTU?) 80% efficiency New Yorker unit (circa 1990's) and the house is ~1,300 sq.ft. There is a single pump on a single zone feeding all circuits which have a mix of newer baseboard radiators (living room & bathrooms) and original-to-the-system fan-coil hydronic convectors (bedrooms, dining room, kitchen). The electrical circuits for the hydronic convectors, however, are split into two zones: 1st floor convectors on one thermostat & 2nd floor convectors on the other.
All the radiators/convectors on both floors have manual air purge valves that are pretty much in useless condition. As such, after I moved in, I had a good plumber/heating guy install shut-off and boiler drain valves in the return risers of each radiator/convector in the basement for easy purging. At the same time, he installed a city water make-up pressure-regulating device set to ~15psi, an automatic air purger (w/automatic purge valve) on the supply side of the boiler, and finally he re-routed the expansion tank to the supply-side. In addition, there is one automatic purge valve in the baseboard of one room upstairs that was there when I moved in (not the unit that needs constant purging, though). There is a file attached showing the layout of my system as I know it (there is only one shut-off/boiler-drain valve shown on the bathroom loop, but they are actually installed on every circuit in the basement risers).
My question is, where is the air keep coming from? I don't see any signs of leaking water anywhere in the system? Could it be air coming in from the city water makeup and there's nothing I can do about it? Any help would be appreciated.
Oh, one less important side question: The boiler is running with a Honeywell L8148E1265 aquastat and it's set at it's lowest setting = 180degF. However, I noticed that it very rarely (only really cold days here in SE Michigan) gets to kick off at it's 195 upper set point (15 deg differential). Would I benefit from lower set point aquastat (say, a Honeywell L8148E1299) or should I leave the system as designed?