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Posted by HeatPro on November 03, 2007 at 12:04:22:
In Reply to: EXPANSION TANK posted by pete on November 03, 2007 at 05:26:51:
Air will go into water through open automatic air vents over a long period. When the expansion tank and system is in balance, the occasion you have doesn't occur; but that depends on all the relationships being perfect, which is unreasonable to expect, so some purging is necessary after a while. It is likely the gauge has failed in that time. The black plastic covers a tire-like air bladder filler.
I close my automatic vent cover a month after doing service so the air isn't introduced and so haven't had a problem on startup for years. (I purposely have no direct connection to a water source, so even that can't fail or leak by in either direction.) This reference can help:
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