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Non-Thermostatic Vacuum Vents?
Gentlemen (and Ladies): I'd like to put this topic out for discussion - especially among you old farts experienced in the art.
In a vacuum-vapor system there is always some kind of a vent at the end of the dry return. I'm thinking, for example, of a Trane, which has separate vents at the ends of the steam and return mains, or a brand such as Dunham, which has a crossover trap and a single vent at the end of the dry return. In the case of Trane (and others) they used a normal float and thermostatic vacuum vent at the end of the steam main and a thermostatic only vacuum "quick vent" at the end of the dry return. My question is this: Why does it need to be thermostatic at all - or does it? No steam should ever get to the end of a dry return (I'm not including trapless systems (like, say, Broomel) where vapor was expected to occasionally make it past the little water seal or tiny check valve. My recollection is that WarrenWebster, for example, and I think even Dunham, had only the vacuum check atop the float assembly that dealt with abnormal conditions, and no thermostatic element at all. This leads me to wonder why we don't just use a (suitably light) check valve at the end of a dry (vacuum) return, and if it's not vacuum, just an open pipe. Your thoughts?