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I found an old Homart adjustable air vent . I believe it was made by Sears and is probably pre 1950's. As pointed out in the LAOSH alot of designs for these have come and gone. This one is sort of unique in that there is a jog in it which keeps it from being strictly at right angles to the radiator. Doing an image search of steam air vents I couldn' t find any others like it. I would be curious to know if people ever see these around on jobs, why they think it was designed that way, and why it was abandoned. It does have a siphon for water, but I would guess this angle might have something to do with improving drainage? As to why this design did not catch on, I'm thinking that the angle makes it a little more tricky to screw into the boss on the rad, and might have resulted in some cross threading issues and boogering up the threads. It does stick out a little further than usual, and perhaps the appearance wasn't aesthetic. I'm not sure they give any consideration to aesthetics for steam air vents , but Sears was one of Americas biggest corporations and you would think they put some thought into this different design and tested it in a lab.
I was having trouble with an overheating rad of about 20 EDR, the first radiator to get heat basically. A varivalve had little effect on the heating and made a lot of noise with my wet steam-especially after a nitetime setback. Then I tried a Hoffmann 40 which didn't help slow things down either(though it did work really well to slow things down on a 50 EDR rad in another location). Then I tried the old Homart (I don't think any air vents were ever thrown away here since about 1910, all stored down in the cellar - this was the only old adjustable one though - clockwise for "slower" and counter for "faster", a screw is located on top for this purpose.) Turning it all the way to the slow end actually helped quite a bit with the overheating , and though it still made noise - it was quieter than the other two. Maybe a Ventrite adjustable would work even better based on the reports on this site, but the Homart was free and I liked the retro feel of it. I was sort of surprised that the vari-valve, which has a range from completely open to closed, wouldn't slow down the radiator heating even when completely closed.