Location of Vent On One Pipe Steam Radiator

Bill_110

Happy thanksgiving.  I was looking at an old Book "Winter Air Conditioning"  with a 1958 copyright. In the steam section they talked about the placement of the air vent on a one pipe steam radiator. They talked about steam being lighter than air, and thus rising to the top of the radiator and forcing the air down, so that a vent placed near the top of the radiator would create a large air pocket at the end opposite the entrance of the steam. 



Dan Holohan in his Q&A about old radiators on this site mentions the same phenomenon and recommends putting the vent about in the middle of the radiator.



The authors of Winter Air Conditioning say "The proper location of a vent valve is the lowest possible point on the radiator at which steam will exist" they then refer to an illustration that shows the vent valve about 20 or 25% up from the bottom, rather than half way. They then go on to caution that "The vent valve must not be located so low that condensate will clog the inlet to the vent valve".  They don't tell you how to determine how high that is or how to determine the "lowest possible point on the radiator at which steam will exist."

I'm curious as to whether there is any contradiction in these views.  I'm thinking that maybe the middle is a sure bet to avoid condensate and still get most of the benefits of venting and heating efficiently?  I'm also wondering if the fact that these authors were writing in the era when large water content boilers probably made drier steam than todays low water content ones might effect that decision.

If one already had an air vent on the wrong side of a radiator or too high up on one, would the best approach be to locate it in the middle, or aggressively go for some lower point (like a 1/4 way up)?  Would it make that much difference?

Steamhead

The radiator should have

a "boss" somewhere between 1/3 and 2/3 of the way up the radiator, which can be drilled and tapped for 1/8" pipe thread. That's where the vent should go.