Reddit reviews Hoffman Steam Air Vent 40, 1/8" Angle Steam Radiator Air Valve 401440
We found 2 Reddit comments about Hoffman Steam Air Vent 40, 1/8" Angle Steam Radiator Air Valve 401440. Here are the top ones, ranked by their Reddit score.
Air Valve (non-vacuum)Float-type ventSingle non-adjustable port1/8" NPT angle connectionMaximum operating pressure 6 psig (0.4 bar)
It's a vent, though it does work as sort of an automated control valve for the radiator, turning off the steam as soon as it reaches the valve to push the steam towards other radiators, and to prevent the steam escaping the system.
Here's an example, though there are many styles. They are usually mounted to the radiator between a third and half the way up on the side opposite the piping.
https://www.amazon.com/Hoffman-Steam-Angle-Radiator-401440/dp/B0057XTJOI
That is almost certainly a single pipe steam radiator. (If there are two pipes on the other side, it's a two pipe steam one. they have other failure modes, let me know if that's the case...) The most likely problem is that the air vent (which is the thing in the middle hole) has failed, and no longer letting air out of the radiator when the boiler is steaming. Remove it; replace it with one of these or other good quality made-in-USA radiator vent. (Chinese made ones are all crap, and will fail within a year.)
Also, check that someone didn't shut the valve off....