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Two valves on each radiator?
Large 100 year old colonial. Hadn’t used the steam in over 10 years. Just replaced ancient oil fired boiler with Independence gas fired unit just as the weather got cold. Also had thermostatic valves placed on nearly all radiators (about 20–it’s a big house). So far things are working quite well. Heat is very even and comfortable and the system is quiet. My question is about the valves.
I understand how the metal cylinder on the top works: I can move the little brass pin to control the speed at which the air is bled out of the radiator and thus control how fast the unit warms up. This allows me to boost the heat in some rooms and back off in others even with a single thermostat for the house. So far, so good. But what is the plastic grey knob for? That has plus and minus direction markings and can screw tighter and looser. But what is it for?