Connect a single pipe steam radiator to a smart thermostat

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But a brief primer. With steam heat -- one pipe or two pipe -- the steam is created by the boiler in the basement, and flows to the radiators via the steam mains. So long as the boiler is running, steam is being created. Once the steam reaches a radiator, it needs to push the air in the radiator out, so it can get in. Disregarding two pipe systems for the moment, to avoid confusion, the air gets out through the vent. As soon as the air starts to leave steam can get in and start to heat the radiator and, through it, the space. When steam reaches the vent, the vent closes. This does not mean the radiator stops heating -- rather, as the steam already in the radiator condenses, more steam can come in and the radiator will continue to heat as long as the boiler is making steam.

One can balance the radiators in various rooms by changing the rate at which air can leave and steam can get in, by changing the vents.

So long as the vent has not closed because steam reached it, you can also reduce the amount of heat which the radiator puts out by closing the vent before steam gets to it. A thermostatic vent will do that. If the vent is closed when the boiler starts the next cycle, that radiator will get very little heat. If the vent then opens during that cycle, that radiator will begin to heat -- until the boiler stops.

If the boiler is not running, the vents -- unless they are closed by, for instance, a thermostatic control -- will all be open. If a vent is controlled by a thermostatic control and it opens while the boiler is off nothing will happen Until the boiler starts again.

So a bottom line: with one pipe steam (and with two, although the process is different) you can reduce the heat given off by the radiator by closing the vent (he valve, in two pipe) with, for instance, a thermostatically controlled vent. You cannot increase the heat, however, unless something else has turned the boiler on.

In some very large two pipe systems, the boiler is essentially always on, and it is possible to control individual radiators by turning their valves on and off -- the steam is always there. In one pipe systems this doesn't work. A little thought on the above comments will show that the boiler must turn off and the system cool off from time to time so that the vents can regain control of their radiators. Most often in residential systems this is done by a master thermostat which senses that the building as a whole needs heat and turs on the boiler -- and the radiators provide the heat. Individual spaces can have individual controls to reduce the heat to that space -- but not to increase it, and even then only if there is a master thermostat for the boiler so that the system runs when the building needs heat. Most one pipe residential systems can be balanced wo that there is no need to further adjust the temperature of individual spaces.

I might add that as much as I love steam heat, and feel that it provides the highest level of comfort of any system, neither it nor hot water heat (which is a little more flexible) nor even central forced air is well suited to an application where the occupants want to raise or lower the temperature of an individual space at will and frequently. One way to overcome this problem is to use the steam to provide what might be called base heating -- say maintaining the building as a whole at the lowest temperature any occupant is likely to desire -- and using either electric resistance heating or individual or mini-split type heat pumps to provide the topping up control. This works well enough, although it is expensive to install, in the "shoulder" seasons. It can become very expensive to run in more extreme winter temperatures, however, where the heat pumps have to resort to electric resistance backup.