The radiators are too big for the space they serve.
There once was a time when people were afraid of the air in a closed room. They thought "vitiated" air (the name the used to describe it) would make them sick so they always kept a window open. The Dead Men knew their clients were going to do this so they compensated by oversizing the radiators on most jobs.
Today, that oversized radiator may be overheating the room it serves. To find out if it is, calculate the heat loss of the room and check this against the size of the radiator. If the radiator is too big, you can solve the problem by installing a thermostatic radiator valve.
If you have a two-pipe system, the TRV replaces the radiator's supply valve. On one-pipe systems, the TRV goes between the air vent and the radiator. This is because you can't throttle the inlet valve on a one-pipe steam radiator.
If you don't want to install a TRV, you can do what the folks who lived during the Victorian era did. Cover the radiator with a heavy-cloth bag, similar to a pillowcase. The cloth cuts down on the convective currents and does a good job of undersizing the radiator. The Victorians went out of their way to make their radiator bags decorative. Many of these bags had fringes and a drawstring that would give the Victorians a way of raising and lowering the bag like a curtain.
You can also box in the radiator or use a commercially available radiator cover. Depending on air pattern they create, a cover can either increase or decrease the heat output of a radiator.
The steam pressure is too high.
At just under one psi, the temperature of steam is about 215 degrees. This is the pressure the Dead Men had in mind when they came up with the charts they used to size steam heating systems.
To size radiation, they used the term EDR, which stands for Equivalent Direct Radiation. In a steam- heating system, one square foot of EDR will put out 240 BTU/Hr. when there is 70-degree air outside the radiator and 215-degree steam on the inside. The Dead Men intended for there to be slightly less than one psi in the radiators on the coldest days of the year.
If you raise the pressure, you will also increase the heat that comes from each square foot EDR. For instance, at 10 psi, each square foot EDR puts out 240 BTU/Hr. That's why raising the pressure often overheats the rooms.
Crank it down!
The pipes aren't insulated.
A bare steam pipe will put out about five times the heat of an insulated pipe. When you remove the insulation from a steam pipe, you increase the heat in the room through which the pipe runs. This often happens in basements. Someone removes the asbestos and the basement overheats. The same can happen anywhere in the building.
Insulate the steam pipes.
The thermostat is out of calibration.
If it is, the burner may be running longer than it should. That will quickly overheat all or part of the building. Make sure you use an ammeter when you're checking the thermostat's calibration. Don't guess at that anticipator's setting.
Check, too, to see if the thermostat has a mercury switch. If it does, make sure the thermostat hangs level on the wall. And check to see if the thermostat is in a cold draft, or if it's hanging on a poorly insulated outside wall. Make the necessary corrections.
The device that controls the firing cycle is defective or in the wrong place.
Larger steam-heated buildings have heat-timing devices. These devices will fill the piping and radiation with steam on a call for heat. Then they'll run the boiler for a certain time based on the outdoor temperature.
Some control manufacturers use a pressuretrol to figure out when steam fills the piping and radiation. It's easy to trick a pressuretrol - all it takes is some dirt in either the pressuretrol or the pigtail. If there's too much heat in the building, check that pressuretrol.
Other heat-timing devices use a thermistor to sense temperature rather than pressure. You usually place the thermistor at the end of the longest steam main, but there are no fixed rules; it varies from building to building. However, if the thermistor is on a main that has a clogged air vent, the burner will run all the time. That's because the trapped air will keep the steam from reaching the thermistor.
Check, too, for thermistors that wind up on cold water lines, drain lines and, yes, even gas lines. It happens.
If you have a gravity-return system, make sure the thermistor is high enough on the main. It needs to be below the "A" or "B" Dimension so the rising condensate doesn't cover and cool it. (See The Lost Art of Steam Heating for a complete discussion of "A" and "B" Dimensions.)
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).