I am writing this from my own experience and not from reading on the web. Some information maybe a little bit off but for a simple mechanical system like the heating system, minor inaccuracies are probably tolerable.
A steam heating system operates by boiling water and turning it to steam. The steam then travels through pipes and gives up heat through cast iron radiators. Steam heating requires no motor to circulate the steam as in the case of water heaters. Because steam can travels up further against gravity, steam heating is a preferred choice for high buildings. The disadvantage is the loss of heat through pipes because hotter objects radiates faster than colder ones.
A steam heating system typically has a safety valve that opens up and vents steam in order to protect the boiler from rupturing. This only happens if the operating pressure threshold is set too high.
An open-loop circuit having four points of contact controls the system. The contacts comprise of the thermostat in a room, a pressure gauge near the heater, a low water sensor, and a pilot light valve. Electrical power for opening gas valves is provided by a thermocouple located next to a pilot flame. If the pilot flame goes out, no energy is available to open the gas valves and thus the system is effectively shut off.
The thermostat closes the loop when temperature in the room is lower than the set temperature. The pressure gauge too, closes the loop when pressure in the boiler is less than the set pressure.
Each room in a home should have radiators which are properly sized for the space of the room. A radiator's heating output can also be controlled using the attached air bleeder. The bleeder is made of a heat sensitive material that opens and closes the radiator in response to the hot air passing through it. By adjusting the air bleeder, one controls the radiator's operating duration while the heating system is on.
Operate the heating system by setting the thermostat to a desired temperature and set the pressure gauge at the lowest value, say 2.5 PSI, and set the high threshold pressure at 1.5 PSI. The extra pressure setting is the amount of hysteresis used to control the firing rate (number of on/off) of the boiler. The current loop is cut off if pressure is greater than 4 PSI. It cuts back in at 2.5 PSI. If the heating system is unable to catch up to the set temperature in a few hours, adjust (loosen) the pressure valve screw in minute increments then wait a few hours. Repeat the procedure until the heating system catches up with the setting on the thermostat.
The bottom of the boiler is connected to a water valve. Open the valve frequently to drain muddy water from the boiler. Add water to the system in small increments. If the boiler is low in water, the gas valve is cut off automatically to prevent damage to the boiler. When both ends of the clear cylinder are open, the indicated water level is equal to that in the boiler. For that reason, the valves on both ends should stay open.
If it is a new radiator, one should flush it with water a few times to remove materials left from the manufacturing process and surface grease added to protect a new unit from rust.
