Steam boilers use heat from the burner to convert water in the boiler to steam. The steam moves from the top of the boiler through the distribution pipes to the radiators. The steam condenses at the radiators, releasing its heat. The condensate returns to the boiler as water.
Before the system starts, the top of the boiler, the distribution pipes, and the radiators are filled with air. As the steam moves through the system, it pushes the air out of the system. When the system shuts down, and the steam all condenses back to liquid, air is let back into the pipes and radiators, and into the top of the boiler.
When the thermostat calls for heat, the burner turns on. The water in the boiler heats up and boils. Steam comes off the top of the water in the boiler, just as it does in a pot on the stove. The burner is controlled by the thermostat and by the pressuretrol. The two devices are in series. The thermostat shuts off the burner when the house gets warm enough. The pressuretrol controls the steam pressure. When the steam pressure in the top of the boiler reaches the upper allowable limit, the burner will shut down, even though the thermostat is still calling for heat. When the steam pressure drops back into normal limits, the burner will come back on if the thermostat is still calling for heat.
As steam starts to form above the water in the boiler, it pushes up on the air above the water and pushes down on the water itself. Let’s assume that we have one psi of steam pressure. If we want to move the steam through the system, we have to get the air vents out of the way. Air vents in the system are open to the atmosphere. These allow the air to be pushed out of the piping and out of the radiators (depending on the type of system) to allow the steam to move into the piping and radiators. These vents remain open to the atmosphere until the steam hits them. The hot steam typically expands a bellows which pushes a valve closed. The air vents allow air to escape but don’t let the steam get out. The steam moves through the system, pushing the air out of the piping and radiators. The steam condenses as it hits the cold pipes and radiators, releasing its heat into the radiator.
The goal is to have the steam hit all of the radiators at about the same time. This allows for even heating of the building. Some air vents on radiators are adjustable. This means that they close at rates that can be adjusted by the homeowner or service technician so that all of the radiators fill with steam at about the same time.
In steam boilers, it is important to monitor how far the water rises in the return pipe. If the water gets pushed back into pipes that should contain steam, it will cause chaos. The steam won’t be able to move and we might have heating problems. The steam slamming into the water can cause water hammer. Boiler water levels may also drop to a point where the boiler shuts off on low water. A component known as the equalizer pipe is used to create equal pressure on the boiler water and the water in the return pipe, so we don’t have this problem.
When enough heat has been added to the house, the thermostat is satisfied. This causes the burner to shut off. No more steam is produced. The steam in the system condenses into water, which runs back into the boiler. As the system cools, the air vents which had been driven closed by the high temperature steam are cooled and the vents reopen. Air from the house comes back in through the vents to fill the pipes and radiators.