Residential Steam Boiler Basics

Residential steam boiler
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The steam boiler is an old convection heating technology dating back over 200 years and is most commonly found in old homes. Because steam boilers operate at a higher temperature than hot-water boilers, they are inherently less efficient than their hot-water cousins and are more finicky to operate.

Like a hot-water heating system, a steam system distributes heat via one or more radiators in each room. But while a hot-water radiator can be a slim wall unit or a baseboard convector, a steam radiator is a big, cast-iron hulk, the kind you see in old apartment buildings. Steam boilers are generally reliable, but they should be serviced routinely for the best performance and longevity.

How a Steam Boiler Works

At the heart of a steam boiler is a cast iron tank filled about halfway with water. A burner below the tank heats the water until it turns to steam. The steam rises up through a system of supply pipes to each radiator in the home. The steam’s heat is transferred to the radiator (and into the room via air convection), causing the steam to condense and turn back into to water. The water travels along return pipes back to the radiator, where it is reheated for the next cycle.

Types of Steam Piping Systems

Most steam boiler systems in homes are either one-pipe or two-pipe systems. One-pipe (or single-pipe) systems use the same pipes to deliver steam to the radiators and to return water to the boiler. The steam rises up and the water falls down through the same pipes. A two-pipe system is relatively more modern and uses separate piping for the steam supply and the water return.

An easy way to identify which type of system you have is to look at a radiator. If there’s only one pipe connected to it, you have a one-pipe system. Two pipes—usually on opposite ends of the radiator—indicate a two-pipe system. Another clue is that one-pipe systems have no temperature control at the radiators; they must be all the way on or all the way off.

Air Vents in Steam Systems

Both one-pipe and two-pipe steam heating systems involve venting to bleed air from the system with each heating cycle. Air is forced out through vents to allow the steam to enter the radiators. One-pipe systems have an air vent or valve on each radiator. These are what cause the hissing sound associated with very old heating systems. With a two-pipe system, air is expelled through a steam trap vent located on the water return piping.  

Steam Boiler Controls

Steam boilers have several critical features to ensure safety and proper operation:

  • Pressure gauge and regulator: Monitors pressure and shuts off the fuel supply to the boiler if the preset pressure level is exceeded
  • Temperature and pressure relief valve: Releases steam if the pressure gauge/regulator fails and pressure builds dangerously high inside the boiler
  • Water level sight glass: Clear glass tube that reveals the water level in the boiler
  • Low-water cutoff: Shuts off the system if the boiler’s water level drops below the minimum level

Maintaining a Steam Boiler

Steam boilers are generally reliable and can run for 15 to 30 years or more, but they need more maintenance and regular checkups than a gas furnace, for example. Many steam boilers should be checked on weekly to make sure the water is at the right level. Too much water wastes energy, and too little water can weaken boiler parts due to overheating. A drop in water level also can indicate chronic problems, such as leaks or valve failures.

Another regular maintenance task is flushing out some of the water to remove sediment and prevent problems with low-water safety valves. This is usually a simple process of draining a bucket of water, then refilling the boiler with the same amount of fresh water. A quick flush and visual inspection of the boiler controls is good weekly practice.

In addition to homeowner maintenance, boilers should be checked by a professional at the beginning of each heating season. Boiler technicians test critical controls and safety features and conduct a thorough visual inspection to troubleshoot potential problems. Since most homes with steam boilers have very old systems that are costly to replace—or to convert to another types of heating system—it really pays to follow a regular maintenance schedule to keep your boiler running as long as possible.