Steam Boiler: Function, Types, and Installation

The Ins and Outs of This Older Heating System

Residential steam boiler
Peter Dazeley / Getty Images

If you have a vintage home that was built before the 1950s, and you still have the original radiators with fins, you probably have a steam boiler system. The steam boiler is an old convection heating technology dating back over 200 years and is most commonly found in older homes. Steam boilers are generally reliable, but they should be serviced routinely for the best performance and longevity.

What Is a Steam Boiler System?

A steam boiler system distributes heat via steam. The boiler furnace heats the water with a gas or oil burner, and the water is turned into steam. The steam is then pushed into the radiators in each room. A steam radiator is a big, cast-iron hulk, the kind you see in old apartment buildings.

The Difference Between Steam and Hot Water Heating Systems

Two types of systems—steam and hot water—can be used with radiators. Steam heating can be a bit less efficient than hot water heating.

  • Steam: Steam boilers circulate steam through pipes connected to radiators, which then condenses back to water and flows back to the boiler for reheating.
  • Hot water: Modern radiator systems circulate hot water to slim wall-mounted radiators or baseboards using electric pumps. The hot water releases its heat within the radiator, and the water that is cooled down returns to the boiler for more heating.

How a Steam Boiler Works

At the heart of a steam boiler is a cast-iron tank filled about halfway with water. Here are the steps steam boiler systems take:

  1. A burner below the tank heats the water until it turns to steam.
  2. The steam rises through a system of supply pipes to each radiator in the home.
  3. The steam’s heat is transferred to the radiator and its fins (and into the room via air convection).
  4. The steam is then condensed and turned back into water.
  5. The water travels along return pipes back to the radiator, where it is reheated for the next cycle.

Steam Piping Systems

Most steam boiler systems in homes are either one-pipe or two-pipe systems. An easy way to identify which type of system you have is to look at a radiator.

  • One-pipe system: Also called a single-pipe system, this method uses the same pipes to deliver steam to the radiators and to return water to the boiler. The steam rises and the water falls through the same pipes. If you see one pipe connected to your radiator, it's a one-pipe system. A one-pipe system does not have a temperature control at the radiators; it must be completely on or completely off to function.
  • Two-pipe system: This system is relatively more modern and uses separate piping for the steam supply and the water return. If you have a two-pipe system, the two pipes will usually be found on opposite ends of the radiator.

Air Vents in Steam Systems

Both one-pipe and two-pipe steam boiler 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, the 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. Take these steps to make sure Since most homes with steam boilers have very old systems that are costly to replace—or to convert to another type of heating system—it pays to follow these three steps to keep your boiler running as long as possible:

  • Check water levels weekly: Many steam boilers should be checked 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.
  • Flush sediment weekly: Another regular maintenance task is a visual inspection of the controls and a quick 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.
  • Call in professionals seasonally: In addition to homeowner maintenance, boilers should be checked by an HVAC 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. However, homeowners should rest assured that the likelihood of a steam boiler explosion in a home is extremely rare today. Safety measures and codes were developed long ago by the American Boiler Manufacturers Association (ABMA) and the American Society of Mechanical Engineers (ASME).