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HVAC Steam Heating Systems

The Old House Web

Parts of this story: Introduction >> Fuel burning units, general >> Forced warm air heating systems >> Forced hot water (hydronic) heating systems >> Electric resistance heating >>Air conditioning systems

steam heat

++CTA++Steam heating systems are seldom installed now in small residential buildings but are still common in many older ones. They are simple in design and operation, but require a higher level of maintenance than modern residential heating systems.

Unless the steam system is in good working order and adequate plans can be made for its upkeep, consider replacing it with a more maintenance-free system.

Assess the condition and operation of steam heating systems as follows:

Boiler. Steam boilers are physically similar to hot water boilers and should be inspected similarly.

Boiler controls. Unlike hot water boilers, steam boilers operate only about three-fourths full of water and at much lower pressures, usually 2 to 5 psi. Steam boilers should be equipped with a water level gauge, a pressure gauge, a high-pressure limit switch, a low water cut-off, and a safety valve.


Activate the boiler and observe the water level gauge that indicates the level of the water in the boiler. The gauge should normally read about half full, though the actual level of the water is not critical as long as the level is showing. If the gauge is full of water, the boiler is flooded and water must be drained from the system. If the gauge is empty, the boiler water level is too low and must be filled (either manually through the fill valve or automatically through the automatic water feed valve, if the boiler has one).

Unsteady, up and down motion of water in the gauge means the boiler is clogged with sediment or is otherwise operating incorrectly and must be repaired. The clarity of the boiler water should be noted when checking the gauge; if the gauge is too dirty to judge the water level, remove and clean it. This test and any resulting work should be done by a service technician.

The high-pressure limit switch turns off the burner when the boiler pressure exceeds a preset level, usually 5 to 7 psi (35 to 48 kPa). It is connected to the boiler by a pigtail-shaped pipe. The low water cut-off shuts down the burner when the boiler water level is too low.


Lower the water level in the boiler and see whether the low water cutoff turns off the burner. Usually this test should be performed by a service technician. The pressure relief valve is designed to discharge when the boiler pressure exceeds 15 psi (103 kPa).

Distribution piping. The steam distribution system consists of distribution piping, radiators, and control valves. Distribution piping may have either a one-pipe or two-pipe configuration.

In a one-pipe system, steam from the boiler rises under pressure through the pipes to the radiators. There it displaces air by evacuation through the radiator vent valves, condenses on the radiators inner surface, and gives up heat. Steam condensate flows by gravity back through the same pipes to the boiler for reheating. The pipes, therefore, must be pitched no less than one inch in ten feet in the direction of the boiler to ensure that the condensate does not block the steam in any part of the system. All piping and radiators must be located above the boiler in a one-pipe system.

One-pipe parallel flow steam heating system

In a two-pipe system, steam flows to the radiators in one pipe and condensate returns in another. A steam trap on the condensate return line releases air displaced by the incoming steam. If the condensate return piping is located below the level of the boiler, it should be brought back up to the level of the boiler and vented to the supply piping in a Hartford Loop. This prevents a leak in the condensate return from emptying the boiler. Two-pipe systems can be balanced by regulating the supply valves on each radiator, and may be converted for use in a hot water heating system (although new, larger-size return piping usually must be installed).

One-pipe counter-flow steam heating system

Two-pipe steam heating system

Distribution piping should be checked for leaks at all valves and connections. Make sure all piping is properly pitched to drain toward the boiler. Pounding may occur when oncoming steam meets water trapped in the system by improperly pitched distribution piping or by shutoff valves that are not fully closed or fully open. Inspect the condition of all piping. Make sure pipes are properly insulated in unheated basements, attics, and crawl spaces.

Radiators and control valves. Steam radiators are made of cast iron and are usually free standing. They are quite durable and, in most cases, can be reused. Radiators should be located on outside walls whenever possible.


Lower the water level in the boiler and see whether the low water cutoff turns off the burner. Usually this test should be performed by a service technician. The pressure relief valve is designed to discharge when the boiler pressure exceeds 15 psi (103 kPa).

Activate the system and inspect the condition of all radiators. Look for signs of water leakage. Feel their surfaces to make sure they are heating uniformly; if they are not, check the radiator air vents and supply valves on a one-pipe system and the radiator supply valves and the steam trap on the condensate return on a two-pipe system.

Often air vents need cleaning and supply valves need tightening, or valve packing needs to be replaced. Pounding near the radiator can often be cured by lifting one edge of the radiator slightly; this reduces condensate blocking in the pipes.

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

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