Steam Heating Systems

Many older buildings were heated with steam before forced water radiators and forced air heating systems came into wide usage. It is actually quite rare for steam heat to be installed in new construction, although new steam boilers are widely available to replace old ones. Steam is a good method for old buildings that contains little or no insulation. Radiators get so hot that they can heat a room and overcome thermal losses resulting from insufficient or missing insulation. In the old days, many steam boilers were coal-fired (many old buildings, such as Brooklyn brownstones still have coal chutes), but nowadays they are gas- or oil-fired. Steam heating systems, when properly maintained, provide efficient operation and excellent heating. Understanding how the systems operate and how the components function is important for proper maintenance.

When the thermostat calls for heat, the burner ignites and begins the heat the water in the boiler. Eventually the water turns to steam and begins to rise through the heat pipes.Air bleeder valves on radiators and quick vents on supply mains stay open purging air from the system until they are contacted by steam, at which point they will close. These vents must be in good working order or else the heating system will not operate effectively. Once a radiator fills with steam, it gets very warm and radiates heat to the room. As the steam hits the radiator, it will condense back into water and gather at the bottom of the radiator. All radiators should be pitched slightly allowing the water to drain back into the heat pipe so that it may return to the boiler. Radiators also have shut-off valves to control heating in each room and to balance the system. Steam boilers should have an equalizer piping arrangement to equalize pressure between the supply and return pipes. A Hartford loop piping arrangement prevents the boiler from running dry in the event of a return pipe leak. The name derives from a Hartford insurance company that had the piping arrangement designed so as to avoid having to pay claims arising from dry boilers that cracked.

Radiators and heat pipes

Steam radiators are made of cast iron and are usually free standing. They are quite durable. Steam radiators are also now available in baseboard style. Radiators should be located on outside walls and under windows whenever possible. Typically, the smallest ones are closest to the boiler, as they will get very hot, and the largest ones are furthest away.

Radiators and steam pipes get hot!Temperatures on these surfaces can easily exceed 300°F.Radiators should have protective gratings in front them to protect residents from contact burn injuries.

Heat pipes should be insulated for the same reason. Also, a significant percentage of heat is lost on uninsulated near-boiler piping. Steam piping is connected by threaded joints which sometimes leak. All piping and radiators should be checked periodically for leakage.

Steam radiators cannot be located on the same level (usually the basement) as the boiler, as steam needs to be able to rise into a radiator. But a forced water loop can be installed to provide heat in such cases.

Asbestos

Asbestos is a naturally occurring mineral that is a known carcinogen and was used in construction up until 1960. The most common use of asbestos was in steam pipe insulation. It looks like white plaster or cardboard and if you look at the edge you can see corrugation similar to that of cardboard.


Asbestos cannot be identified visually and needs laboratory testing for confirmation. See the Asbestos page under the Environmental Issues section for more details.

Boiler controls

Unlike hot water boilers, steam boilers operate only about half to 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.

The water level gauge, or sight glass, allows viewing of the water level in the system.Ideally, the level should be about in the middle but it will tend to be higher when the system is not running. The glass should be cleaned if it is dirty.

The pressure gauge shows the operating pressure of the boiler. A steam boiler typically operates in the 2 - 5 psi range.

The high-pressure limit switch, or pressuretrol, will automatically shut the boiler down if the system pressure gets too high.

The low water cut off will automatically shut the boiler down if the water in the boiler falls below a certain level. If the water level in a boiler gets too low, boiler damage can result.

The safety relief valve will purge steam in the event that pressure in the boiler gets too high. It is installed as a backup to the high-pressure limit switch.

An oil burner is shown as the heat source but many steam systems are heated with gas as well.

Some boilers are equipped with a low water auto feed that will automatically add water when the level drops too low. These units work great but can sometimes jam, resulting in no water getting added, so the level still needs to be checked regularly. Below is a typical auto-feed unit

Maintenance of steam systems

Check the sight glass regularly to be sure that the boiler is not low or overly full. The low water cut-off should be operated weekly. A valve is opened lowering the level of water in the boiler. When the water lowers to a certain point, the boiler should shut down automatically. Be sure to refill the boiler to its previous level. Even if the boiler has an automatic feed, the water level and low cut-off should still be checked.

Boilers should be partially drained and refilled periodically to remove sediment that has accumulated. Accumulated sediment can interfere with the efficient operation of the system.

Have your heating technician show you any other maintenance which should be performed weekly for continued operation of your steam system.

Conversion from steam to forced hot water

Sometimes a steam system is converted to a forced hot water system. This most often occurs when it is time to replace a steam boiler and is presented as an option by the heating company replacing the boiler. It has been stated that a forced hot water will run more efficiently than a steam system because a forced hot water system will usually heat the water to 180°F whereas a steam system heats the water past its boiling point, which is 212°F. Although this is true, a steam system is required to heat about half a boiler full of water, whereas a forced hot water system must heat all of the water in the completely filled system -- including all pipes and radiators. This amount of water is many times the volume of water in a steam system, especially when you consider that the steam radiators and steam pipes are very large in capacity and rarely replaced during the conversion. Also, pipes and radiators in normal forced hot water system are much smaller in capacity, and although a baseboard or wall convector radiator in a forced water system will not give off as much heat as a cast iron radiator in a steam system, the amount of water required to heat the system is considerably less.

The other consideration is that houses containing steam systems are usually old and thus not likely to have been designed with heating efficiency in mind. There may be little or no insulation in the walls and the windows and exterior doors may be leaky as well if they have not been updated. So, complex calculations involving, room sizes, radiator sizes, and weather-tightness should be carefully performed prior to converting from steam to hot water, which in many cases is beyond the skill of an installer. All in all it may turn out that the conversion does not result in greater efficiency and may result in reduced comfort.