Heating System Workings...Steam!


Steam Boilers

In a steam system, water heated in the boiler turns to steam, and the steam moves through radiators to heat the home.

While there are many similarities between a steam boiler and a hot water boiler, there are a few differences, including devices found in a steam boiler system that may be unfamiliar.

A Steam Boiler includes:
* Steam valves. Located on the radiators, these valves vent the air in the pipes to make room for the steam. If you notice white calcium buildup around a steam valve, it means steam is escaping.

* Gauge glass. This shows the water level in the boiler. The normal level is usually in the center of the glass. The water level in the glass will fluctuate slightly when the burner runs.

* Low-water cutoff. Required by code, this device shuts off the burner if the water level in the boiler falls too low.

* Automatic water feeder. This optional device, which is sometimes found on steam systems, automatically adds water to the boiler if the level gets too low. Even if a system has an automatic water feed, the homeowner should still check the boiler’s water level on a regular basis.
* Pressure relief valve. The pressure relief valve is a safety device to prevent the system from over-pressurizing.

* Steam limit control. Sometimes called a pressuretrol, the steam limit control turns off the burner when sufficient steam pressure is achieved.

Boiler Designs

Some older oil boilers you may see have been converted from coal. Due to antiquated designs, these systems are plagued by high draft loss and poor heat transfer. Newer oil-fired boilers are more efficient, in part because of low-mass construction and reduced water storage. This results in less heat loss.

The most common design for steel boilers uses fire tubes. Combustion gases flow into these long tubes, which are surrounded by water. The oil burner fires into the combustion chamber, which has a dry base design.

A dry base design means there is no water surrounding the firebox. All the water is contained in the upper section of the boiler. Other steel boilers have a wet base design in which water surrounds the combustion chamber. This design is more efficient because the hot gases pass through the fire tubes twice before they are vented.


The wet base design is also called a two-pass system. Like steel boilers, cast-iron boilers also have dry base and wet base designs. In a cast-iron boiler, the hot gases rising out of the firebox pass over the outside of each of the boiler’s cast sections through flue channels which are located between the cast sections. The gases are then vented through the flue pipe to the chimney.

All boilers have internal passages for the combustion gases. If the passages are too wide, the heat transfer rate will be low. Combustion gas passages that are too wide are often a problem in boilers converted from coal to oil.

Efficiency Ratings

Cast-iron and steel boilers are tested to verify their heating capacity and efficiency. Boiler ratings are published by The Hydronics Institute. Rating indicators for each boiler model include hot water output in Btus per hour and the Annual Fuel Utilization Efficiency (AFUE) ratio. AFUE is determined by a testing procedure specified by the U.S. Department of Energy. All heating equipment manufactured after 1980 is required to have a label indicating its AFUE. The AFUE ratio is a measurement of a heating system’s seasonal efficiency, taking into account how well the system performs over an entire season of starts and stops. AFUE should not be confused with combustion efficiency, which indicates how well the burner converts oil into heat.

In many oil-heated homes, there are tags attached to the equipment that may indicate combustion efficiency. If the combustion efficiency is below 78, you may want to consult with an Oilheat professional. While the unit doesn’t have to be replaced if its combustion efficiency is less than 78, a careful analysis may indicate that you could save energy and money by upgrading the burner, furnace or boiler. Such an assessment would require a review of your household’s heating oil bills, and the cost of new equipment, including installation, to determine if upgrading to new equipment is justified.

The age of your boiler may determine its efficiency. On average, efficiency ratings for conventional boilers 30 years old or older, are in the mid-60% range. However, new boilers have efficiency ratings from 81% - 92%.