Knowledge Base


boilers.jpgOil-fired boilers are more versatile than you think. 

While furnaces heat and distribute air, boilers heat and distribute water. One of the advantages of an oil-fired boiler is that it can heat a home's hot water along with the indoor temperature. A separate water heating unit is not needed. Another advantage of a boiler is that it heats a home very evenly. What's more, because of new technology, such as small, flexible air ducts, homeowners with boilers can now install a central air conditioning system more easily than in the past.

Additionally, boilers can be used in homes that have duct systems by means of a hydro-air system. In a hydro-air system, heated water is sent to an air handler to warm the house. The concept is much like that of a central cooling system where refrigerant is sent to an air handler to cool the house. The hot water in the hydro-air system warms the air in the air handler and the heated air blows through the ducts to warm the home.

Hot Water Boilers

In a hot water (hydronic) boiler system, heated water from the boiler is pumped by the circulator through radiators, radiant tubing or baseboards. With hot water heating, dividing the system into separate heating areas of the home is easily accomplished. Zoning provides efficient, comfortable, trouble-free, inexpensive heating. Radiant heating, one of the oldest forms of hot water heating, has made a comeback in recent years due to its warmth and efficiency. Advances in technology have made radiant piping more durable and affordable. Plastic tubing (which is a more economical choice than other piping materials) has made it conveniently possible for homeowners to have their floors, walls, driveways and pools heated hydronically. Hydronic boiler systems can distribute hot water through:

  • Radiators
  • Baseboards
  • Convectors
  • Vents (hydro-air systems)

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 that prevents 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.


The Condensing Boiler

One of the newer technologies in Oilheat is the condensing, oil-fired warm air boiler, which features two heat exchangers. Efficiency ratings of 95 percent have been achieved, so operating costs can be much lower. The ultra-high efficiency of the condensing boiler is achieved by lowering the stack temperature to the point where the steam in the flue gases turns back into water.

Boiler Designs

Originally, many oil boilers were converted from coal-burning machinery. While becoming more and more rare, some of these antiques are still being used, and 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 percent range. However, new boilers have efficiency ratings from 85 percent - 95 percent.