What is an Oil Furnace?

Kate Monteith

An oil furnace is a heating system used to provide hot water or heated indoor air. Oil furnaces use pressurized oil mist to ignite a flame and provide heat. Oil is not an especially clean-burning fuel, the primary reason why most people prefer electric or natural gas heaters today. These furnaces are still available for sale, however, and many are still functioning in older homes and public buildings.

Oil furnaces use pressurized oil mist to ignite a flame and provide heat.
Oil furnaces use pressurized oil mist to ignite a flame and provide heat.

The oil furnace might be more accurately called a pressure burner. The heating function takes place in a combustion chamber, where highly pressurized oil is sprayed across an electric spark to create a flame. The resulting heat can be easily regulated by the amount of oil mist being sprayed into the chamber. Oil is kept in a tank and replenished whenever the supply runs low.

A propane tank.
A propane tank.

One of the most important considerations of all when buying a heater is to consider the overall heating efficiency of the system. The efficiency of furnaces is measured by the percentage of Annual Fuel Utilization Efficiency (AFUE). An oil furnace is considered a very good heater if it achieves a rating of 80% or more. The AFUE rating for gas heaters can be much higher, however, with 95% efficiency ratings in top of the line furnaces.

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When compared to natural gas, oil is not the cleanest of petroleum products. Fuel oil contains a great deal of sulfur that can build up and cause malfunctions in a furnace system. Soot from the oil can clog the spray nozzle and build up in the combustion chamber, severely affecting the AFUE percentage. Experts in heating, ventilation and air conditioning (HVAC) recommend that oil furnaces be professionally cleaned at least once each year.

An HVAC technician is qualified to remove the soot deposits that accumulate inside an oil furnace. The annual cost of a service call can add up over a number of years, especially when compared to the cleaner alternatives of gas or electric furnaces. In fact, many HVAC technicians no longer accept service calls for oil furnaces.

Years ago, the oil furnace was a reliable heating system for locations without gas service. Today, natural gas service has reached into almost every urban location. Anyone without gas service can easily and cheaply tap into a portable propane tank for heat. Electric heat is also readily available. Essentially, electric and natural gas furnaces are overtaking the home heating industry, and the use of oil furnaces is slowly declining.

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