Fuel Oil Furnace Flue Types

Oil Furnace Operation

• Oil-powered furnaces use a motor assembly and oil tank unit to deliver hot combustion gases to a furnace’s heat exchanger compartment. The motor assembly operates within an oil burner unit that heats and sprays oil onto the interior of a combustion chamber. The hot gases from the oil circulate up through the heat exchanger unit and into a home’s duct and vent system. By-product combustion gases are pulled into the flue from the heat exchanger and into the outside air.

Combustion Processes

• The furnace combustion processe relies on air currents--or drafts--that move air and gas materials around the combustion chamber. Air drafts also pull by-product combustion gases into the chimney flue. Oil furnaces produce extremely hot temperatures compared to other types of furnaces. In order to generate this heat, combustion chambers include a liner that’s designed to ignite oil droplets and produce hot gases. These conditions require chimney flues that can withstand extreme temperatures.

Class A Chimney Design

• Class A chimney designs work with woodstove and fireplace insert appliances in addition to oil furnaces. Class A chimney constructions include a double-walled stainless steel passage with fire-resistant insulating materials tucked between the two walls. As different home designs use different chimney configurations, Class A flues come in a range of sizes.

  As chimney flues typically sit within close proximity of a furnace’s combustion chamber, oil-powered flue types must withstand extreme heat conditions. Class A chimneys tolerate a zero-clearance requirement in terms of proximity, though building code requirements may mandate a minimum of one-inch clearance.

Type L Chimney Design

• Type L chimneys consist of double-walled steel that acts as its own insulating material. Unlike Class A chimney flues, Type L designs allow for an airspace in-between steel walls as opposed to insulation fillings. The airspace helps to improve draft flow through the flue chamber, which provides an added safety feature for venting oil-powered appliances and furnaces. For both Class A and Type L chimney flues, the top of the chimney must extend from 2 to 3 feet above the roof surface, depending on the slope of the roof and any nearby surrounding structures.