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Oil Furnace Basics

Oil fired furnaces are the second most common heating type in North America. In some areas, oil furnaces are more common than gas. People living in the country usually can only choose between oil and propance as a fuel source.

Two types of oil burners usually heat air or water. The most common is the high pressure or gun-type burner. The other is a vaporizing or pot-type burner. When the thermostat calls for heat, a high pressure or gun-type oil burner pumps oil through a nozzle producing an oil mist. A blower mixes the oil mist with air and propels the air-oil mixture into a combustion chamber. A high-voltage spark created by two electrodes then ignites the air-oil mixture. In a vaporizing or pot-type burner, an oil control valve opens to allow oil to pool in a pot. A blower or natural draft adds the air needed to support combustion. An electric spark then ignites the oil. The heat of the burning oil causes the oil in the pool to vaporize and mix with the air. The vaporized oil-air mixture then ignites and the cycle continues. The vaporizing burner requires a higher grade of oil that vaporizes easily for efficient operation.

Periodically, oil-fired heating systems need adjustment to keep them running in the safest, most efficient and least costly manner. This publication is designed to help you understand how oil-fired heating systems work (both forced air and hot water systems), what periodic preventive maintenance tasks the units should have, and what special tasks and problems might arise that would need the attention of a heating contractor.

All oil-fired heating systems are composed of a heat producing source (a furnace in the case of forced air systems and a boiler for hot water systems) heat exchanger; a distribution system (ducts in the case of forced air systems, pipes in hot water systems); a burner by-products elimination system (exhaust stack, flue or chimney); and control mechanisms (such as the thermostat and master switch). Understanding how these various parts operate and work together is an important part of any preventive maintenance program.

A basic oil-fired, forced-air heating system consists of a burner fed by heating oil from a storage tank, usually located inside the house, firing into a combustion chamber in the furnace. The combustion gases pass through the furnace, where they give up heat across a heat exchanger. They are then exhausted to the outside through a flue pipe and chimney. For most systems, a barometric damper, acting as a mixingvalve in the flue pipe, downstream of the furnace proper, isolates the burner from changes in pressure at the chimney exit by pulling varying quantities of heated room air into the exhaust. A circulating fan passes cool house air from the cold air return ducts over the furnace heat exchanger, where it is warmed up, then moved into the hot air ducts, which distribute the heated air throughout the house.

The heat exchanger is the medium of heat transfer and separates the burned fuel from the air that moves through the house. Rust or cracking will allow CO gas to enter your home and usually requires furnace replacement.

When a heat exchanger cracks or fails, there are two points to consider involving air pressure. When cracks and rust, etc. are located at air pressure points, such as at turns and/or at the top of the heat exchanger (where the direction of the exhaust is not directed toward the flue), you may get combustion gases forced into the flow of the air in the plenum and ductwork.

Trouble Shooting Oil Furnace

Basic trouble shooting for when your oil furnace won't run. If your furnace fails it a good possibility that it's an electrical or fuel problem. First make sure that the furnace is switched on and that no circuit breakers have been thrown and no fuses blown. If this is not the problem, the motor may have overloaded or overheated. Let it sit for half an hour before pressing the power reset button. Should the problem persist, it's likely fuel-based. Make sure that the fuel tank is not empty; top it off if need be. Should this not be the case, then there's likely a fuel blockage. Clean out the oil pump strainer and the oil filter. Professional help will be required should the fuel line itself need to be cleaned.

Combustion Chamber Crack

The temperature of combustion can approach 4000oF, which is above the melting point of steel and cast iron, the materials used to construct boilers and furnace heat exchangers. Combustion chambers are capable of also acting as "heat sinks" or "Reservoirs of Heat"; this provides additional energy to the medium being heated, allowing longer periods between burner firings during heating operation.

Regular maintenance and cleaning by a professional oil furnace technician can extend the life of your furnace and ensure it is operating at its optimum efficiency.

Using a pleated air filter with electrostatic strips are the minimum standard recommended by most furnace experts. The expanded surface area allows for greater air movement which does not restrict operation of fan unit and lower furnace efficiency.

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Furnace Efficiency

The efficiency of an oil fired forced air furnace is a combination of the design of the furnace itself and the burner efficiency. Let’s look at the efficiencies:

Old Furnace Standard Burner

The seasonal efficiency is about 60%. This means for each dollar you spend on oil, you are getting sixty cents worth of heat into the home.

Old Furnace with an Upgraded Burner

An old furnace can be improved by replacing the burner. New burners mix the air and oil better to create more efficient combustion and higher efficiency. This burner is called a flame retention head burner, named after the more compact shape of the flame that ejects from the burner. This upgrade may achieve a seasonal efficiency of about 70% to 78%.

Modern Furnace

A modern furnace with a modern burner (flame retention head) will have a seasonal efficiency of about 78% to 86%.

Mid Efficiency Furnace

A mid efficiency furnace utilizes more advanced technology for the burner, called a high static burner. These furnaces achieve a seasonal efficiency of 83% to 89%.

Oil Leaks

Because of the possibility of an oil leak, some insurance companies will require that your oil tank be replaced if it is more than 20 years old, even if it appears to be in perfect condition.

There may be regulations in your area that require upgrades such as replacement of the oil feed tube, double walled tanks, spill containment.

An oil tank that is buried in the ground may eventually leak. The problem is that not only is this likely to go un-noticed but it can create an enormous and costly environmental mess. This is beyond the scope of a home inspection. Consult the local oil supplier or have an environmental assessment done.

Disadvantages of Oil

 

  • Requires delivery
  • Requires more maintenance than gas or electric systems
  • Older equipment can be noisy, dirty, and difficult to maintain
  • Risk of an oil leak and environmental contamination
  • Occasional odors
  • Risk of puffbacks, where soot is released into a house
  • Electricity to run oil burner adds to cost of operation