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Oil-fired Forced-air Heating Systems

List of Models: Oil-fired forced air furnaces.
ENERGY STAR technical specifications

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.

Oil-fired forced-air furnace

Oil-fired forced-air furnace

Note that there are two entirely separate air movement paths within the furnace.

  • The combustion path supplies air to the burner and follows the hot combustion gases through the heat exchanger and flue pipe to the chimney and out of the house.
  • The second path circulates and heats the air within the house.

A third air path, external to the furnace, is the dilution air pulled through the barometric damper.

In most houses, the quantity of dilution air drawn through the barometric damper is much greater than required for combustion and can represent from 3 to 15 percent of the total heat loss in the house. Thus, anything that reduces this dilution airflow without compromising the performance of the furnace will lead to increased fuel savings and efficiency.

Many oil furnaces can be sidewall-vented by using an additional induced draft fan, normally located downstream of the furnace and barometric damper, located on the inside wall of the house. Some of these have a long run time after the burner shuts off in order to purge the furnace system of any combustion gases. The use of the barometric damper and long purge time both tend to reduce efficiency.

Some newer furnaces might have an optional direct connection for outside air for combustion (sealed combustion) instead of using indoor air. Care must be taken if this approach is followed. On a cold winter day, if the air is not warmed somewhat before it reaches the burner, it could cool the fuel oil and cause start-up problems.

A potentially more serious problem occurs when the oil is stored in tank outside the house, rather than in the heated space. When the outside temperature gets very cold, the oil in the tank cools down as well. The oil can become very viscous (thick), and you may not be able to get the oil from the tank to the burner – hence, no heat. Even if it does finally get to the burner, it may be so thick that your oil burner cannot atomize it properly and you will get poor combustion, significant sooting and potential burner failure. If you have an outside tank, consider some form of heating either from the tank or the line, and install a smaller holding tank within the house to help prevent these problems. Even better, consider bringing the tank inside the house.

Furnace Efficiency

Ever since the fuel price scare of the early 1970s, the industry has been working to improve the efficiency of furnaces and boilers. The introduction of improved burners with flame-retention heads was the first major step in boosting the efficiency of conventional oil-fired heating equipment. High-static burners have further enhanced the efficiency capability of oil heating systems. Some manufacturers have produced a “mid-efficiency” class of oil furnace designed to make the most of the superior performance of these new high-static burners. Other companies have developed condensing furnaces that cool the combustion gases enough to recover the heat that is normally lost in the form of water vapour. New technologies now allow appliances to efficiently integrate two different functions, such as space and water heating, simultaneously. A furnace's energy efficiency performance over a heating season is called the Seasonal Energy Utilization Efficiency (SEUE). This SEUE is expressed as a percentage. The higher the percentage, the more efficient the furnace. For furnaces with no standing pilot light, the SEUE is equivalent to the Annual Fuel Utilization Efficiency (AFUE).

ENERGY STAR qualified, oil-fired furnaces have an AFUE of 85 percent or more. To improve electrical efficiency, choose one with a high-efficiency circulating fan motor. Some high-efficiency furnaces using a variable-speed direct current (DC) motor to run the air circulation fan, can significantly reduce electricity consumption while providing better heat distribution. Usually, you can locate the ENERGY STAR symbol on the back of the manufacturer's brochures, beside the EnerGuide rating box. Use the EnerGuide rating to determine the AFUE rating and locate the ENERGY STAR symbol to ensure you have the most efficient product available.

ENERGY STAR qualifications

Mid-efficiency Furnaces

In addition to an improved high-static oil burner, a mid-efficiency oil furnace typically features a low-mass combustion chamber (usually ceramic fibre). The hot combustion gases pass through a superior heat exchanger that enables the circulating house air to extract more heat, and have a lower flue gas temperature. The need for a barometric damper and the large requirement for exhaust gas dilution by house air have been eliminated in the most efficient designs, and the furnace exhaust can be side-wall vented through a double-walled stainless flue pipe.

Mid-efficiency furnaces may have a seasonal efficiency of 83 to 89 percent and may use 28 to 33 percent less fuel than an old conventional furnace producing the same amount of heat.

Oil-fired forced-air furnace

The mid-efficiency furnace must keep the exiting gases above a certain temperature to prevent water vapour in the flue gas from condensing inside the furnace or venting system, where it can cause corrosion and other serious problems. The exit temperature of the combustion gases can be as low as 150°C (302°F).

Benefits of a good standard-efficiency furnace are:

  • much lower combustion and dilution air requirements
  • more power to exhaust the combustion products (an advantage in newer, tighter housing)
  • a safety shut-off in case of raft problems
  • a more effective venting system

Condensing Oil Furnaces

Heat is carried away not only in the high temperature of the flue gases, but also in the water vapour they contain. The water vapour that is produced when fuel is burned holds a substantial amount of latent heat – for oil this is just over 6%, while for natural gas it is over 11% of all the energy in the fuel. This is because oil produces only half the water vapour of gas when it is burned.

A condensing furnace uses an extra heat exchanger made of stainless steel to extract more heat from the combustion gases before they leave the furnace, dropping the exit temperature to between 40°C and 50°C (104°F and 122°F). This results in water vapour from the flue gas condensing inside the heat exchanger and releasing its latent heat to the house air circulating through the furnace. At this point, the combustion gases are so cool that they require only a narrow, certified, vent pipe that goes out the side wall of the house, instead of up the chimney. The condensate runs to a drain outlet.

With oil containing only half the hydrogen of natural gas, the potential for efficiency improvements by condensing the flue gas is much lower for oil than for natural gas – the dew point is lower, so the furnace has to work harder to condense less. Also, with higher sulphur levels, the condensate is corrosive, so that any condensing heat exchanger for oil must be even more corrosion-resistant. Oil combustion also produces a certain amount of soot, which can concentrate the acidic condensate as "acid smut" at certain points on the heat exchange surface, making things even more difficult.

Manufacturers have been working to resolve the issues mentioned above. They use a very high quality heat exchanger and a high-static burner. As these are relatively new to the oil-funace marketplace, it may still be too early to tell if all the long-term concerns with oil-fired condensing systems have been resolved, but the products and installations to date look very promising.

Many heating industry experts believe that new technology furnaces, available in small enough sizes to fit the needs of even super-insulated houses, will be the next major development in oil heating. This can come about in two ways – either by developing alternative oil burner technology or by integrating the functions of different home energy requirements, such as space and water heating systems.

Carbon monoxide detectors

Because modern houses are more airtight and have more powerful air-exhausting systems, there is a greater chance that combustion products – sometimes containing deadly carbon monoxide gas – will linger inside your house and build up to dangerous levels. A certified carbon monoxide detector located close to fuel-fired equipment (such as furnaces, fireplaces, space heaters, wood stoves and gas or propane refrigerators) will signal a potentially dangerous situation that must be corrected immediately.

Symptoms of low-level carbon monoxide poisoning are similar to those of the flu – headaches, lethargy and nausea. If your carbon monoxide detector goes off, leave your home immediately, call your gas distribution company and seek medical attention.

If you operate a conventional wood-burning fireplace (which can often leak carbon monoxide), install a carbon monoxide detector near the fireplace.

Reducing Condensation Problems in the House

More efficient heating systems, combined with better draft-proofing and insulation, can result in less air infiltration. This in turn may lead to excess moisture in the house, particularly on cold winter days. Heavy condensation on the inside of windows and dampness or mould growth on walls or ceilings are indications of too much moisture.

If these are not corrected, serious structural damage will eventually occur; luckily, indoor condensation problems can be solved:

  • Because most of the indoor humidity arises from regular household activities (such as showering and cooking), your first step should be to reduce the amount of moisture from these sources. You can do this, for example, by ensuring that your clothes dryer vents to the outside, using lids on pots when cooking, and keeping showers short.
  • You should consider installing exhaust fans in the bathroom and kitchen, vented directly to the outside.
  • You should also check the humidifier setting on your furnace, if it is equipped with one. In fact, it may not be necessary to have a humidifier in a more airtight house.
  • Finally, you should talk to a contractor about installing a heat recovery ventilator (HRV) that will increase the ventilation in your house and decrease humidity without wasting energy. HRV is a very good choice if good air quality is desired.

Improving the efficiency of existing oil heating system