Oil-fired Forced-air Heating Systems

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Oil-fired Forced-air Heating Systems

A basic oil-fired, forced-air heating system consists of a burner fed by heating oil, usually from a storage tank inside or outside 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.

A barometric damper, acting as a valve in the flue pipe, 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


Note that there are two separate air movement paths.

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; it also includes air drawn through the barometric damper.
he other path circulates and heats the air within the house.
In many houses, the quantity of air drawn through the barometric damper is much greater than required for combustion and can represent as much as 10 to 15 percent of the total heat loss in the house. Thus, anything that reduces this airflow without compromising the performance of the furnace will lead to increased fuel savings and efficiency.

Some newer furnaces might have an optional direct connection for outside air for combustion (sealed combustion) instead of using indoor air. However, 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 similar 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 get poor combustion. 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.

High-efficiency Furnaces
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. Now manufacturers have produced a “high-efficiency” class of oil furnace designed to make the most of the superior performance of these new 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

Standard-efficiency Furnaces
In addition to an improved high-static oil burner, a non-condensing, standard-efficiency furnace features an improved 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. The most efficient designs have eliminated the need for a barometric damper and the large requirement for the dilution of exhaust gas by house air.

Standard-efficiency oil furnace


The standard-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).

A number of oil-fired systems approved for use in Canada can be vented directly out the side wall of the house, thereby eliminating the need for a chimney.

One sidewall-venting system uses the forced draft of a high-static burner to expel combustion gases. Others go further and use sealed combustion with a high-static burner.

Another sidewall-venting system uses an induced draft fan. This fan is normally located downstream of the furnace at the inside wall of the house and pulls the gases from the furnace out of the house through a small exhaust vent. Some of these sidewall systems require dilution air from the house or have a long run time after the burner shuts off in order to purge the furnace system of any combustion gases.

Sidewall venting tends to reduce efficiency.

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
Standard-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.

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 eveloping alternative oil burner technology or by integrating the functions of different home energy requirements, such as space and water heating systems.

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 produced by the burning of natural gas holds a substantial amount of latent heat – about 11 percent of all the energy in the fuel. Oil, on the other hand, contains only half the water vapour of gas; thus, oil has much less energy tied up in the form of latent heat.

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). As a result, water vapour from the flue gas condenses inside the heat exchanger and releases 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 plastic 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. The fact that 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, makes things even more difficult. This type of furnace is only marginally more efficient than a well-designed standard-efficiency furnace.

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.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.