HPAC Magazine

Where there is smoke… there is a maintenance issue

November 1, 2012 | By Adam Dras

New oilburners should not produce smoke.

Smoke and soot, which are nothing more than unburned carbon, are created by outdated burner designs and incorrect burner service and adjustment. Smoke production is unnecessary and must be eliminated, because it reduces efficiency, increases service calls, and is a nuisance to homeowners. It can be prevented using modern burners, and through careful adjustment of burners using combustion test equipment.

Excessive smoke wastes fuel because it deposits soot on the heat exchanger surfaces (see Figure 1). This insulates the heat exchanger, limiting its ability to extract the heat from the combustion gases. A layer of soot only 1/8″ thick can reduce heat absorption by over eight per cent.

Efficiency loss caused by a smoky burner occurs as the soot slowly builds up. Soot also affects the reliable operation of the burner. If it builds up on the cad cell or the bimetal of the stack relay, it can act like a flame failure and cause the control to lock out on safety, creating an unnecessary service call.

Overfiring can cause smoke. If a unit is overfired, the burner will create heat faster than the heating system can distribute to the building. When this happens, the burner short cycles (goes on and off frequently for short periods of time). The problem is that older oilburners create smoke when they start and stop.

Up to two thirds of all the smoke produced by burners made before the year 2000 is produced on start up and shut down. Therefore, properly sized nozzles will produce less frequent burner cycling and less smoke.


Sulfur exists in varying degrees in all fossil fuels. The sulfur content of heating oil ranges from 0.5 per cent to 0.05 per cent by weight.

When burned, the sulfur mixes with oxygen to form sulfuric oxide (SOx). It reacts with the water vapour in the combustion gases to create sulfuric acid aerosol. When the acid condenses (at about 66C-93C); (150F to 200F), it adheres to the flue pipe and heat exchanger surfaces in a film and reacts with the iron in the pipe and heat exchanger wall. This creates iron sulfates, the light-yellow to rust-coloured crusty scale you find clinging to the heat exchanger.

Scale buildup downgrades efficiency by one to four per cent over the year. It also blocks flue passages, restricting air flow and increasing smoke and soot. Sulfur levels in heating oil are gradually being reduced, so this will be less of a problem in the future.

Carbon monoxide

Carbon monoxide, or CO, is a toxic gas that can occur in homes and buildings where combustion by-products are generated, not properly vented and allowed to accumulate. CO is a colourless, odourless, tasteless poison. Carbon monoxide is readily absorbed into the body and can impair the oxygen-carrying capacity of the blood (hemoglobin).

Impairment of the body’s hemoglobin results in less oxygen to the brain, heart, and tissues. Even short-term overexposure to carbon monoxide can be critical or fatal to people with heart and lung diseases, and to the young or the elderly. It may also cause headaches and dizziness and other significant medical problems in healthy people. At low concentrations, CO can go undetected and contribute to nagging illnesses, and can compound pre-existing health problems.

Carbon monoxide is a result of incomplete combustion due to unburned fuel. During combustion, carbon in the fuel oxidizes through a series of reactions to form carbon dioxide (CO2). However, 100 per cent conversion of carbon to CO2 is rarely achieved under field conditions and some carbon only oxidizes to the intermediate step, carbon monoxide or CO. Carbon monoxide is usually produced by insufficient combustion air. However, excess air and mismatched oil to air patterns and ratios can also reduce flame temperature to a point where CO is produced. So, adding too much air to clean up a smoky fire can create CO. When any part of the flame is reduced below 609C (1128F), CO will be produced. Flame impingement also results in lower flame temperature and CO production.

Carbon monoxide is released into homes by vent blockage, flue pipe damage, heat exchanger cracks, and restricted air supply into the house. This last problem is progressively getting worse as new homes become tighter in their construction, and many homeowners are weather stripping and insulating their older homes.

Most homes have a number of devices such as exhaust fans, clothes dryers, and fireplaces, that remove air from the home. This suction is often stronger than the suction of the heating system’s chimney or power vent. This backdrafting causes the emissions from the heating system, the water heater, gas ovens, gas stoves, gas dryers, and wood stoves or fireplaces to enter the living area and elevate CO levels.

Oilheat’s CO warning signs

If you see smoke near the burner, dark smoke coming from the chimney, or smell a sharp raw oil smell, the burner is probably producing unacceptable levels of carbon monoxide. With insufficient combustion air, oilburners usually produce elevated smoke levels before high CO levels are reached. This smoke is a warning signal.

The result is that the danger from high CO levels is much lower from oilburners than any other hydrocarbon burner. However, if oilburners are operated with too much combustion air, it chills the flame and creates CO with no smoke. Improper nozzle to air patterning can also produce CO.   <>

This material has been supplied by the Canadian Oil Heat Association. It is excerpted from Today’s Oilheat Technician’s Manual 3rd Edition, which is available at www.coha.ca.

Ambient CO limits (Recommended)

0 ppm. This level is most desirable, but cannot always be achieved due to cigarettes, candles, and appliances such as gas stoves.

1-9 ppm. Normal levels within the home.

10-35 ppm. Advise occupants, check for symptoms (slight headache, tiredness, dizziness, and nausea or flu like symptoms), check all appliances, including the furnace, water heater and boiler, check for other sources including internal combustion engine operation in attached garages.

36-99 ppm. Recommend fresh air, check for symptoms, ventilate the space, recommend medical attention.

100+ ppm. Evacuate the home (including yourself!) and contact emergency medical services (911). Do not attempt to ventilate the space. Short-term exposure to these levels can cause permanent physical damage.



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