Fuel Oil Properties and Combustion Theory

Introduction

This chapter forms the theoretical foundation of your trade as an oil heating systems technician. The Red Seal exam evaluates not only your ability to install and repair equipment, but also your in-depth understanding of the properties of the fuel you handle and the physico-chemical principles that govern its combustion. Mastering these concepts is essential for diagnosing problems, optimizing equipment efficiency, and ensuring the safety of occupants and property. This chapter covers key definitions, critical fuel oil properties, Canadian standards requirements, combustion calculations, and common pitfalls to avoid on the exam.

Physical and Chemical Properties of Fuel Oil

Definition and Classification

Fuel oil is a liquid fuel derived from crude petroleum through fractional distillation. For residential and light commercial heating applications in Canada, light fuel oil No. 1 and fuel oil No. 2 are primarily used. Fuel oil No. 2 is the most common for domestic boilers and furnaces. It is identified by the standard CAN/CGSB-3.2 (Fuel oil, light) which specifies its physico-chemical characteristics.

Property	Fuel Oil No. 1 (Kerosene)	Fuel Oil No. 2 (Domestic)
Density (kg/L at 15 °C)	0.78 – 0.82	0.84 – 0.88
Kinematic viscosity (cSt at 40 °C)	1.0 – 2.0	1.9 – 3.6
Flash point (°C)	≥ 38	≥ 38
Pour point (°C)	≤ -40	≤ -6
Higher heating value (MJ/L)	≈ 37.5	≈ 38.2
Sulfur content (% mass)	≤ 0.5	≤ 0.5

Viscosity

Viscosity is the resistance of a fluid to flow. It is critical for the atomization of fuel oil in the burner. Oil that is too viscous will not atomize properly, producing coarse droplets that burn incompletely, generate soot, and reduce efficiency. The viscosity of fuel oil No. 2 increases as temperature decreases. In very cold weather, it can become too viscous to pump properly. This is why outdoor tanks must be installed with a return line and sometimes with a line heater. The standard CSA B139 (Installation code for oil-burning equipment) requires that fuel oil be sufficiently fluid to ensure adequate atomization at the expected ambient temperature.

Flash Point and Pour Point

The flash point is the minimum temperature at which fuel oil vapors ignite in the presence of a pilot flame or spark. For fuel oil No. 2, it is at least 38 °C. This property determines the classification of the liquid as flammable (Class II) and influences storage and handling requirements. The pour point is the lowest temperature at which fuel oil will still flow. For fuel oil No. 2, it is approximately -6 °C, which explains supply problems in very cold weather in Canada's northern regions.

Heating Value

The heating value is the amount of heat released by the complete combustion of a unit mass or volume of fuel. A distinction is made between the higher heating value (HHV), which includes the latent heat of condensation of the water vapor produced, and the lower heating value (LHV), which excludes this heat. For fuel oil No. 2, the HHV is approximately 38.2 MJ/L and the LHV approximately 35.8 MJ/L. Conventional heating appliances do not recover condensation heat (combustion gases are exhausted at high temperature), so efficiency is calculated on the basis of the LHV. Condensing appliances, which are rarer with oil, can recover a portion of this latent heat.

Density and Specific Mass

The density of fuel oil No. 2 is approximately 0.85 kg/L at 15 °C. It varies slightly with temperature: fuel oil expands when heated, so its density decreases. This property is important for volume calculations in tanks and for fuel metering. The exam may ask you to convert volumes to masses or vice versa. For example, a 1000 L tank contains approximately 850 kg of fuel oil No. 2.

Combustion Theory

The Three Elements of the Fire Triangle

Combustion is an exothermic chemical reaction between a fuel and an oxidizer (oxygen from the air). For it to occur, three elements must be present simultaneously: fuel, oxidizer (oxygen), and heat (ignition source). This is the fire triangle. Removing any one of these elements extinguishes the fire. In an oil burner, combustion is controlled and continuous: the fuel is atomized, mixed with air, and ignited by a high-voltage electrode.

Chemical Combustion Reaction

The complete combustion of fuel oil (approximated by the formula C₁₂H₂₆ for fuel oil No. 2) produces carbon dioxide (CO₂) and water vapor (H₂O):

C₁₂H₂₆ + 18.5 O₂ → 12 CO₂ + 13 H₂O + heat

In practice, air contains approximately 21% oxygen and 79% nitrogen (by volume). Nitrogen does not participate in the reaction but absorbs heat and is exhausted with the combustion gases. The theoretical (stoichiometric) amount of air required to completely burn 1 kg of fuel oil No. 2 is approximately 14.4 kg of air (or about 11.9 m³ of air at 15 °C). By volume, this represents approximately 14.4 m³ of air per litre of fuel oil.

Complete vs. Incomplete Combustion

Complete combustion occurs when all the carbon in the fuel is oxidized to CO₂ and all the hydrogen to H₂O. The products are odourless (except water vapor) and the flame is blue and transparent. Incomplete combustion occurs when there is a lack of oxygen or poor air-fuel mixing. It produces carbon monoxide (CO), carbon (soot), unburned hydrocarbons, and hydrogen. The flame becomes yellow and sooty. CO is a deadly toxic gas: it binds to blood hemoglobin 200 times more readily than oxygen, causing cellular asphyxiation.

Excess Air

In practice, it is impossible to achieve a perfect mixture between fuel and air. Therefore, excess air must be supplied to ensure complete combustion. Excess air is expressed as a percentage above the stoichiometric amount. For a properly adjusted oil burner, typical excess air is 10 to 15%. Too little excess air (< 5%) produces soot and CO. Too much excess air (> 25%) cools the flame, reduces efficiency (heat is carried away in the combustion gases), and increases stack losses.

Combustion Gas Analysis

Combustion gas analysis is the technician's essential diagnostic tool. The parameters measured are:

Parameter	Optimal Value (Oil)	Significance
CO₂ (%)	10 – 12%	Indicator of combustion quality
O₂ (%)	3 – 5%	Indicator of excess air
CO (ppm)	< 100 ppm	Indicator of incomplete combustion
Gas temperature (°C)	150 – 250 °C	Indicator of stack losses
Smoke spot number (Bacharach scale)	0 – 1	Indicator of soot formation

The CO₂ content is inversely proportional to excess air: the higher the excess air, the more diluted the CO₂. A CO₂ of 12% indicates approximately 10% excess air. A CO₂ of 10% indicates approximately 30% excess air. Combustion efficiency can be estimated using the simplified formula:

Efficiency (%) ≈ 100 − (heat losses from flue gases + excess air losses)

Heat losses from flue gases are calculated from the combustion gas temperature and CO₂. A rule of thumb: each 15 °C increase in flue gas temperature above ambient temperature increases losses by approximately 1% at 10% CO₂.

The Bacharach Smoke Spot Number

The Bacharach smoke spot number is a visual measurement of the amount of soot in the combustion gases. A determined volume of gas is drawn through a filter paper, and the resulting stain is compared to a scale from 0 (white) to 9 (black). For a properly adjusted oil burner, the number should be 0 or 1. A higher number indicates incomplete combustion, improper air adjustment, a worn nozzle, or misaligned electrodes.

Combustion Equipment and Adjustments

The Oil Burner: Main Components

The typical oil burner (forced-air burner) includes:

The motor that drives the fan and pump.

The fan (blower) that supplies the combustion air.

The pump that draws oil from the tank and pressurizes it (typically 100 psi or 690 kPa).

The spray nozzle that atomizes the oil into fine droplets.

The ignition electrodes that produce the spark to ignite the mixture.

The ignition transformer (or electronic igniter) that supplies the high voltage (10,000 V).

The flame controller (photocell or cadmium cell) that detects the presence of the flame and shuts down the burner in case of flame failure.

The Spray Nozzle

The nozzle is a critical component. It is characterized by:

The flow rate in gallons per hour (GPH) at a given pressure (typically 0.75 to 1.25 GPH for residential applications).

The spray angle (typically 60°, 70°, or 80°).

The spray pattern type (solid, hollow, semi-solid).

The nozzle flow rate is proportional to the square root of the pressure. If you double the pressure, the flow rate increases by √2 ≈ 1.41. The formula is:

Q₂ = Q₁ × √(P₂ / P₁)

Where Q is the flow rate and P is the pressure. For example, a 1.00 GPH nozzle at 100 psi will produce 1.15 GPH at 132 psi (√(132/100) ≈ 1.15).

Burner Adjustment

Burner adjustment consists of adjusting:

55.The pump pressure (generally 100 psi, but check the manufacturer's specifications).

56.The combustion air flow (via the air shutter and damper position).

57.The electrode position (typical gap of 3.2 mm (1/8 in) and distance from the nozzle).

58.The nozzle alignment within the flame tube.

After each adjustment, a combustion gas analysis must be performed to verify that CO₂ is in the optimal range (10-12%), CO is below 100 ppm, and the smoke spot number is 0 or 1.

Applicable Canadian Standards and Codes

CSA B139 — Installation Code for Oil-Burning Equipment

The standard CSA B139 (Installation code for oil-burning equipment) is the primary reference for the installation, maintenance, and repair of oil-burning equipment in Canada. It covers:

Installation of storage tanks (rules 4.1 to 4.12).

Piping and connections (rules 5.1 to 5.10).

Installation of burners and appliances (rules 6.1 to 6.15).

Ventilation and venting of combustion products (rules 7.1 to 7.8).

Safety requirements (rules 8.1 to 8.6).

For example, Rule 4.3 of CSA B139 requires that outdoor tanks be installed on stable supports and that the bottom of the tank be at a minimum distance of 150 mm (6 in) from the ground. Rule 6.8 requires that the burner be installed so that it is accessible for maintenance.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1-21) applies to the electrical installations of oil-burning equipment. Rule 8-200 concerns the connections of motors and fixed appliances. Rule 26-100 deals with conductors and installation methods for control circuits. The technician must ensure that:

The burner is connected to a dedicated circuit with an appropriately sized circuit breaker or fuses.

The ground wire is properly connected.

Connections comply with the requirements of Rule 4-024 (tightening of connections).

CSA B149.1 — Natural Gas and Propane Code

Although this code primarily concerns gas, CSA B149.1 is relevant to the oil technician because it addresses the ventilation of rooms where combustion appliances are installed. Rule 8.2 of CSA B149.1 specifies the combustion air supply requirements for appliances installed in confined spaces. These requirements are similar to those of CSA B139.

CAN/CGSB-3.2 Standard

The standard CAN/CGSB-3.2 specifies the requirements for light fuel oil (No. 1 and No. 2). It defines physico-chemical properties, sampling methods, and test methods. The technician must be familiar with this standard to understand the specifications of the fuel being used.

Practical Calculations and Conversions

Unit Conversions

Unit	Equivalent
1 imperial gallon (imp gal)	4.546 L
1 US gallon (US gal)	3.785 L
1 pound (lb)	0.454 kg
1 psi	6.895 kPa
1 inch (in)	25.4 mm
1 foot (ft)	0.305 m
1 BTU	1.055 kJ
1 therm	100,000 BTU

Consumption Calculation

The hourly consumption of a burner can be calculated from the appliance's input rating and the heating value of the fuel:

Consumption (L/h) = Input (kW) / (HHV (MJ/L) × Efficiency)

For example, a boiler rated at 100,000 BTU/h (29.3 kW) with an efficiency of 80% will consume:

Consumption = 29.3 / (38.2 × 0.80) ≈ 0.96 L/h

Calculating the Volume of a Cylindrical Tank

For a horizontal cylindrical tank, the partially filled volume can be calculated using trigonometric formulas. In practice, gauges or conversion tables are used. The exam may ask you to estimate the remaining volume in a tank based on the measured fuel height. For a 1000 L (250 imp gal) tank with a diameter of 1.2 m, a fuel height of 0.3 m corresponds to approximately 25% of the total volume, or 250 L.

Calculating Combustion Air Dilution

The amount of combustion air required is approximately 14.4 m³ per litre of fuel oil. For a burner consuming 1 L/h, approximately 14.4 m³/h of combustion air is therefore required. If the appliance is installed in an enclosed room, the ventilation must supply this amount of air plus an allowance for general ventilation. CSA B139 requires a combustion air opening of at least 550 mm² per kW of appliance input (Rule 7.2).

Maintenance and Diagnostic Procedures

Burner Commissioning Procedure

92.Check the condition of the tank and piping (leaks, corrosion).

93.Purge the oil line to remove air.

94.Check the pump pressure (typically 100 psi).

95.Install a new nozzle of the appropriate size.

96.Check the electrode gap and position.

97.Set the air shutter to the manufacturer's recommended initial position.

98.Start the burner and perform a combustion gas analysis.

99.Adjust the combustion air to achieve a CO₂ of 10-12% and CO < 100 ppm.

100.Check the smoke spot number (0 or 1).

101.Measure the flue gas temperature and calculate the efficiency.

Diagnosing Common Faults

Symptom	Probable Cause	Check
Burner does not start	No electrical supply, faulty thermostat, flame controller in lockout	Check the circuit breaker, thermostat, reset button
Burner starts but shuts down after a few seconds	Photocell detects flame too late or not at all, electrodes mispositioned	Check the photocell, electrodes, nozzle
Yellow and sooty flame	Insufficient excess air, worn or clogged nozzle, pump pressure too low	Analyze the gases, replace the nozzle, check the pressure
Soot buildup in the combustion chamber	Improper air adjustment, unsuitable nozzle, insufficient draft	Analyze the gases, check the chimney
Burner knocks or vibrates	Air in the oil line, faulty pump, pressure too high	Purge the line, check the pump

Summary

Fuel oil No. 2 is the standard fuel for residential heating in Canada. It is defined by the standard CAN/CGSB-3.2.

Viscosity, flash point, and heating value are the most important properties for burner operation.

Complete combustion produces CO₂ and water. Incomplete combustion produces toxic CO and soot.

10 to 15% excess air is necessary for optimal combustion. Combustion gas analysis (CO₂, O₂, CO, temperature, smoke spot number) is the essential diagnostic tool.

Combustion efficiency depends on flue gas temperature and excess air. A CO₂ of 10-12% and a temperature of 150-250 °C indicate proper adjustment.

CSA B139 is the installation code for oil-burning equipment. The Canadian Electrical Code, Part I governs electrical installations.

Calculations for consumption, nozzle flow rate, and tank volume are essential skills.

The commissioning procedure includes checking the pump, nozzle, electrodes, and performing a combustion gas analysis.

Pitfalls to Avoid

114.Confusing imperial gallons and US gallons: in Canada, the gallon is imperial (4.546 L). Using the US gallon (3.785 L) in a consumption calculation will give a result that is wrong by approximately 20%.

115.Forgetting that nozzle flow rate is specified at a given pressure: a 1.00 GPH nozzle at 100 psi will produce a different flow rate at another pressure. Use the formula Q₂ = Q₁ × √(P₂ / P₁).

116.Neglecting the Bacharach smoke spot number: a CO₂ in the normal range does not guarantee clean combustion. The smoke spot number must be checked systematically.

117.Confusing HHV and LHV: the efficiencies of conventional appliances are calculated on the basis of the LHV. Using the HHV overestimates efficiency.

118.Ignoring the flue gas temperature: a temperature that is too high (> 300 °C) indicates excessive stack losses. A temperature that is too low (< 120 °C) can cause condensation and corrosion in the chimney.

119.Forgetting the ventilation requirements of CSA B139: an inadequately ventilated room can create a negative pressure that disrupts draft and causes incomplete combustion.

120.Not checking electrode alignment: improper electrode positioning is a common cause of hard starting and rattling.

121.Using an inappropriate nozzle size: a nozzle that is too large increases consumption and can overheat the appliance. A nozzle that is too small reduces input and can cause soot accumulation.

122.Confusing the rules of CSA B139 with those of the Electrical Code: CSA B139 deals with mechanical installation, while the Electrical Code (Part I) deals with electrical connections. Both apply simultaneously.

123.Forgetting to purge the oil line after servicing: air in the line causes erratic burner operation and can damage the pump.

124.Not accounting for outdoor temperature for storage: fuel oil No. 2 can become too viscous in very cold weather. Outdoor lines must be protected or equipped with a heater.

125.Neglecting the flame controller: a faulty flame controller can allow the burner to operate without a flame, causing an accumulation of unburned oil and an explosion risk.

Fuel Oil Properties and Combustion Theory