Gas Properties, Combustion, and Venting Principles

Gas Properties, Combustion, and Venting Principles

Introduction

This chapter forms the theoretical foundation of the gasfitter trade. Before you touch a tool, you must understand what you are working with: gas, its nature, its behaviour, and the chemistry of its combustion. The Red Seal exam assesses not only your ability to install pipes, but also your understanding of the physical and chemical principles that ensure the safety of installations. A gasfitter who does not understand combustion is a public hazard. This chapter covers the physical properties of gases, combustion requirements, appliance characteristics, and venting system design principles according to the Natural Gas and Propane Installation Code (CSA B149.1) .

1. Physical Properties of Gases

1.1 States of Matter and Definition of a Gas

A gas is a state of matter where molecules are in constant, free motion, occupying the entire volume of their container. Unlike liquids and solids, gases have neither fixed shape nor fixed volume. This fundamental property explains why a gas leak spreads rapidly throughout an entire building.

Key terms to master:

1.2 Fundamental Gas Laws

Three laws govern the behaviour of gases. You must know them by heart and be able to apply them in practical situations.

Boyle's Law (pressure-volume relationship): At constant temperature, the volume of a gas is inversely proportional to its absolute pressure. Formula: P₁ × V₁ = P₂ × V₂.

Practical application: If you compress a gas volume of 2 m³ at a pressure of 100 kPa into a tank where the pressure becomes 200 kPa, the volume will be reduced to 1 m³. This law explains why propane tanks can contain a large quantity of liquefied gas under pressure.

Charles's Law (volume-temperature relationship): At constant pressure, the volume of a gas is directly proportional to its absolute temperature. Formula: V₁/T₁ = V₂/T₂ (temperatures in Kelvin).

Practical application: An outdoor propane tank exposed to sunlight will see its internal pressure increase. This is why tanks must never be filled beyond 80% of their capacity — the thermal expansion of the liquid could cause dangerous overpressure.

Gay-Lussac's Law (pressure-temperature relationship): At constant volume, the pressure of a gas is directly proportional to its absolute temperature. Formula: P₁/T₁ = P₂/T₂.

1.3 Density and Relative Density

The relative density (or specific gravity) of a gas is the ratio between the mass of a given volume of that gas and the mass of the same volume of air, under the same conditions of temperature and pressure. Air has a relative density of 1.0.

Safety implication: Natural gas rises; natural gas detectors must therefore be installed near the ceiling. Propane sinks; propane detectors must be installed near the floor. A propane leak in a basement can create an explosive accumulation without being detected by smell if ventilation is insufficient.

1.4 Heating Value

The heating value (or calorific value) is the amount of heat released by the complete combustion of a given quantity of gas. It is expressed in megajoules per cubic metre (MJ/m³) for natural gas and in megajoules per litre (MJ/L) for liquid propane.

Higher Heating Value (HHV): Total heat released, including the latent heat of condensation of the water vapour produced.

Lower Heating Value (LHV): Heat released without recovering the heat of condensation.

Trap to avoid: Vent sizing calculations use the HHV, while efficiency calculations for condensing appliances use the LHV. Never mix the two.

1.5 Flammability Limits

The flammability limits (or explosibility limits) define the range of concentration of a gas in air within which combustion can occur.

Interpretation: Below the LFL, the mixture is too lean to burn (not enough fuel). Above the UFL, the mixture is too rich (not enough oxygen). The zone between the two is the explosive zone.

Practical application: A propane concentration of 2.1% in the air of a basement is sufficient to cause an explosion. One litre of vaporized liquid propane produces approximately 270 litres of gas — a very small quantity of liquid can create an explosive atmosphere in a large room.

1.6 Ignition Temperature

The ignition temperature (or auto-ignition point) is the minimum temperature at which a gas ignites spontaneously without an external ignition source.

Implication: Any hot surface (such as a defective electric motor or an overheated electrical wire) can ignite a gas-air mixture. This is why gas installations require proper clearances and adequate ventilation.

2. Combustion

2.1 The Chemical Combustion Reaction

Combustion is an exothermic chemical reaction between a fuel (hydrocarbon) and an oxidant (oxygen from the air). For methane (the main component of natural gas):

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

For propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + heat

Complete combustion: All carbon is converted to carbon dioxide (CO₂) and all hydrogen to water (H₂O). This is the ideal combustion, producing maximum heat and no dangerous pollutants.

Incomplete combustion: There is a lack of oxygen. The carbon is partially converted to carbon monoxide (CO), a toxic and odourless gas. The flame becomes yellow and produces soot.

2.2 The Three Elements of Combustion

Combustion requires three simultaneous elements: fuel, oxygen (air), and heat (ignition source). This is the fire triangle. Removing any single element extinguishes the fire.

Combustion air: Atmospheric air contains approximately 21% oxygen and 79% nitrogen. Nitrogen does not participate in combustion but absorbs heat and is vented with the combustion products.

2.3 Theoretical Air and Excess Air

Theoretical air (stoichiometric): The exact amount of air required to completely burn a given volume of gas.

Excess air: In practice, more air is supplied than the theoretical air to ensure complete combustion. The excess air varies according to the type of burner:

Trap to avoid: Too much excess air cools the flame and reduces efficiency. Not enough excess air produces CO. Balance is essential.

2.4 Combustion Products

The combustion products of natural gas and propane are:

2.5 Combustion Analysis

Analysis of combustion products is an essential diagnostic tool. Instruments measure:

Combustion ratio: The CO₂/CO ratio is an indicator of quality. A ratio greater than 1000:1 indicates good combustion.

2.6 Combustion Efficiency

Combustion efficiency is the ratio between the useful heat produced and the total heat released by the fuel.

Gross thermal efficiency: Does not account for the latent heat of condensation. Typically 75 to 85% for a conventional appliance.

Net thermal efficiency: Accounts for the recovery of condensation heat. Typically 90 to 97% for a condensing appliance.

Stack loss: The heat lost in the vented combustion products. It depends on the flue gas temperature and the excess air.

Simplified formula: Stack loss (%) = K × (T_flue − T_ambient) / CO₂

Where K is a constant (approximately 0.36 for natural gas and 0.38 for propane).

3. Burner and Appliance Characteristics

3.1 Atmospheric Burners

The atmospheric burner is the most common type in residential appliances. Primary air is drawn in by the Venturi effect of the gas jet. Secondary air is supplied by natural convection around the flame.

Characteristics:

Components of an atmospheric burner:

3.2 Forced Draft Burners

Forced draft burners use a fan to supply combustion air under pressure. They allow better control of the air-gas ratio and more efficient combustion.

Advantages:

3.3 Premix Burners

Premix burners mix the gas and air before injection into the combustion chamber. They offer the best combustion control and the lowest NOₓ emissions.

3.4 Flame and Its Structure

A properly adjusted gas flame has three zones:

Flame colour:

Flashback: The flame travels back into the Venturi and burns at the injector. Cause: mixture too lean, gas pressure too low, or damaged injector.

Lift-off: The flame detaches from the burner head. Cause: gas pressure too high, or excess primary air.

3.5 Ignition and Flame Detection

Ignition systems:

Flame detection:

4. Combustion Product Venting Principles

4.1 Natural Draft

Natural draft is the upward movement of combustion products in a chimney, caused by the density difference between hot gases (less dense) and cold outside air (more dense).

Draft formula: Draft (Pa) = H × (ρ_ext − ρ_int) × g

Where:

Factors affecting draft:

4.2 Types of Venting Systems

Category I (natural draft): Appliances with negative pressure in the vent. Flue gases are vented by natural draft. Example: atmospheric water heater.

Category II (forced draft, negative pressure): Appliances with an exhaust fan downstream of the heat exchanger. Negative pressure in the vent. Example: condensing boiler.

Category III (forced draft, positive pressure): Appliances with a fan upstream of the heat exchanger. Positive pressure in the vent. Example: condensing water heater.

Category IV (positive pressure, condensing): Condensing appliances with a fan. Positive pressure and condensation in the vent. Example: high-efficiency condensing furnace.

4.3 Venting Systems — Materials and Requirements

Type B vent (double wall): For natural draft appliances. The inner wall in stainless steel or aluminium resists corrosion. The air space between the walls insulates and keeps the flue gases hot.

Type C vent: Triple wall, used for indoor installations where the vent passes through habitable spaces.

PVC/CPVC vent: For condensing appliances (Categories II and IV). Resists acidic condensate corrosion. Maximum temperature limited (approximately 60 °C for PVC, 90 °C for CPVC).

Stainless steel vent: For high temperatures and corrosive environments. Used for high-efficiency appliances.

Requirements according to CSA B149.1:

4.4 Vent Sizing

Vent sizing is critical. A vent that is too small cannot vent the combustion products. A vent that is too large cools the flue gases, causes condensation, and reduces draft.

Sizing factors:

Rule of thumb: The vent diameter must be at least equal to the diameter of the appliance vent outlet. For natural draft appliances, the vent must be sized according to the tables in CSA B149.1 (Annex B).

Trap to avoid: Never reduce the diameter of a vent. Never connect two appliances to a vent sized for only one.

4.5 Connecting Multiple Appliances

CSA B149.1 (Rule 8-210) allows connecting multiple appliances to a common vent under certain conditions:

4.6 Condensation in Vents

Condensation occurs when the flue gas temperature drops below the dew point (approximately 55 °C for natural gas, 50 °C for propane). The consequences:

Prevention:

4.7 Combustion Air Ventilation

Combustion air must be supplied in sufficient quantity. CSA B149.1 (Rules 8-300 to 8-310) requires:

Trap to avoid: Ventilation openings must be calculated based on the actual free area, not the gross area. A ventilation grille with louvers reduces the free area by 40 to 60%.

5. Safety and Regulations

5.1 Leak Detection

Detection methods:

Strict prohibition: Never use an open flame to detect a gas leak. This is a dangerous practice and is prohibited by the code.

5.2 Purging of Piping

Purging consists of evacuating the air from a new or repaired pipe before putting it into service. Air in a gas pipe creates a potentially explosive mixture.

Purging procedure:

Trap to avoid: Never purge a pipe inside a building. The vented gas must be directed outdoors.

5.3 Pressure Testing

CSA B149.1 (Rule 5-200) requires a pressure test of all piping before it is put into service:

Procedure: Pressurize the piping with air or an inert gas, close the source, and observe the gauge. Any pressure drop indicates a leak. Locate and repair the leak, then repeat the test.

5.4 Clearances and Safety Distances

CSA B149.1 specifies minimum clearances between appliances and combustible materials:

6. Traps to Avoid

7. Summary

8. Exam Tips

This chapter gives you the theoretical foundations. The next chapter will cover the components of gas distribution systems, installation techniques, and commissioning procedures.