Gas Properties, Combustion, and Venting Principles

Properties of Gases, Combustion, and Venting Principles

Chapter Introduction

This chapter provides the theoretical foundation for the practice of a Class A gasfitter. Mastering the physical and chemical properties of gases, combustion mechanisms, and venting principles is essential not only for passing the Red Seal exam but also for designing, installing, and maintaining safe systems. In Canada, all these concepts are governed by the Canadian Electrical Code, Part I (for the electrical aspects of appliances) and, most importantly, by CSA B149.1 (Natural Gas and Propane Installation Code). The rules cited in this chapter refer to the most recent version of CSA B149.1, unless otherwise indicated.

1. Physical and Chemical Properties of Gases

1.1 Natural Gas (NG)

Natural gas is a mixture of light hydrocarbons, composed primarily of methane (CH₄) at approximately 85–95%, with varying amounts of ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), nitrogen (N₂), and carbon dioxide (CO₂). It is delivered by pipeline at a typical pressure of 7 to 60 kPa (1 to 8.7 psig) for residential and commercial installations.

Key characteristics:

Relative density: 0.60 (relative to air = 1.00). Natural gas is lighter than air.

Higher heating value (HHV): approximately 37.5 MJ/m³ (1,000 BTU/ft³) at standard Canadian conditions (15 °C, 101.325 kPa).

Flammability limits: 4% to 15% by volume in air.

Auto-ignition temperature: approximately 540 °C (1,000 °F).

Odour: Natural gas is odourless in its pure state. An odorant (mercaptan) is added at a concentration of 18 to 20 mg/m³ for olfactory detection.

1.2 Propane (C₃H₈)

Propane is a liquefied petroleum gas (LPG) that is liquid under moderate pressure and vaporizes at room temperature. It is stored and transported as a liquid in pressurized tanks.

Key characteristics:

Relative density (vapour): 1.52 (heavier than air).

Relative density (liquid): 0.51 (relative to water = 1.00).

Higher heating value: approximately 93.1 MJ/m³ of vapour (2,500 BTU/ft³) or 25.3 MJ/L of liquid.

Flammability limits: 2.1% to 9.5% by volume in air.

Auto-ignition temperature: approximately 480 °C (900 °F).

Vapour pressure: at 21 °C, the pressure in a tank is approximately 830 kPa (120 psig). At −42 °C, propane boils at atmospheric pressure.

1.3 NG vs. Propane Comparison

Property	Natural Gas (CH₄)	Propane (C₃H₈)
Relative density (vapour)	0.60	1.52
Relative density (liquid)	N/A	0.51
HHV (MJ/m³)	37.5	93.1
Lower flammability limit	4%	2.1%
Upper flammability limit	15%	9.5%
Auto-ignition temperature	540 °C	480 °C
Typical supply pressure	7–60 kPa	2.7–3.5 kPa (regulated)
Behaviour in the event of a leak	Rises upward	Accumulates at floor level

Important trap: Liquid propane escaping from a tank vaporizes by absorbing heat. A liquid propane leak can cause cold burns (cryogenic burns) on the skin. Additionally, liquid propane that vaporizes produces approximately 270 volumes of vapour for every 1 volume of liquid.

1.4 The Ideal Gas Law and Corrections

For pipe sizing calculations, you must apply the ideal gas law:

P × V = n × R × T

Where:

P = absolute pressure (kPa abs)

V = volume (m³)

n = number of moles

R = gas constant (8.314 J/mol·K)

T = absolute temperature (K = °C + 273.15)

Volume correction: The volume of a gas varies with temperature and pressure. To correct a volume measured at conditions different from standard conditions (15 °C, 101.325 kPa), use:

V₂ = V₁ × (P₁/P₂) × (T₂/T₁)

Where pressures are absolute and temperatures are in Kelvin.

Calculation example: A flow meter indicates 10 m³/h of natural gas at 5 °C and 110 kPa abs. What is the flow rate at standard conditions?

V₂ = 10 × (110/101.325) × (288.15/278.15) = 10 × 1.0856 × 1.0359 = 11.25 m³/h

2. Combustion

2.1 Definition and Chemical Equations

Combustion is an exothermic chemical reaction between a fuel (the gas) and an oxidizer (oxygen from the air). For complete combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O + 890 kJ/mol

For propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + 2,220 kJ/mol

2.2 Complete vs. Incomplete Combustion

Complete combustion: All the carbon in the fuel is oxidized to CO₂ and all the hydrogen to H₂O. Efficiency is maximized and the products are harmless (with the exception of CO₂, which is a greenhouse gas).

Incomplete combustion: Occurs when oxygen is insufficient. Products include:

Carbon monoxide (CO) — deadly toxic gas

Carbon (soot)

Aldehydes and other organic compounds

Incomplete combustion equation for methane:

2CH₄ + 3O₂ → 2CO + 4H₂O

2.3 Combustion Air: Theoretical Quantity and Excess Air

Theoretical (stoichiometric) air is the exact amount of air required to completely burn the gas. For methane, the air-to-gas ratio is approximately 9.5:1 by volume. For propane, it is approximately 23.8:1.

In practice, excess air is always supplied to ensure complete combustion. Typical values:

Appliance type	Excess air (%)	Max CO₂ in flue products (%)
Atmospheric burner (residential)	40–60%	9–10%
Forced-air burner	20–40%	10–12%
Premix burner	10–20%	11–12%
Condensing boiler	10–15%	9–11%

Excess air formula:

Excess air (%) = (measured O₂ / (20.9 − measured O₂)) × 100

Where measured O₂ is the percentage of oxygen in the combustion products.

2.4 Flue Gas Analysis

Flue gas analysis is an essential diagnostic tool. The parameters measured are:

O₂: should be between 3% and 8% for most atmospheric appliances

CO₂: an indicator of combustion efficiency (the higher it is, the less excess air)

CO: should be below 100 ppm (0.01%) for properly adjusted residential appliances

Flue gas temperature: the lower it is, the higher the efficiency

Combustion efficiency:

η = 100 − (flue losses)

Flue losses are calculated as:

Losses (%) = K × (flue temperature − ambient air temperature) / CO₂

Where K is a constant that depends on the type of gas (approximately 0.38 for natural gas and 0.42 for propane).

2.5 The Flame: Structure and Characteristics

A properly adjusted atmospheric burner flame displays:

Inner cone (preheat zone): blue-green, well defined

Outer cone (reaction zone): blue, semi-transparent

Flame height: 10 to 15 mm for residential burners

Characteristics of a good flame:

Blue and stable

No yellow (except for infrared burners)

No lifting (liftoff)

No flashback

Common flame problems:

Symptom	Probable cause	Correction
Yellow flame	Insufficient primary air	Open the primary air shutter
Lifting flame (liftoff)	Excess primary air, pressure too high	Close the shutter, reduce pressure
Flashback	Mixture too rich, flame speed too high	Reduce pressure, check the orifice
Unstable flame	Air currents, fluctuating pressure	Stabilize the supply, check the regulator

2.6 Flame Speed and Stabilization

Flame speed (the rate of propagation of the flame front through the air-gas mixture) is approximately 0.3 to 0.5 m/s for methane and 0.4 to 0.6 m/s for propane. For a stable flame, the mixture velocity at the burner outlet must be slightly higher than the flame speed.

If the mixture velocity is too high → flame lift-off

If the mixture velocity is too low → flashback

Flame stabilization is ensured by flame holders or stabilization ports that create recirculation zones.

3. Ventilation and Combustion Air Supply

3.1 CSA B149.1 Requirements (Rule 8.2 and subsequent)

CSA B149.1 requires that every gas appliance be installed in a space where combustion air is sufficient. Three methods are recognized:

Method A — Air from inside: The appliance is installed in a space with sufficient volume. The required volume is calculated according to Rule 8.2.2:

Required volume (m³) = (Total heat input in kW × 3.5) / 10

Or more simply: 3.5 m³ per kW of installed capacity (for a ceiling height of 2.4 m, this is equivalent to approximately 1.5 m² of floor area per kW).

Method B — Air from outside (direct openings): Two openings are required, each with a minimum free area of 550 mm² per kW (Rule 8.4.2). One opening must be within 300 mm of the ceiling (high) and the other within 300 mm of the floor (low).

Method C — Air from outside (vertical or horizontal ducts): The duct cross-sectional area is calculated according to Rule 8.4.3. For a vertical duct, the free area is 550 mm² per kW; for a horizontal duct, it is 1,100 mm² per kW.

3.2 Calculating Opening Sizes

Example: A 30 kW water heater and a 45 kW furnace are installed in a mechanical room. Air is supplied through two direct openings to the outside.

Total input = 30 + 45 = 75 kW

Area of each opening = 75 × 550 = 41,250 mm² = 412.5 cm²

For a rectangular opening 200 mm high:

Width = 41,250 / 200 = 206 mm (round up to 210 mm)

Important: The free area accounts for grilles and screens. A reduction factor of 25% is commonly applied for standard metal grilles. Always verify the actual free area specified by the grille manufacturer.

3.3 Confined and Unconfined Spaces

A space is considered unconfined if its volume is greater than 4.8 m³ per kW of installed input (Rule 8.2.1). In this case, no ventilation openings are required.

A space is confined if its volume is less than this threshold. Ventilation openings are then mandatory.

Example: A 60 kW furnace is installed in a basement measuring 6 m × 8 m × 2.4 m.

Volume = 6 × 8 × 2.4 = 115.2 m³

Volume required for unconfined status = 60 × 4.8 = 288 m³

115.2 < 288 → The space is confined → ventilation is required.

4. Venting of Combustion Products

4.1 Types of Venting Systems

CSA B149.1 (Rule 8.10 and subsequent) distinguishes several types of venting systems:

Type	Description	Pressure in vent	Typical applications
Type A	Masonry chimney	Negative	Atmospheric appliances
Type B	Double-wall vent (insulated)	Negative	Residential atmospheric appliances
Type C	Single-wall vent	Negative	Connector between appliance and chimney
Type D	Positive-pressure vent	Positive	Direct-vent, condensing appliances
Type E	Special vent (certified)	Variable	Per certification

4.2 Vent Sizing Rules

Vent sizing must follow the tables in CSA B149.1 (Annex B) or the detailed calculation method. The fundamental principles:

121.The vent area must be sufficient to carry the flue gas flow without spillage.

122.Vent height influences draft: the taller the vent, the stronger the draft.

123.The minimum diameter for a residential atmospheric appliance is 100 mm (4 in).

124.The connector between the appliance and the chimney must have an upward slope of at least 6% (Rule 8.10.3.2).

Practical rule for draft:

Draft (Pa) = 0.7 × H × (ρ_air − ρ_flue) × g

Where:

H = vent height (m)

ρ_air = density of ambient air (kg/m³)

ρ_flue = density of flue gas (kg/m³)

g = 9.81 m/s²

4.3 Connecting Multiple Appliances

When multiple appliances are connected to a single chimney (Rule 8.10.4), the following requirements apply:

Each appliance must have its own individual connector before joining the common vent.

Connectors must be offset vertically by at least 300 mm.

The common vent cross-sectional area must be sized for the total input of all appliances.

Power-vented appliances (direct vent) must never be connected to a common natural-draft chimney with atmospheric appliances.

4.4 Direct Venting (Sealed Systems)

Direct-vent appliances (Type D) draw combustion air from outside and exhaust combustion products through a concentric or separate vent. Specific rules (CSA B149.1, Rule 8.11) include:

The termination must be at least 300 mm above the expected snow level (minimum 300 mm above grade).

The minimum distance between the termination and any building opening is 300 mm for concentric vents and 1 m for separate vents.

Terminations must not be installed under a balcony, porch, or overhang.

The distance between two direct-vent terminations on the same building must be at least 1 m if they are on the same plane.

4.5 Condensation in Vents

Condensation of combustion products occurs when the flue gas temperature drops below the dew point (approximately 55–60 °C for natural gas). The consequences:

Corrosion of unprotected metal vents

Acidic water droplets (pH 3–4) that can damage materials

Partial blockage of the vent from deposits

Solutions:

Use vents certified for condensation (stainless steel, special plastic)

Insulate vents in unheated spaces

Install a condensate drain with a trap

4.6 Condensing Appliances

Condensing boilers and water heaters recover the latent heat from the water vapour in the flue gas. Their efficiency can reach 95–98% (HHV) compared to 80–85% for conventional appliances.

Specific requirements (CSA B149.1, Rule 8.12):

Venting in corrosion-resistant material (PVC, CPVC, polypropylene, stainless steel)

Vent sloped toward the appliance for condensate drainage

Condensate neutralization if the pH is below 5.5 (per local requirements)

Termination at least 300 mm above the expected snow level

5. Practical Calculations for the Exam

5.1 Calculating Heat Input

Heat input (kW) = Volume flow rate (m³/h) × HHV (MJ/m³) / 3.6

Example: A meter indicates 4.5 m³/h of natural gas (HHV = 37.5 MJ/m³).

Heat input = 4.5 × 37.5 / 3.6 = 46.9 kW

5.2 Converting Between Natural Gas and Propane

To convert an appliance from natural gas to propane (or vice versa), the orifices must be replaced because the heating value and density differ.

Orifice sizing formula:

D₂ = D₁ × √(HHV₁ × ρ₂ / (HHV₂ × ρ₁))

Where D = orifice diameter, HHV = higher heating value, ρ = relative density.

Example: A 3.5 mm natural gas orifice must be replaced for propane.

D_propane = 3.5 × √(37.5 × 1.52 / (93.1 × 0.60)) = 3.5 × √(57 / 55.86) = 3.5 × 1.010 = 3.54 mm

In practice, propane orifices are generally smaller in diameter than natural gas orifices for the same input, because propane has a higher volumetric heating value.

5.3 Gas Pressure Calculations

Pressure drop in piping depends on flow rate, diameter, equivalent length, and gas density. The tables in CSA B149.1 (Annex A) provide maximum flow rates for different lengths and diameters.

General rule: The maximum allowable pressure drop is:

0.5 kPa (2 in water column) for residential low-pressure installations (7 kPa)

10% of the service pressure for higher-pressure installations

5.4 Equivalent Length

Equivalent length accounts for pressure losses in fittings and accessories. Typical values in equivalent lengths of straight pipe:

Fitting	Equivalent length (m)
90° elbow (standard)	0.3 to 0.6 m
90° elbow (long radius)	0.2 to 0.4 m
45° elbow	0.15 to 0.3 m
Tee (straight through)	0.3 to 0.5 m
Tee (branch)	1.0 to 1.5 m
Shut-off valve	0.3 to 0.6 m
Regulator	0.5 to 1.0 m

Total equivalent length = Actual length + Sum of equivalent lengths of fittings

6. Safety and Leak Detection

6.1 Gas Leak Detection

Leak detection methods according to CSA B149.1 (Rule 5.8):

187.Soap solution: Application of a soapy water solution to joints and fittings. The formation of bubbles indicates a leak.

188.Electronic detector: A device calibrated to detect hydrocarbons.

189.Manometer: Pressure leak test (pressure test).

Leak test: Piping must be subjected to a test pressure of 1.5 times the service pressure or 350 kPa (50 psig), whichever is greater, for at least 15 minutes without pressure drop (Rule 5.8.2).

6.2 Purging of Piping

Purging of new or repaired piping must be done with gas, never with compressed air (risk of explosive mixture). Purging must be done to the outside of the building, to a location where the gas can safely disperse.

Practical rule: Purging is complete when the odour of gas is detectable at the outlet, or when the oxygen concentration is below 1% (measured with an analyzer).

6.3 Carbon Monoxide Detectors

CO is an odourless, colourless, and toxic gas. Installation requirements for CO detectors are generally defined by provincial building codes and CSA standards (CSA 6.19). For the exam, remember:

CO is produced during incomplete combustion

The occupational exposure limit is 25 ppm (8-hour average)

Symptoms of poisoning appear at 100 ppm (headaches, nausea)

Above 400 ppm, CO is lethal in less than one hour

7. Traps to Avoid

202.Confusing relative density and absolute density: Relative density is unitless (ratio to air or water). Natural gas has a relative density of 0.60 (lighter than air), propane 1.52 (heavier).

203.Forgetting to convert pressures to absolute pressure: In ideal gas law calculations, always use absolute pressure (gauge pressure + 101.325 kPa).

204.Neglecting the free area of grilles: Ventilation grilles reduce the effective area. A calculation based on gross area is a common error.

205.Confusing flammability limits: NG: 4–15%, Propane: 2.1–9.5%. Propane has a narrower range but a lower lower limit.

206.Forgetting the 6% slope on vent connectors: This upward slope is mandatory for atmospheric appliances.

207.Connecting a direct-vent appliance to a common natural-draft chimney: This is a serious violation of CSA B149.1.

208.Using HHV instead of LHV: Efficiency calculations in Canada use HHV (unlike Europe where LHV is used). HHV includes the latent heat of condensation of water vapour.

209.Ignoring water temperature in condensation calculations: The dew point of flue gas is approximately 55–60 °C for natural gas. Below this temperature, condensation occurs.

210.Confusing ventilation requirements for confined spaces: 4.8 m³/kW for unconfined status, 3.5 m³/kW for opening calculations (550 mm²/kW per opening).

211.Forgetting the diversity factor: When sizing piping for multiple appliances, a diversity factor may be applied, but never for appliances that operate continuously (water heaters, furnaces).

8. Summary

Natural gas (methane) is lighter than air (density 0.60), with an HHV of approximately 37.5 MJ/m³ and flammability limits of 4 to 15%.

Propane is heavier than air (density 1.52), with an HHV of approximately 93.1 MJ/m³ and limits of 2.1 to 9.5%. It is stored as a liquid under pressure.

Complete combustion produces CO₂ and water; incomplete combustion produces toxic CO and soot.

Excess air is necessary for complete combustion: 40–60% for atmospheric burners.

Flue gas analysis measures O₂, CO₂, CO, and temperature to assess efficiency and safety.

Ventilation of confined spaces requires openings calculated at 550 mm²/kW (Method B) with two openings (high and low).

Venting of combustion products must comply with vent types (A, B, C, D, E) and the sizing rules of CSA B149.1.

Condensing appliances require corrosion-resistant vents and a condensate drain.

Sizing calculations use the ideal gas law, equivalent lengths, and the flow tables in CSA B149.1.

Safety relies on leak detection (soap solution, pressure test at 350 kPa for 15 minutes) and CO prevention.

9. Review Questions

226.What is the relative density of propane compared to air? What does this imply for leak detection?

227.Calculate the ventilation opening size required for an 85 kW boiler supplied by two direct openings to the outside.

228.A natural gas appliance produces 80 ppm of CO in its flue gas. Is this acceptable? Justify your answer.

229.What is the difference between a confined and unconfined space according to CSA B149.1?

230.Why can a condensing appliance not be connected to a standard Type B chimney?

231.A 500 L propane tank is filled to 80% of its capacity. How much propane vapour does this represent at 15 °C?

232.What is the heat input of an appliance that consumes 3.2 m³/h of natural gas (HHV = 37.5 MJ/m³)?

233.What are the three leak detection methods recognized by CSA B149.1?

234.A 6 m tall vent exhausts flue gas at 180 °C into ambient air at 20 °C. Is the draft sufficient?

235.What are the minimum termination distances for a concentric direct vent relative to building openings?

This chapter covers the essential concepts for the "Properties of Gases, Combustion, and Venting Principles" section of the Red Seal exam. For complete preparation, also consult CSA B149.1 (full text), the sizing tables in Annex A, and the official Red Seal study guides.