Gas Systems and Combustion Safety

Chapter Introduction

This chapter covers all the knowledge required for the Red Seal exam concerning gas systems and combustion safety. You will find the fundamental principles, essential calculations, Canadian regulatory requirements, and typical exam traps. Mastering this content is mandatory to pass the "Gas and Combustion" section of the exam, which typically represents 15 to 20% of the questions.

1. Physical and Chemical Properties of Combustible Gases

1.1 Natural Gas (NG)

Natural gas is primarily composed of methane (CH₄) (85 to 97%), with smaller amounts of ethane, propane, butane, nitrogen, and carbon dioxide. Its key properties:

Property	Typical Value
Relative density (air = 1)	0.60 to 0.65
Higher heating value (HHV)	37.5 to 43.0 MJ/m³
Lower heating value (LHV)	33.7 to 38.7 MJ/m³
Lower explosive limit (LEL)	4 to 5% volume in air
Upper explosive limit (UEL)	14 to 15% volume in air
Auto-ignition temperature	Approximately 540 °C
Flame speed	0.3 to 0.4 m/s

Natural gas is lighter than air — it accumulates at the ceiling in the event of a leak. This property determines the placement of detectors and the ventilation strategy.

1.2 Propane (C₃H₈)

Propane is a liquefied petroleum gas (LPG) stored under pressure. Its properties:

Property	Typical Value
Relative density (air = 1)	1.52 to 1.55
HHV	93.2 MJ/m³ (gaseous) or 25.5 MJ/L (liquid)
LEL	2.1% volume in air
UEL	9.5% volume in air
Auto-ignition temperature	450 to 500 °C
Boiling point	−42 °C at atmospheric pressure

Propane is heavier than air — it accumulates at floor level. Detectors must be placed near the floor.

1.3 Butane (C₄H₁₀)

Butane has a relative density of approximately 2.0 and a boiling point of −0.5 °C. It is less commonly used in Canada for residential heating, but you will find it in certain portable applications.

1.4 Heating Value and Conversion

The higher heating value (HHV) includes the latent heat of condensation of the water vapour produced by combustion. The lower heating value (LHV) excludes this heat. For sizing calculations, the HHV is generally used for natural gas.

Conversion formula:

LHV = HHV − (2.44 MJ/kg × mass of water produced per kg of fuel)

Calculation example:

An appliance consumes 2.5 m³/h of natural gas with an HHV of 38 MJ/m³. The input power is:

P = 2.5 × 38 = 95 MJ/h = 95 ÷ 3.6 = 26.4 kW

2. Combustion: Principles and Efficiency

2.1 The Chemical Reaction of Combustion

Complete combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

For complete combustion, you need:

1 volume of methane for 2 volumes of oxygen

Air contains 21% oxygen, so approximately 9.5 volumes of air are needed for 1 volume of methane (stoichiometric ratio)

2.2 The Three Elements of Combustion

32.Fuel (gas)

33.Oxidizer (oxygen from the air)

34.Ignition source (spark, pilot flame, hot surface)

The absence of any single element prevents combustion. This is the principle of the fire triangle.

2.3 Products of Combustion

Complete combustion produces:

Carbon dioxide (CO₂)

Water vapour (H₂O)

Nitrogen (N₂) — non-reactive, but present in the air

Incomplete combustion produces:

Carbon monoxide (CO) — deadly toxic

Soot (unburned carbon)

Aldehydes and other organic compounds

2.4 Excess Air

Excess air is the additional air beyond the stoichiometric volume required. It is expressed as a percentage:

Type of Appliance	Typical Excess Air
Atmospheric burner (residential)	40 to 60%
Forced-air burner	10 to 20%
Premix burner	5 to 15%
Condensing boiler	10 to 30%

Formula:

Excess air (%) = [(CO₂ max / CO₂ measured) − 1] × 100

Where CO₂ max is the theoretical CO₂ for perfect combustion (approximately 11.7% for natural gas, 13.8% for propane).

2.5 Flue Gas Analysis

Flue gas analysis is performed with a combustion analyzer that measures:

Parameter	Acceptable Value (Natural Gas)
O₂	3 to 6% (depending on the appliance)
CO₂	8 to 11%
CO	< 100 ppm (ideally < 25 ppm)
Flue gas temperature	Varies by appliance
Draft	0.01 to 0.05 in H₂O (2.5 to 12.5 Pa)

Flue gas temperature and efficiency:

Combustion efficiency (%) = 100 − [K × (Tflue − Tambient)]

Where K is a constant that depends on the fuel (approximately 0.37 for natural gas).

Example:

Tflue = 180 °C, Tambient = 20 °C

Efficiency = 100 − [0.37 × (180 − 20)] = 100 − 59.2 = 40.8% — this result indicates a serious problem (likely a fouled heat exchanger or massive excess air).

3. Gas Supply Systems

3.1 Gas Piping

The Natural Gas and Propane Installation Code (CSA B149.1) governs the installation of gas piping. Essential points:

Black steel pipes (schedule 40) are the standard for fixed installations

Copper is permitted for interior lines (type K or L) with pressure limits

Polyethylene (PE) is used for underground installations, never indoors

Fittings must be threaded (NPT) or welded — never soft-soldered

Key rule (CSA B149.1, Article 4.4): All piping must be supported at maximum intervals of:

1.5 m for 1/2 in and 3/4 in pipes

2.0 m for 1 in pipes and larger

3.2 Sizing of Piping

Sizing is done using the pressure drop method. The maximum allowable pressure loss is 0.5 in H₂O (125 Pa) for low-pressure systems (up to 7 in H₂O).

Sizing factors:

Equivalent length (including fittings: each 90° elbow ≈ 0.6 m equivalent)

Required flow rate (m³/h)

Gas density

Supply pressure

Simplified table for natural gas (pressure 7 in H₂O, drop 0.5 in H₂O):

Length (m)	1/2 in Pipe	3/4 in Pipe	1 in Pipe
10	2.8 m³/h	6.5 m³/h	12.5 m³/h
20	1.9 m³/h	4.5 m³/h	8.8 m³/h
30	1.5 m³/h	3.6 m³/h	7.1 m³/h

Rule of thumb: For each 90° elbow, add 0.6 m to the total length. For each tee, add 1.2 m.

3.3 Pressure Regulators

The pressure regulator reduces the gas pressure to a usable level for the appliance. Types:

Type	Inlet Pressure	Outlet Pressure
Low-pressure regulator	Up to 14 in H₂O	7 in H₂O (residential)
Medium-pressure regulator	14 in H₂O to 5 psi	7 to 14 in H₂O
High-pressure regulator	5 to 100 psi	Variable

Regulator characteristics:

Outlet pressure: set at the factory or on site

Capacity: maximum flow rate in m³/h

Set point: outlet pressure at zero flow

Droop: the difference between the set point and the pressure at maximum flow

Exam trap: The outlet pressure of a residential low-pressure regulator is 7 in H₂O (1.74 kPa), not 11 in H₂O.

3.4 Shut-off Valves

Manual shut-off valve: mandatory at each appliance, within 1.8 m of the appliance

Automatic shut-off valve: operated by thermostat or electronic control

Emergency shut-off valve: outside the building or at the main entrance

CSA B149.1 requirement: Each appliance must have an accessible manual shut-off valve installed upstream of the flexible connector.

4. Gas Appliances: Classification and Requirements

4.1 Classification by Venting Type

Category	Description	Vent Pressure
Category I	Natural draft, negative pressure	Negative
Category II	Fan-assisted, negative pressure, condensing	Negative
Category III	Fan-assisted, positive pressure	Positive
Category IV	Fan-assisted, positive pressure, condensing	Positive

4.2 Classification by Venting System Type

Type B: Chimney vented, combustion air from the room

Type C: Sealed combustion venting and combustion air (direct vent)

Type D: Mechanical venting, combustion air from the room

4.3 Burners

Atmospheric burner:

Mixes primary air through the Venturi effect

Secondary air is supplied by natural convection

Used in most residential water heaters and furnaces

Forced-air burner:

A fan supplies the combustion air

Better mixture control

Used for large boilers and industrial applications

Premix burner:

Complete air/gas mixing before the nozzle

Maximum efficiency, low NOₓ emissions

Used in modern condensing boilers

4.4 Orifices (Injectors)

The orifice diameter determines the gas flow rate. The relationship is:

Q = C × d² × √(ΔP)

Where:

Q = flow rate (m³/h)

C = discharge coefficient (manufacturer's constant)

d = orifice diameter (mm)

ΔP = differential pressure (Pa)

Rule of thumb: The flow rate is proportional to the square of the orifice diameter and the square root of the pressure.

Example:

A 2.0 mm orifice at 7 in H₂O delivers 1.5 m³/h. What diameter is needed for 3.0 m³/h at the same pressure?

Q₂/Q₁ = (d₂/d₁)²

3.0/1.5 = (d₂/2.0)²

2 = (d₂/2.0)²

d₂ = 2.0 × √2 = 2.83 mm

4.5 Gas Valves and Safety Devices

Combination valve:

Manual shut-off valve

Pressure regulator

Safety valve (opens on call for heat)

Modulating control valve

Flame detection devices:

Thermocouple: generates a millivoltage (25 to 30 mV) when heated by the pilot flame

Thermopile: several thermocouples in series, produces 750 mV to power the main valve

Flame ionization detection: measures the conductivity of the flame (used in electronic systems)

UV detection: ultraviolet sensor for industrial burners

Safety device response times:

Device	Maximum Shut-off Time
Thermocouple	90 seconds
Flame ionization detection	0.8 to 4 seconds
UV detection	0.5 to 2 seconds

5. Venting of Combustion Products

5.1 Chimneys and Vent Connectors

CSA B149.1 requirements for chimneys:

Minimum diameter: calculated based on the appliance input and chimney height

Minimum height: 0.9 m above the point of roof penetration, and 0.6 m above any obstacle within a 3 m radius

Materials: stainless steel (316L for condensing appliances), galvanized steel (prohibited for condensing appliances), concrete, masonry

Simplified sizing table (masonry chimney, natural draft appliance):

Appliance Input (kW)	Minimum Chimney Diameter
Up to 30	150 mm (6 in)
30 to 60	200 mm (8 in)
60 to 100	250 mm (10 in)

5.2 Draft

Draft is the force that removes the combustion products. It is created by the density difference between the hot gases and the outside air.

Theoretical draft (Pa) = 0.0342 × H × P × (1/Tair − 1/Tflue)

Where:

H = chimney height (m)

P = atmospheric pressure (Pa)

T = temperatures in Kelvin

Measured draft: measured with a manometer at the appliance vent connection. Typical values:

Minimum draft: 2.5 Pa (0.01 in H₂O)

Optimal draft: 5 to 12 Pa (0.02 to 0.05 in H₂O)

Excessive draft: > 25 Pa (0.10 in H₂O) — risk of flame distortion

5.3 Condensing Vent Systems

Condensing appliances (Category IV) use PVC, CPVC, or 316L stainless steel venting. The vent must be:

Airtight and watertight

Resistant to acids (pH 3 to 5)

Sloped toward the appliance (condensate return)

Requirement: Condensate must be neutralized (pH 6 to 9) before discharge to the sewer, according to local requirements.

5.4 Combustion Air

CSA B149.1, Article 8.4: Combustion air can be supplied from:

172.Inside the building (openings to adjacent rooms)

173.Outside (direct openings or ducts)

Sizing of openings (outside air):

Free area method: 1 cm² per 4.4 kW of total input (minimum 100 cm²)

Room volume method: 1 cm² per 4.4 kW for high and low openings

Rule of thumb: For a 30 kW appliance, you need at least 30 ÷ 4.4 = 6.8 cm², rounded up to the 100 cm² minimum.

6. Safety: Leak Detection and Carbon Monoxide

6.1 Gas Leak Detection

Detection methods:

182.Soap solution: apply to fittings, observe for bubbles — the most reliable method

183.Electronic detector: measures the gas concentration in the air

184.Manometer: check for pressure drop in the piping

Pressure test (CSA B149.1, Article 4.8):

New system: 500 kPa (5 bar) for 15 minutes, or 50 kPa (0.5 bar) for 30 minutes

Existing system: 35 kPa (0.35 bar) for 15 minutes

Test procedure:

189.Isolate the system with a shut-off valve

190.Pressurize with air or nitrogen

191.Observe the pressure drop for the required duration

192.If the drop exceeds 0.1 kPa, locate the leak with soap solution

6.2 Carbon Monoxide (CO)

CO is a colourless, odourless, and toxic gas. It binds to hemoglobin 200 to 300 times more strongly than oxygen.

Sources of CO:

Incomplete combustion (lack of air)

Cracked heat exchanger

Blocked venting

Improperly adjusted appliance

Exposure symptoms:

Concentration (ppm)	Effects
35	Headache after 2 to 3 hours
100	Headache, fatigue after 1 hour
200	Dizziness, nausea after 30 minutes
400	Unconsciousness after 30 minutes
1200+	Rapid death

Requirement: Every combustion appliance must be checked for CO emissions during maintenance. A CO level > 100 ppm in the combustion products indicates a problem requiring immediate correction.

6.3 Room Ventilation

Ventilation requirements (CSA B149.1):

Rooms containing gas appliances must have adequate ventilation

Ventilation openings: 1 cm² per 4.4 kW, distributed high and low

Garages: appliances elevated a minimum of 450 mm above the floor

7. Essential Calculations for the Exam

7.1 Unit Conversions

Unit	Equivalence
1 in H₂O	249 Pa
1 kPa	1000 Pa
1 psi	6.895 kPa
1 MJ	0.2778 kWh
1 m³ of natural gas	38 MJ (HHV)
1 L of liquid propane	25.5 MJ

7.2 Calculating Appliance Input

P (kW) = Q (m³/h) × HHV (MJ/m³) ÷ 3.6

Example:

A water heater consumes 1.8 m³/h of natural gas (HHV = 38 MJ/m³).

P = 1.8 × 38 ÷ 3.6 = 19 kW

7.3 Calculating Efficiency

Efficiency (%) = (Output power ÷ Input power) × 100

Example:

A furnace has an input of 30 kW and an output of 27 kW.

Efficiency = (27 ÷ 30) × 100 = 90%

7.4 Calculating Maximum CO₂

CO₂ max (%) = 100 ÷ (1 + stoichiometric excess air)

For natural gas: CO₂ max ≈ 11.7%

For propane: CO₂ max ≈ 13.8%

7.5 Calculating Combustion Air Flow Rate

Air flow rate (m³/h) = Gas flow rate (m³/h) × Air/gas ratio

Stoichiometric air/gas ratio:

Natural gas: 9.5:1

Propane: 23.8:1

Example:

An appliance consumes 2.0 m³/h of natural gas. Stoichiometric air = 2.0 × 9.5 = 19 m³/h. With 50% excess air: 19 × 1.5 = 28.5 m³/h of air required.

8. Applicable Standards and Codes

8.1 CSA B149.1 — Natural Gas and Propane Installation Code

This code is the primary reference for the installation of gas appliances in Canada. Key points:

Article 3.2: Qualification of installers

Article 4.4: Piping and connections

Article 4.8: Pressure testing

Article 5.8: Appliances — general requirements

Article 6.3: Venting of combustion products

Article 8.4: Combustion air and ventilation

8.2 CSA B149.2 — Propane Storage and Handling Code

This code covers the installation of propane tanks and associated systems.

8.3 CSA B149.3 — Code for the Field Approval of Fuel-related Components

This code covers safety requirements for solid, liquid, and gas fuel-related appliances and equipment.

8.4 Canadian Electrical Code, Part I

This code covers the electrical installations of gas appliances. Key points:

Rule 8-200: Connection of appliances — accessible disconnecting means

Rule 26-700: Heating appliances — wiring requirements

Rule 26-702: Thermostats and controls

8.5 Other Relevant Standards

CSA 2.1: Gas-fired heating appliances

CSA 4.1: Gas-fired water heaters

CSA 6.19: Flexible connectors

ULC S102: Flame spread test

9. Maintenance and Troubleshooting Procedures

9.1 Annual Inspection of a Gas Appliance

260.Visual inspection: burners, heat exchanger, venting, ventilation

261.Gas pressure test: check the inlet and outlet pressure of the regulator

262.Combustion analysis: O₂, CO₂, CO, flue gas temperature

263.Draft check: manometer at the vent connection

264.Safety test: verify the operation of the thermocouple, safety valve

265.Cleaning: burners, orifices, heat exchanger

266.Detector check: CO and smoke

9.2 Troubleshooting: Common Symptoms

Symptom	Probable Cause	Check
No flame	Valve closed, defective thermocouple	Check the valve, test the thermocouple (25 mV)
Yellow flame	Lack of primary air	Adjust the air shutter, clean the orifices
Flame blow-off	Excessive draft	Check the draft, install a draft stabilizer
High CO	Incomplete combustion	Adjust the air, check the heat exchanger
Appliance keeps shutting off	Weak thermocouple, poor draft	Test the thermocouple, check the draft

9.3 Thermocouple Pressure Test

270.Disconnect the thermocouple from the valve

271.Connect a multimeter (mV)

272.Heat the tip with the pilot flame

273.Expected reading: 25 to 30 mV (minimum 15 mV to open the valve)

Traps to Avoid

276.Confusing the densities: Natural gas is lighter than air (0.6), propane is heavier (1.5). Detectors and ventilation must be placed accordingly.

277.Forgetting the HHV/LHV conversion: The exam uses the HHV for natural gas (38 MJ/m³) and propane (93 MJ/m³). Do not use the LHV (33.7 and 85.8 MJ/m³) unless otherwise indicated.

278.Neglecting the equivalent length: When sizing piping, you must add the equivalent lengths of fittings. A system with 10 elbows may require a larger diameter pipe.

279.Confusing appliance categories: Category I = natural draft, Category IV = fan-assisted with condensation. The venting systems are not interchangeable.

280.Using PVC for a Category III appliance: PVC is reserved for Category IV (condensing) appliances. Category III appliances require stainless steel.

281.Forgetting the response time of safety devices: A thermocouple can take up to 90 seconds to close the valve. Flame ionization detection is much faster (0.8 to 4 seconds).

282.Calculating excess air without the measured CO₂: The formula Excess air = [(CO₂ max / CO₂ measured) − 1] × 100 requires the measured CO₂, not the theoretical CO₂.

283.Ignoring the 450 mm rule in garages: Gas appliances in garages must be elevated a minimum of 450 mm above the floor.

284.Confusing inlet pressure and outlet pressure: A low-pressure regulator has an outlet of 7 in H₂O, but the inlet can be at 14 in H₂O or higher.

285.Neglecting the piping pressure test: The test must be done with air or nitrogen, never with natural gas or propane.

Summary

Gas properties: Natural gas (methane) is lighter than air (density 0.6), propane is heavier (density 1.5). The HHV of natural gas is approximately 38 MJ/m³, and that of propane is 93 MJ/m³.

Combustion: Complete combustion requires an adequate air supply. Typical excess air is 40 to 60% for atmospheric burners. Maximum CO₂ is approximately 11.7% for natural gas and 13.8% for propane.

Piping: Sizing is done using the pressure drop method (0.5 in H₂O max). The equivalent length includes fittings. Black steel pipes are the standard.

Regulators: The standard outlet pressure for a residential regulator is 7 in H₂O. The regulator capacity must be checked against the maximum flow rate.

Venting: Category I appliances use natural draft, Categories II and IV are fan-assisted with condensation, and Category III is fan-assisted with positive pressure. The venting must be matched to the category.

Safety: CO is a deadly toxic gas. Appliances must be checked annually with a combustion analyzer. CO in the combustion products must be < 100 ppm.

Codes: CSA B149.1 is the primary code for gas installation. The Canadian Electrical Code, Part I, governs the electrical aspects. Compliance with both is mandatory.

Key calculations: Input (kW) = flow rate (m³/h) × HHV (MJ/m³) ÷ 3.6. Efficiency = (output power ÷ input power) × 100. Air flow rate = gas flow rate × air/gas ratio × (1 + excess air).

Self-Assessment Questions

298.What is the relative density of propane compared to air?

a) 0.6 b) 1.0 c) 1.5 d) 2.0

Answer: c) 1.5

301.An appliance consumes 3.0 m³/h of natural gas. What is its input in kW (HHV = 38 MJ/m³)?

a) 31.7 kW b) 38.0 kW c) 45.6 kW d) 114 kW

Answer: a) 31.7 kW (3.0 × 38 ÷ 3.6)

304.What is the theoretical maximum CO₂ for natural gas?

a) 9.5% b) 11.7% c) 13.8% d) 15.0%

Answer: b) 11.7%

307.What is the standard outlet pressure of a residential low-pressure regulator?

a) 3.5 in H₂O b) 7 in H₂O c) 11 in H₂O d) 14 in H₂O

Answer: b) 7 in H₂O

310.Which material is prohibited for venting condensing appliances?

a) PVC b) CPVC c) 316L stainless steel d) Galvanized steel

Answer: d) Galvanized steel

313.What is the minimum elevation of a gas appliance in a garage?

a) 150 mm b) 300 mm c) 450 mm d) 600 mm

Answer: c) 450 mm

316.What is the maximum shut-off time for a thermocouple?

a) 4 seconds b) 30 seconds c) 90 seconds d) 120 seconds

Answer: c) 90 seconds

319.A 2.5 mm orifice delivers a flow rate of 2.0 m³/h. What flow rate will a 3.5 mm orifice deliver at the same pressure?

a) 2.8 m³/h b) 3.9 m³/h c) 4.2 m³/h d) 5.0 m³/h

Answer: b) 3.9 m³/h (2.0 × (3.5/2.5)² = 2.0 × 1.96)

This chapter covers all the essential knowledge for the "Gas and Combustion" section of the Red Seal exam. Review the tables, master the calculations, and memorize the key values. Good luck with your preparation!

Gas Systems and Combustion Safety