Heating, Ventilation, and Air Conditioning (HVAC) Systems

Chapter Introduction

This chapter covers heating, ventilation, and air conditioning (HVAC) systems installed in transport trailers. As a trailer technician, you must master the diagnosis, maintenance, and repair of these systems, which ensure driver comfort and cargo preservation. The Red Seal exam requires a thorough understanding of thermodynamic principles, electrical circuits, refrigeration systems, and applicable safety standards.

Fundamental Principles of Applied Thermodynamics

Heat Transfer

Heat always moves from a zone of high temperature to a zone of lower temperature. Three modes of transfer are relevant in HVAC:

Conduction: direct transfer through a solid material. Thermal conductivity (k) is expressed in W/(m·K). Aluminum (k ≈ 205 W/(m·K)) conducts heat well; insulating foam (k ≈ 0.03 W/(m·K)) resists it.

Convection: transfer by movement of a fluid (air or liquid). Trailer heat exchangers use forced convection via fans.

Radiation: transfer by electromagnetic waves. The sun radiates through trailer walls, contributing to the thermal load.

Essential Units of Measurement

Quantity	SI Unit	Imperial Unit	Conversion
Temperature	°C	°F	°F = (°C × 9/5) + 32
Heat	Joule (J)	BTU	1 BTU ≈ 1055 J
Power	Watt (W)	BTU/h	1 BTU/h ≈ 0.293 W
Pressure	kPa	psi	1 psi ≈ 6.895 kPa
Airflow	L/s	ft³/min (CFM)	1 CFM ≈ 0.472 L/s

Thermal Load

The total thermal load (Q) of a trailer is calculated as follows:

Q = Q_transmission + Q_infiltration + Q_solar + Q_internal

Q_transmission = U × A × ΔT, where U is the heat transfer coefficient (W/(m²·K)), A is the surface area (m²), and ΔT is the temperature difference (°C or K).

Q_infiltration: heat introduced by air leaks around doors, seals, and cable penetrations.

Q_solar: heat gain from radiation through walls and roof, especially for refrigerated trailers.

Q_internal: heat generated by cargo, motors, lighting, and occupants.

Calculation Example: An insulated trailer 12 m long, 2.5 m wide, and 2.7 m high (total surface area ≈ 105 m²) with a U coefficient of 0.4 W/(m²·K) experiences a ΔT of 30 °C between inside and outside.

Q_transmission = 0.4 × 105 × 30 = 1260 W

This basic calculation allows you to size the air conditioning or heating system.

Heating Systems

Forced Air Heating (Fuel)

Transport trailers commonly use forced air heaters running on diesel. The principle is as follows:

25.Fuel is injected into a combustion chamber.

26.Combustion air is drawn in by a dedicated fan.

27.Circulation air (trailer air) passes around the combustion chamber in a heat exchanger.

28.Heated air is distributed through a network of ducts.

Key Components:

Burner: atomizes the fuel and mixes it with air.

Glow plug (or igniter): ignites the mixture.

Flame sensor: detects the presence of the flame; shuts off fuel if no flame is present.

Thermostat: regulates temperature by starting/stopping the burner.

Overheat limit switch: shuts down the system if temperature exceeds a threshold (typically 90 °C).

Diagnostic Procedure:

36.Check fuel supply (level, filters, pump).

37.Test the glow plug (typical resistance: 0.5 to 2 Ω).

38.Measure supply voltage (12 V or 24 V depending on the system).

39.Inspect the flame sensor (ionization probe or thermocouple).

40.Check air ducts for obstructions.

Electric Heating

Some trailers use electric resistance heating elements for heating. The power dissipated is calculated using Joule's law:

P = I² × R = V² / R

Example: A 1000 W element operating at 120 V AC draws a current of:

I = P / V = 1000 / 120 = 8.33 A

The resistance of the element is:

R = V² / P = 120² / 1000 = 14.4 Ω

Trap to Avoid: Measuring the resistance of a cold element gives a value lower than the hot resistance (the temperature coefficient of nichrome is positive). A cold measurement of 12 Ω for a nominal 14.4 Ω element is normal.

Heat Exchangers and Safety

The heat exchangers of fuel heaters must be inspected regularly for cracks. A crack allows carbon monoxide (CO) to contaminate the trailer air. CO is odorless and deadly.

Applicable Standard: CSA B149.1 (Natural Gas and Propane Installation Code) applies to systems using propane. Requirements include:

Adequate ventilation of the combustion chamber (Rule 5.4).

Minimum clearance between the heater and combustible materials (Rule 5.8).

Venting of combustion products to the outside (Rule 5.10).

Air Conditioning Systems

Mechanical Refrigeration Cycle

The vapor-compression refrigeration cycle has four stages:

58.Compression: The compressor draws in low-pressure refrigerant gas and compresses it to high pressure. The temperature rises significantly (80 to 100 °C).

59.Condensation: The hot high-pressure gas circulates through the condenser where it releases heat to the outside air and condenses into a liquid.

60.Expansion: The high-pressure liquid passes through the expansion valve (thermostatic or fixed orifice) where its pressure drops suddenly.

61.Evaporation: The low-pressure refrigerant evaporates in the evaporator, absorbing heat from the trailer air.

Pressure-Temperature Chart

Each refrigerant has a unique pressure-temperature relationship. The following table gives typical pressures for R-134a:

Temperature (°C)	Pressure (kPa)
-10	185
0	293
10	414
20	572
30	770
40	1017
50	1318

Practical Application: If the low-side suction pressure is 293 kPa, the evaporation temperature is 0 °C. If the temperature of the air entering the evaporator is 25 °C, the superheat is 25 °C - 0 °C = 25 °C. Normal superheat is between 5 and 10 °C.

Air Conditioning System Components

Component	Function	Common Fault	Symptom
Compressor	Compresses refrigerant	Internal leak, faulty clutch	No cooling, noise
Condenser	Dissipates heat	Blocked fins, leak	Excessive high pressure
Expansion valve	Reduces pressure	Blockage, incorrect adjustment	Low pressure too low
Evaporator	Absorbs heat	Frosting, obstruction	Low airflow, insufficient cooling
Accumulator	Protects compressor from liquid slugging	Moisture saturation	Frosting, fluctuating pressure
Filter drier	Retains moisture and particles	Saturation	High high pressure, inefficiency

Refrigerants and the Environment

R-134a (tetrafluoroethane) is the most common refrigerant in recent trailers. R-404A is used in refrigerated trailers. R-22 (chlorodifluoromethane) is being phased out due to its ozone depletion potential (ODP = 0.055).

Canadian Regulation: The Ozone-depleting Substances Regulations (ODSR) prohibit the sale of R-22 to uncertified persons. Technicians must hold certification from a recognized body (for example, nationally, certification is issued by organizations accredited by Environment and Climate Change Canada).

Best Handling Practices:

Recover refrigerant before opening any circuit (legal requirement).

Use a scale to weigh the exact charge.

Evacuate the circuit to a vacuum of 500 microns (500 µmHg) or less before recharging.

Check for leaks with an electronic detector or UV dye.

Pressure Diagnosis

Situation	Low Pressure	High Pressure	Probable Cause
Normal	200-300 kPa	1200-1800 kPa	—
Undercharge	Too low	Too low	Refrigerant leak
Overcharge	Too high	Too high	Excess refrigerant
Blocked condenser	Normal	Too high	Dirty fins, faulty fan
Blocked expansion valve	Too low	Too low	Clogged filter, moisture
Worn compressor	Too high	Too low	Faulty internal valves

Ventilation Systems

Natural and Mechanical Ventilation

Transport trailers require ventilation to:

Remove moisture (condensation, wet cargo).

Renew air for occupants.

Remove flammable vapors (regulations for dangerous goods transport).

Types of Ventilation:

Natural: roof vents, side grilles, floor openings. Airflow depends on the pressure difference between inside and outside.

Mechanical: exhaust or supply fans. Airflow is measured in L/s or CFM.

Ventilation Rate Calculation

The required airflow for a trailer is calculated using the formula:

Q = V × N

Where V is the interior volume (m³) and N is the number of air changes per hour.

Example: A trailer measuring 12 m × 2.5 m × 2.7 m has a volume of 81 m³. For 4 air changes per hour:

Q = 81 × 4 = 324 m³/h = 90 L/s

Refrigerated Trailer Ventilation

Refrigerated trailers carrying fresh produce (fruits, vegetables) require controlled ventilation to remove ethylene and CO₂ produced by product respiration. Modern systems use motorized dampers that open periodically to introduce fresh air.

Applicable Standard: CSA B149.1 also applies to ventilation systems in trailers using propane gas for heating or refrigeration.

HVAC Electrical Systems

Power Supply

Trailer HVAC systems operate on:

12 V DC: tractor vehicle circuit (lighting, controls, fans).

24 V DC: some European systems and refrigeration units.

120 V AC: shore power for electric systems.

230 V AC: some large-capacity refrigeration units.

Circuit Protection

The Canadian Electrical Code, Part I (CE Code) (CSA C22.1 standard) applies to electrical installations in vehicles and trailers. Relevant rules include:

Rule 8-200: Calculation of circuit nominal current. The maximum current must be determined based on the load.

Rule 14-100: Overcurrent protection. Each ungrounded conductor must be protected by a fuse or circuit breaker.

Rule 24-100: Requirements for vehicles and mobile equipment. Conductors must be mechanically protected.

Protection Calculation: A 500 W fan motor at 120 V AC draws:

I = P / V = 500 / 120 = 4.17 A

The circuit breaker must be sized at 125% of the nominal current (Rule 8-200):

I_protection = 4.17 × 1.25 = 5.2 A → choose a 6 A or 7.5 A circuit breaker.

Relays and Controls

HVAC systems use relays to switch large loads (motors, compressors). A typical relay:

Coil: 12 V DC, resistance of 80 to 120 Ω, current of 100 to 150 mA.

Contacts: capacity of 30 A at 12 V DC.

Relay Diagnosis:

117.Measure coil resistance (80-120 Ω).

118.Apply voltage to the coil and check contact continuity (resistance < 0.5 Ω).

119.Verify no continuity between contacts at rest.

Sensors and Thermostats

Electronic thermostats use thermistors (NTC or PTC) to measure temperature. An NTC thermistor (negative temperature coefficient) sees its resistance decrease as temperature increases.

Example: An NTC thermistor rated 10 kΩ at 25 °C has a resistance of 5 kΩ at 50 °C. The control circuit converts this resistance into a voltage signal for the controller.

Testing Procedure:

124.Disconnect the sensor.

125.Measure resistance at room temperature (compare to the manufacturer's curve).

126.Heat the sensor with a heat gun and verify the resistance change.

127.Replace if the value is out of tolerance (±5%).

Preventive Maintenance and Procedures

Periodic Inspection

Interval	Operation
Every trip	Check belts, electrical connections, visible leaks
Monthly	Clean air filters, check refrigeration system pressures
Quarterly	Inspect heat exchangers, test overheat limit switches
Annually	Recharge refrigerant if necessary, replace filter drier, complete leak test

Refrigerant Recharge Procedure

132.Recovery: Recover all remaining refrigerant into an approved cylinder.

133.Leak Test: Pressurize the circuit with dry nitrogen to 2000-3000 kPa and check with soap solution or an electronic detector.

134.Evacuation: Pull a vacuum to 500 microns or less for at least 30 minutes.

135.Recharge: Weigh the exact charge according to manufacturer specifications (indicated on the nameplate).

136.Verification: Start the system and check pressures, superheat, and subcooling.

Superheat and Subcooling

Superheat: difference between the refrigerant temperature at the evaporator outlet and the evaporation temperature. Normal: 5 to 10 °C.

Subcooling: difference between the condensation temperature and the liquid temperature at the condenser outlet. Normal: 5 to 8 °C.

Formulas:

Superheat = T_evaporator_outlet - T_saturation_low_pressure

Subcooling = T_saturation_high_pressure - T_condenser_outlet

Example: The low pressure is 293 kPa (R-134a, saturation at 0 °C). The measured temperature at the evaporator outlet is 8 °C.

Superheat = 8 - 0 = 8 °C → within the normal range.

Canadian Standards and Regulations

Canadian Electrical Code, Part I

This code (CSA C22.1 standard) governs electrical installations in vehicles and trailers. Key points for technicians:

Rule 8-200: Sizing of conductors and protections.

Rule 10-200: Grounding and bonding.

Rule 24-100: Specific requirements for vehicles and trailers.

Rule 24-102: Protection of conductors against mechanical damage.

Rule 24-104: Motor control and protection devices.

CSA B149.1 (Natural Gas and Propane Installation Code)

This standard applies to propane heating systems in trailers:

Rule 5.4: Combustion chamber ventilation.

Rule 5.8: Clearances to combustible materials.

Rule 5.10: Venting of combustion products.

Rule 5.14: Safety devices (overheat limit switch, flame detector).

Ozone-depleting Substances Regulations

This federal regulation (under the Canadian Environmental Protection Act):

Prohibits the sale of refrigerants to uncertified persons.

Requires refrigerant recovery before any repair.

Limits refrigerant emissions into the atmosphere.

Requires record-keeping of refrigerant quantities purchased and used.

Traps to Avoid

166.Confusing pressure and temperature: Refrigerant pressure depends on ambient temperature. A system at rest will have balanced pressures corresponding to ambient temperature, not operating pressures.

167.Neglecting the power factor: For AC circuits, real power is P = V × I × cos(φ). A motor with a power factor of 0.8 draws more current than the simple calculation indicates.

168.Forgetting the 125% rule: Conductors and protections must be sized at 125% of the nominal current for continuous loads (Rule 8-200 of the Canadian Electrical Code).

169.Recharging without repairing the leak: Recharging a leaking system is not only ineffective but also illegal under the ODSR. Always locate and repair the leak before recharging.

170.Using a replacement refrigerant without checking compatibility: Refrigerant mixtures (such as R-134a with R-12) can damage the compressor and are prohibited.

171.Ignoring overheat limit switches: Never bypass an overheat limit switch. It is a critical safety device that prevents fires.

172.Measuring thermistor resistance while hot: The thermistor must be at room temperature for a reliable measurement. A hot measurement gives misleading values.

173.Confusing the high and low pressure sides: The high-pressure side is between the compressor and the expansion valve. The low-pressure side is between the expansion valve and the compressor.

174.Forgetting to check ground connections: A poor ground connection can cause intermittent symptoms that are difficult to diagnose.

175.Not documenting interventions: The ODSR requires record-keeping. Complete documentation facilitates future diagnosis.

Summary

Trailer HVAC systems include heating (diesel forced air or electric), air conditioning (vapor-compression refrigeration cycle), and ventilation (natural or mechanical).

Thermal load is calculated using Q = U × A × ΔT for transmission, plus solar, infiltration, and internal gains.

The refrigeration cycle has four stages: compression, condensation, expansion, evaporation.

Refrigeration system pressures are compared to the pressure-temperature chart values for the refrigerant used.

Normal superheat is 5 to 10 °C; normal subcooling is 5 to 8 °C.

The Canadian Electrical Code, Part I (CSA C22.1) governs electrical circuits; Rule 8-200 requires protection at 125% of nominal current.

CSA B149.1 applies to propane and natural gas systems.

The Ozone-depleting Substances Regulations require technician certification and refrigerant recovery.

Diagnostic procedures include measuring pressures, temperatures, resistances, and voltages.

Regular preventive maintenance prevents costly breakdowns and ensures the safety of occupants and cargo.

Final Exam Tip: Master unit conversions (especially °C ↔ °F and kPa ↔ psi), basic formulas (Ohm's law, Joule's law, thermal load calculation), and normal superheat and subcooling values. These elements consistently appear in Red Seal exam questions.

Heating, Ventilation, and Air Conditioning (HVAC) Systems