Refrigerators and Freezers

Chapter Introduction

This chapter covers the full range of theoretical and practical knowledge required for the Red Seal exam concerning domestic and light commercial refrigerators and freezers. You must master refrigeration cycles, components, diagnostics, repairs, and applicable safety standards. This chapter is structured to follow the logical progression of system analysis: physical principles, components, cycles, then service procedures.

Fundamental Thermodynamic Principles

Heat Transfer

A refrigerator does not "create" cold; it moves heat from an enclosed space to the outside. The three modes of heat transfer are:

Conduction: transfer through a solid (walls, coils). Thermal conductivity (k) is expressed in W/(m·K). Polyurethane insulation has a k of approximately 0.022 W/(m·K).

Convection: transfer through fluid movement (air in the cabinet, refrigerant in the tubes).

Radiation: electromagnetic transfer (losses through walls, condensation on surfaces).

The heat load of an enclosure is calculated as the sum of the heat gains: conduction through the walls (Q = U × A × ΔT), air infiltration when doors are opened, sensible and latent heat from food items placed inside, heat generated by electric defrost, and heat dissipated by the compressor and fan.

Pressure and Temperature

The pressure-temperature relationship of the refrigerant is the foundation of diagnostics. Each refrigerant has a unique saturation curve. For R-134a, at 0 °C, the absolute pressure is approximately 293 kPa (2.93 bar). For R-600a (isobutane), at 0 °C, it is approximately 103 kPa. This difference explains why R-600a systems use smaller-displacement compressors and longer capillary tubes.

Absolute pressure = gauge pressure + atmospheric pressure (101.3 kPa at sea level). All superheat and subcooling calculations use gauge pressures converted to saturation temperatures via the refrigerant chart.

Superheat and Subcooling

Superheat: the difference between the actual vapour temperature at the evaporator outlet and the saturation temperature corresponding to the suction pressure. Typical value: 5 to 8 °C for direct expansion systems.

Subcooling: the difference between the saturation temperature of the liquid at the condensing pressure and the actual liquid temperature at the condenser outlet. Typical value: 5 to 10 °C.

Superheat formula: ΔT_superheat = T_actual_vapour − T_saturation(P_suction)

System Components

Compressor

The compressor is the heart of the system. Types encountered:

Type	Application	Characteristics
Hermetic	Domestic refrigerators	Motor and compressor in a single welded shell. Not serviceable on site.
Semi-hermetic	Light commercial	Bolted, accessible for valve or gasket replacement.
Rotary	Small systems	Quiet, efficient, sensitive to contaminants.
Reciprocating (piston)	Domestic and commercial	Robust, high starting torque.

The start relay (potential or current type) and the thermal overload protector (klixon) are critical components. The potential relay uses the counter-electromotive force (CEMF) of the start winding to open the contact when the motor reaches approximately 75% of its rated speed. The thermal overload protector opens the circuit if the winding temperature exceeds approximately 120 °C.

Condenser

The condenser dissipates heat. Types:

Static (plate or wire-and-tube): cooled by natural convection. Requires a minimum ventilation clearance of 50 mm at the back and sides.

Dynamic (fan-cooled): an axial or tangential fan forces air through the coil. The condenser fan and evaporator fan are often coupled on the same motor in small appliances.

The condenser must be kept clean. A 1 mm accumulation of dust can reduce efficiency by 15 to 20%.

Evaporator

The evaporator absorbs heat from the cabinet. It operates at a temperature 5 to 8 °C below the target cabinet temperature. For a refrigerator at 4 °C, the evaporator operates at approximately −4 °C. For a freezer at −18 °C, the evaporator operates at approximately −26 °C.

The heat exchange surface and fin spacing determine capacity. Plate evaporators (roll-bond) are common in small appliances. Finned-tube evaporators are used in frost-free freezers with automatic defrost.

Capillary Tube and Expansion Valve

The capillary tube is the most common expansion device in domestic appliances. It is a copper tube with an internal diameter of 0.6 to 2.0 mm and a length of 1.5 to 4.5 m. It maintains the pressure difference between the condenser and evaporator through friction. It is often soldered in a heat exchange relationship with the suction line to improve subcooling.

The thermostatic expansion valve (TXV) is used on light commercial appliances. It maintains a constant superheat. The sensing bulb is attached to the evaporator outlet. The bulb pressure (P_bulb) opposes the sum of the evaporation pressure (P_evap) and the spring force (P_spring). Balance: P_bulb = P_evap + P_spring.

Accumulator and Filter-Drier

The accumulator (liquid reservoir) is placed on the suction line to prevent liquid from reaching the compressor. The filter-drier is placed on the liquid line. It contains a molecular sieve (zeolite) that adsorbs residual moisture (maximum 50 ppm after installation). A saturated filter-drier causes an increase in pressure differential and localized frosting.

The Complete Refrigeration Cycle

The Four Stages

42.Compression: the compressor draws in low-pressure vapour (approximately 100 to 200 kPa for R-134a) and compresses it to high pressure (800 to 1,200 kPa). The discharge temperature reaches 60 to 90 °C.

43.Condensation: the superheated vapour releases its heat to the condenser and liquefies. Subcooling occurs in the lower portion of the condenser.

44.Expansion: the subcooled liquid passes through the capillary tube. The pressure drops suddenly, a portion of the liquid vaporizes (flash gas), and the temperature drops to the evaporation temperature.

45.Evaporation: the liquid boils, absorbing heat from the cabinet. The vapour returns to the compressor.

Defrost Cycle

Frost-free appliances use a periodic defrost cycle. The defrost timer (electromechanical or electronic) interrupts the cooling cycle every 6 to 12 hours for a duration of 15 to 30 minutes. During defrost:

The compressor stops.

The defrost heater (200 to 600 W) heats the evaporator.

The defrost thermostat (bimetal) cuts power to the heater when the evaporator temperature reaches approximately 10 to 15 °C.

The safety klixon (thermal fuse) cuts the circuit if the temperature exceeds 70 °C.

Defrost water flows through a drain to the collection pan located above the compressor, where it evaporates using the compressor's heat.

Refrigerants and Oils

Common Refrigerants

Refrigerant	Application	GWP	Pressure at 25 °C (kPa)	Remarks
R-134a	Domestic and commercial	1,430	665	Banned in new domestic appliances in Canada since 2020
R-600a (isobutane)	Domestic (new)	3	350	Flammable (A3), max charge 150 g
R-290 (propane)	Light commercial	3	950	Flammable (A3)
R-1234yf	New systems	4	680	Mildly flammable (A2L)
R-12	Older appliances	10,900	650	Banned, replaced by R-134a

R-600a is now the standard for domestic refrigerators sold in Canada. It is flammable: any service work requires specialized tools (non-sparking wrenches, forced ventilation, leak detector calibrated for hydrocarbons). The maximum charge is 150 g for domestic appliances per CSA C22.2 No. 60335-2-24.

Lubricating Oils

Mineral oil: for R-12 and R-22 (older systems).

POE (polyolester) oil: for R-134a and R-1234yf. Hygroscopic: absorbs moisture from the air. An opened container must be resealed immediately.

PAG (polyalkylene glycol) oil: for automotive systems, rarely used in appliances.

AB (alkylbenzene) oil: for R-600a and R-290, compatible with hydrocarbons.

POE oil must never be mixed with mineral oil. A complete system flush is required when changing refrigerants.

Electrical Systems

Power and Control Circuits

The power circuit includes: the power cord, thermostat, start relay, thermal overload protector, compressor, fans, and defrost heaters. The control circuit (on electronic models) includes: the electronic control board, temperature sensors (NTC thermistors), door switch, and control panel.

Thermostats

Bulb thermostat (mechanical): the capillary bulb is attached to the evaporator. The bellows actuates an electrical contact. Typical differential: 3 to 5 °C.

Electronic thermostat: an NTC thermistor (negative temperature coefficient) measures the temperature. The control board compares the value to the setpoint and commands the compressor. Thermistors have a resistance of approximately 10 kΩ at 25 °C and 30 kΩ at 0 °C.

Electrical Measurements

Winding resistance: measured between compressor terminals. For a single-phase compressor: R_common-start ≈ 2 to 4 Ω, R_common-run ≈ 1 to 2 Ω. The start winding resistance is always higher than the run winding.

Starting current: 5 to 8 times the rated current for 0.1 to 0.5 seconds.

Running current: 0.8 to 1.5 A for a domestic refrigerator.

The start capacitor (if present) is 10 to 100 µF. The run capacitor is 2 to 5 µF. A defective capacitor causes hard starting or compressor hum.

Diagnostics and Troubleshooting

Symptoms and Probable Causes

Symptom	Possible Causes	Verification
Compressor does not start	Defective relay, open overload protector, thermostat, wiring	Test relay continuity, measure winding resistance
Compressor hums	Failed start capacitor, open start winding, low voltage	Measure capacitor, check voltage (120 V ± 10%)
Insufficient cooling	Low refrigerant charge, dirty condenser, failed fan, door gasket	Check pressures, clean condenser, test gasket
Excessive frost	Defective thermostat, door not sealing, failed defrost heater	Verify defrost cycle, measure heater resistance
Water leak	Clogged defrost drain, cracked collection pan	Clean drain, inspect pan
Abnormal noise	Worn compressor, fan rubbing, vibrating capillary tube	Locate source, replace part

Systematic Diagnostic Procedure

81.Visual inspection: condition of the cord, receptacle, condenser, and door gaskets.

82.Electrical checks: supply voltage, component continuity, winding resistances.

83.Thermal checks: cabinet temperature, evaporator temperature, condenser temperature (should be warm, 40 to 50 °C).

84.Pressure checks: using a manifold, only if the system is accessible (service ports). For hermetic systems without ports, diagnosis is done through temperatures and currents.

85.Defrost check: advance the timer, verify the heater, defrost thermostat, and safety klixon.

Leak Testing

The detection method depends on the refrigerant:

Electronic leak detector: for all refrigerants, minimum sensitivity of 5 g/year.

Soap solution: for accessible fittings, effective for significant leaks.

UV lamp: with injected dye, for locating slow leaks.

Dry nitrogen: for pressure testing (maximum 2,500 kPa for R-134a systems, 1,500 kPa for R-600a).

Evacuation must reach 500 microns (0.5 Torr) or less, maintained for 30 minutes. Insufficient vacuum leaves moisture that forms acids and sludge in the system.

Applicable Codes and Standards

Canadian Electrical Code

The Canadian Electrical Code, Part I (CE Code) (C22.1) applies to the electrical installation of appliances. Key points:

Rule 2-024: appliances must be installed in accordance with the manufacturer's instructions.

Rule 26-700: household appliances must be connected to a dedicated 15 A or 20 A circuit, with grounding.

Rule 8-200: calculation of minimum loads for branch circuits. For residences, a 15 A circuit can supply multiple receptacles, but a refrigerator should ideally be on a dedicated circuit.

Rule 26-720: receptacles in kitchens must be protected by a ground fault circuit interrupter (GFCI) if they are within 1.5 m of a sink.

CSA Standards for Appliances

CSA C22.2 No. 60335-2-24: safety of refrigerators, freezers, and ice makers. Requirements for R-600a systems: ventilated compressor compartment, safety switch, "flammable" marking.

CSA C22.2 No. 0: general requirements for electrical appliances.

CSA B52: standard for mechanical refrigeration systems (commercial installations). Requirements for ventilation, clearances, and overpressure protection.

Halocarbon Regulations

The Ozone-depleting Substances and Halocarbon Alternatives Regulations (Environment and Climate Change Canada) require:

Mandatory refrigerant recovery during any service work.

Technician certification (environmental section exam).

Record keeping for quantities of refrigerant recovered, recycled, or destroyed.

Prohibition of intentional release into the atmosphere.

CSA B149.1

The CSA B149.1 Code (natural gas and propane) applies if the appliance uses a flammable refrigerant (R-600a, R-290) and the room must be ventilated in the event of a leak. Requirements include: gas leak detector, mechanical ventilation, and minimum distance between ignition sources and potential leak points.

Service Procedures

Compressor Replacement

116.Unplug the appliance and verify the absence of voltage.

117.Recover the refrigerant into a certified cylinder.

118.Cut the lines with a torch (oxy-acetylene or nitrogen) while purging with nitrogen to prevent oxidation.

119.Remove the compressor and replace it with an identical model (capacity, refrigerant type, oil).

120.Braze the connections with copper-phosphorus alloy (15% phosphorus) for copper-to-copper, or silver alloy (30 to 45%) for copper-to-steel.

121.Pressure test with nitrogen (1,500 kPa) to verify leak tightness.

122.Evacuate to 500 microns for 30 minutes.

123.Charge the refrigerant according to the nameplate (exact mass ± 5 g).

124.Verify temperatures and current after 30 minutes of operation.

Capillary Tube Replacement

The capillary tube is sized according to the charge and refrigerant. A tube that is too long increases the condensing pressure and reduces flow. A tube that is too short causes compressor overheating and poor performance. The standard length is specified in the manufacturer's manuals. The tube must be inserted into the suction line for at least 300 mm for the heat exchanger.

Handling R-600a

Work in a ventilated area (at least 6 air changes per hour).

Use non-sparking tools (copper-beryllium alloy wrenches).

Unplug the appliance and wait 5 minutes before any service work (stabilized pressure and temperature).

Recover the refrigerant into a dedicated cylinder (maximum pressure 1,500 kPa).

Never use oxygen for pressure testing.

Verify the absence of leaks with a detector calibrated for propane.

Practical Calculations

Heat Load Calculation

Q_total = Q_walls + Q_infiltration + Q_food + Q_defrost + Q_fans

Example: a 400 L refrigerator with 2 m² of wall area, U coefficient = 0.3 W/(m²·K), ΔT = 25 °C between the interior (4 °C) and exterior (29 °C).

Q_walls = 0.3 × 2 × 25 = 15 W

Infiltration from door openings is estimated at 20% of the wall load. Food placed inside (5 kg at 20 °C cooled to 4 °C): Q = m × c × ΔT = 5 × 3.5 × 16 = 280 kJ, or approximately 8 W over one hour of operation.

The total load is approximately 25 to 30 W. The compressor must have a cooling capacity of 60 to 80 W for a duty cycle of 40 to 50%.

Duty Cycle

Duty cycle = (run time) / (run time + off time) × 100%

A normal duty cycle is 30 to 50% for a refrigerator, 60 to 80% for a freezer. A duty cycle above 80% indicates a problem (low charge, dirty condenser, defective gasket).

Pressure Conversion

1 kPa = 0.145 psi

1 psi = 6.895 kPa

1 bar = 100 kPa

1 micron = 0.001 Torr = 0.133 Pa

Common Pitfalls to Avoid

Confusing gauge and absolute pressure: saturation charts use absolute pressure. At 0 °C, R-134a is at 293 kPa absolute, or approximately 192 kPa gauge at sea level.

Forgetting the thermostat differential: a thermostat does not switch at the exact setpoint temperature. The compressor stops at the setpoint and restarts at setpoint + differential (3 to 5 °C).

Neglecting condenser cleanliness: a dirty condenser causes high pressure, compressor overheating, and increased energy consumption.

Using a manifold on a hermetic system without service ports: piercing the line is prohibited. Use indirect methods (temperatures, currents).

Charging an R-600a system with a standard manifold: hoses must be specific to hydrocarbons (free of residual mineral oil).

Not waiting for pressure stabilization: after any service work, wait at least 30 minutes before measuring pressures.

Confusing the defrost klixon and the defrost thermostat: the former is a thermal fuse (70 °C), the latter is a bimetal (10 to 15 °C).

Forgetting to check the door gasket: a worn gasket causes humid air infiltration, excessive frost, and higher energy consumption.

Not respecting evacuation times: a 500-micron vacuum must be maintained for 30 minutes. A rapid pressure rise indicates a leak or moisture.

Using an uncalibrated leak detector: the detector must be calibrated for the refrigerant being searched for. An R-134a detector will not detect R-600a.

Summary

The refrigerator moves heat from the interior to the exterior through a vapour-compression cycle.

The four essential components are: compressor, condenser, expansion device (capillary tube or TXV), and evaporator.

Superheat (5 to 8 °C) and subcooling (5 to 10 °C) are the two key diagnostic parameters.

R-600a (isobutane) is the standard refrigerant for new domestic appliances; it is flammable and requires specific precautions.

Automatic defrost uses a timer, heater, defrost thermostat, and safety klixon.

Electrical measurements (winding resistances, current, capacitors) allow diagnosis of faults without opening the refrigeration circuit.

The Canadian Electrical Code (C22.1), CSA C22.2 No. 60335-2-24, and the Halocarbon Regulations govern installations and service work.

Refrigerant recovery is mandatory; environmental certification is required to handle refrigerants.

The normal duty cycle is 30 to 50% for a refrigerator and 60 to 80% for a freezer.

A systematic diagnostic approach (visual, electrical, thermal, pressures) avoids errors and unnecessary replacements.

Self-Assessment Questions

176.What is the typical superheat at the outlet of a direct expansion evaporator?

177.What is the role of the safety klixon in a defrost system?

178.Why does R-600a require special precautions?

179.What is the typical start winding resistance of a hermetic compressor?

180.What is the normal duty cycle of a freezer?

181.Which CSA standard covers the safety of domestic refrigerators?

182.What is the maximum nitrogen pressure test for an R-134a system?

183.What does a 500-micron vacuum mean?

184.What is the role of the accumulator in the refrigeration circuit?

185.Which rule of the Canadian Electrical Code concerns branch circuits for household appliances?

This chapter covers the essential knowledge for the Red Seal exam in appliance servicing. The numerical values are typical orders of magnitude; exact values depend on the manufacturer and model. Always consult technical manuals and refrigerant data sheets for precise values.

Refrigerators and Freezers