City & Guilds Level 3 Diploma in Refrigeration, Air Conditioning and Heat Pump Systems (C&G 6187-02) Overview
This study guide covers the six core subjects for the City & Guilds Level 3 Diploma in Refrigeration, Air Conditioning and Heat Pump Systems. It is designed for technicians seeking to demonstrate advanced knowledge and practical skills in system design, installation, commissioning, maintenance, fault diagnosis, environmental compliance, and heat pump technology. The notes are anchored to official sources including ASHRAE, IMC, IECC, ACCA, City & Guilds, UK F-gas regulations, REFCOM, and BESA. Candidates should verify specific pass marks, eligibility, and regulatory details with the awarding body.
For Technical Conquer practice planning, this module is tracked as 80 questions over about 120 minutes with a listed pass mark of 70%. Treat those numbers as practice baselines and verify the current official format before scheduling.
How This Guide Is Organized
The sections below turn the syllabus into studyable subject blocks. Read a subject first, explain the must-know ideas without notes, then use questions, flashcards, and mind maps to test whether the knowledge holds under field-style pressure.
- Advanced Thermodynamics and Vapor Compression Cycles
- Complex Electrical Systems and Control Circuitry
- System Design, Installation, and Commissioning
- Service, Maintenance, and Fault Diagnosis
- Environmental Protection and Refrigerant Management
- Heat Pump Technology and Renewable Integration
Exam Snapshot and Readiness Target
Format: 80 questions, 120 minutes (practice baseline); official format may vary - verify with City & Guilds.
Candidate level: Technician-level - advanced knowledge for employment-ready service and installation roles.
Readiness target: 70% pass mark (practice baseline); confirm official threshold with City & Guilds.
Most candidates should budget at least 36+ focused study hours, then adjust upward for unfamiliar equipment, code, regulatory, commissioning, controls, or calculation-heavy content.
Advanced Thermodynamics and Vapor Compression Cycles
Syllabus Focus
- Thermodynamic principles: laws, properties of refrigerants, pressure-enthalpy diagrams
- Vapor compression cycle analysis: COP, isentropic efficiency, subcooling, superheat
- Multi-stage and cascade systems
- Refrigerant selection and performance characteristics
Key Notes
- The vapor compression cycle consists of compression, condensation, expansion, and evaporation. Understanding the pressure-enthalpy (P-h) diagram is essential for analyzing cycle performance and diagnosing faults.
- Coefficient of Performance (COP) = Cooling effect / Work input. For heat pumps, COP_heating = (Cooling effect + Work input) / Work input.
- Subcooling increases system efficiency by ensuring liquid refrigerant enters the expansion device; typical subcooling is 5-10°F (3-6°C). Superheat at the evaporator outlet should be 8-12°F (5-7°C) to prevent liquid slugging.
- Multi-stage compression with intercooling improves efficiency for low-temperature applications. Cascade systems use two separate cycles with different refrigerants for very low temperatures (e.g., -80°C).
- Refrigerant selection considers ODP, GWP, safety classification (ASHRAE 34), and thermodynamic properties. Common HFCs (e.g., R-410A) are being phased down under F-gas regulations; low-GWP alternatives (e.g., R-32, R-290) are increasing.
Must Know
- Interpret P-h diagrams to determine enthalpy values, calculate COP, and identify cycle inefficiencies.
- Calculate superheat and subcooling from temperature and pressure measurements.
- Understand the impact of evaporator and condenser temperatures on system performance.
- Know the safety classifications of refrigerants (A1, A2L, A3, B1, etc.) and their implications for system design.
Field and Exam Application
- Field: Use P-h diagram analysis to diagnose low cooling capacity - e.g., high superheat indicates low refrigerant charge or restricted evaporator.
- Field: When retrofitting a system from R-22 to R-407C, adjust TXV superheat setting and check oil compatibility.
- Field: For a walk-in freezer, calculate required compressor displacement using volumetric efficiency and desired evaporator temperature.
High-Yield Distinctions
- Subcooling is measured at the condenser outlet; superheat is measured at the evaporator outlet.
- Isentropic efficiency of a compressor = (h2s - h1) / (h2 - h1), where h2s is the enthalpy after isentropic compression.
- Flash gas occurs when liquid refrigerant flashes to vapor after the expansion device, reducing system efficiency.
- In a cascade system, the evaporator of the high-temperature cycle cools the condenser of the low-temperature cycle.
Common Pitfalls
- Confusing subcooling with superheat - always verify measurement points.
- Assuming a fixed superheat setting works for all refrigerants - TXV settings are refrigerant-specific.
- Neglecting pressure drop in suction and discharge lines when analyzing P-h diagrams.
- Using the wrong refrigerant properties (e.g., using R-134a data for R-404A).
Review Tasks
- Draw a P-h diagram for a simple vapor compression cycle and label all key points.
- Calculate COP for a system given evaporator and condenser temperatures and compressor power.
- Compare the performance of R-410A and R-32 at the same operating conditions.
- Explain the purpose of an intercooler in a two-stage compression system.
Complex Electrical Systems and Control Circuitry
Syllabus Focus
- Electrical safety: lockout/tagout, safe isolation, PPE
- Reading and interpreting electrical schematics and wiring diagrams
- Motor controls: starters, contactors, relays, overloads, variable frequency drives (VFDs)
- Control systems: thermostats, pressure switches, PLCs, DDC, and BAS integration
Key Notes
- Safe isolation procedure: identify source, isolate, lock, test for dead, prove tester. Always use a calibrated voltage tester.
- Motor starters include direct-on-line (DOL), star-delta, and soft starters. VFDs provide variable speed control and energy savings.
- Overload protection: thermal overload relays (bimetallic) or electronic overloads protect motors from overcurrent. Set to 115-125% of motor FLA.
- Control circuits use 24V AC for safety (e.g., thermostat to contactor coil). Line voltage (120/230V) powers motors and compressors.
- Building Automation Systems (BAS) use DDC controllers with sensors (temp, humidity, pressure) to optimize HVAC operation. Communication protocols include BACnet, Modbus, and LonWorks.
Must Know
- Identify components on a schematic: contactor coil, normally open/closed contacts, overload, transformer, and control relay.
- Perform safe isolation and prove dead before working on electrical circuits.
- Set overload protection correctly based on motor nameplate FLA.
- Troubleshoot a motor that fails to start: check control voltage, contactor coil, overload status, and motor windings.
Field and Exam Application
- Field: A condenser fan motor trips on overload - measure current and compare to FLA; check for binding or high head pressure.
- Field: Install a VFD on a supply fan and program ramp-up time to avoid duct pressure spikes.
- Field: Diagnose a compressor that cycles on and off - check low-pressure control setting and refrigerant charge.
High-Yield Distinctions
- A contactor is used for high-current loads; a relay is for low-current control signals.
- Star-delta starter reduces inrush current to 1/3 of DOL; used for motors >5 HP.
- VFD output voltage is not sinusoidal; use a true RMS meter for accurate readings.
- BAS points: analog input (AI), digital input (DI), analog output (AO), digital output (DO).
Common Pitfalls
- Failing to lock out/tag out before working on live circuits.
- Misreading schematic symbols - e.g., confusing normally open with normally closed.
- Setting overloads too high, causing motor burnout; or too low, causing nuisance trips.
- Assuming a VFD can be installed without proper harmonics mitigation (line reactors).
Review Tasks
- Draw a basic control circuit for a single-phase compressor with a thermostat and high-pressure switch.
- Explain the difference between a contactor and a relay.
- List the steps for safe isolation of a 3-phase chiller.
- Describe how a VFD controls motor speed and saves energy.
System Design, Installation, and Commissioning
Syllabus Focus
- Load calculations: heat gain/loss, Manual J, cooling/heating loads
- Duct design: Manual D, static pressure, friction loss, duct sizing
- Refrigerant piping design: line sizing, oil return, pressure drop
- Commissioning process: pre-start checks, startup, performance verification, documentation
Key Notes
- Manual J load calculation accounts for building envelope, windows, occupancy, lighting, and equipment. Oversizing leads to short cycling and poor humidity control.
- Duct design per Manual D: total static pressure includes supply, return, fittings, and equipment. Friction rate typically 0.08-0.12 in.wg/100 ft.
- Refrigerant line sizing: suction line sized for low pressure drop (2°F equivalent) and oil return velocity (min 500 fpm horizontal, 1000 fpm vertical). Liquid line sized for 1°F drop.
- Commissioning: verify equipment nameplate, electrical connections, refrigerant charge, airflow, and controls. Document all readings and compare to design specifications.
- Installation best practices: support piping, avoid traps, use proper brazing techniques with nitrogen purge to prevent oxidation.
Must Know
- Perform a Manual J load calculation for a residential space.
- Size ductwork using friction loss charts and Manual D procedures.
- Select refrigerant line sizes based on tonnage and length.
- Complete a commissioning checklist: check voltage, amperage, pressures, temperatures, airflow, and control operation.
Field and Exam Application
- Field: A new split system has high head pressure - check condenser airflow, refrigerant charge, and non-condensables.
- Field: During commissioning, measure total external static pressure and compare to fan curve to verify airflow.
- Field: For a long line set (100 ft), calculate additional refrigerant charge and adjust TXV superheat.
High-Yield Distinctions
- Manual J is for residential; Manual N is for commercial load calculations.
- Duct friction loss increases with velocity squared; doubling velocity quadruples friction loss.
- Refrigerant oil return is critical in long vertical risers - use double risers for variable capacity systems.
- Commissioning is not complete until all setpoints are verified and system operates through all modes.
Common Pitfalls
- Oversizing equipment based on square footage alone - always perform load calculation.
- Ignoring duct leakage - seal ducts to reduce energy loss.
- Using incorrect refrigerant line sizes causing excessive pressure drop or oil return issues.
- Skipping nitrogen purge during brazing - leads to copper oxide formation and compressor failure.
Review Tasks
- Calculate cooling load for a 2000 sq ft house with given U-values and design conditions.
- Size a supply duct for 400 CFM at 0.1 in.wg/100 ft friction loss.
- Determine refrigerant line sizes for a 5-ton R-410A system with 75 ft equivalent length.
- Create a commissioning checklist for a rooftop unit.
Service, Maintenance, and Fault Diagnosis
Syllabus Focus
- Preventive maintenance: cleaning coils, changing filters, lubricating motors, checking belts
- Fault diagnosis: using gauges, thermometers, multimeters, leak detectors
- Common faults: refrigerant leaks, compressor failure, electrical faults, control failures
- Troubleshooting methodology: symptom analysis, systematic checks, repair verification
Key Notes
- Preventive maintenance schedule: monthly filter change, quarterly coil cleaning, annual refrigerant leak check, and electrical connection tightening.
- Diagnostic tools: manifold gauges for pressure/temperature, clamp meter for current, thermocouple for temperature, electronic leak detector for refrigerant leaks.
- Common compressor failure causes: liquid slugging, floodback, overheating, electrical faults, and contamination.
- Systematic troubleshooting: 1) Verify power and controls, 2) Check pressures and temperatures, 3) Analyze superheat/subcooling, 4) Inspect components, 5) Test and repair.
- Leak detection methods: electronic detector, UV dye, bubble solution, pressure test with nitrogen.
Must Know
- Interpret gauge readings to diagnose low charge, overcharge, restriction, or non-condensables.
- Perform a refrigerant leak test using nitrogen pressure test and electronic detector.
- Identify compressor electrical faults: open winding, short to ground, locked rotor.
- Replace a faulty capacitor and verify motor start-up.
Field and Exam Application
- Field: A system has low suction pressure and low superheat - indicates low evaporator airflow or frozen coil.
- Field: High head pressure with high subcooling suggests overcharge or condenser restriction.
- Field: Compressor draws high amps and trips overload - check for tight bearings, high discharge pressure, or electrical issue.
High-Yield Distinctions
- Low superheat + low suction pressure = low airflow or evaporator restriction.
- High superheat + low suction pressure = low refrigerant charge or suction line restriction.
- High subcooling + high head pressure = overcharge or condenser restriction.
- Low subcooling + low head pressure = undercharge or compressor valve issue.
Common Pitfalls
- Adding refrigerant without checking for leaks first.
- Replacing a compressor without finding the root cause of failure.
- Using a leak detector near a fresh refrigerant cylinder - false positive.
- Ignoring safety: recover refrigerant properly, wear PPE, and avoid contact with refrigerant.
Review Tasks
- List the steps to diagnose a no-cooling call on a split system.
- Explain how to measure and interpret superheat and subcooling.
- Describe the procedure for recovering refrigerant from a system.
- Create a preventive maintenance checklist for an air-cooled chiller.
Environmental Protection and Refrigerant Management
Syllabus Focus
- Ozone depletion and global warming: ODP, GWP, regulations
- F-gas regulations: containment, leak checking, record keeping, certification
- Refrigerant recovery, recycling, and reclaiming
- Alternative refrigerants: natural refrigerants (CO2, ammonia, hydrocarbons), low-GWP HFOs
Key Notes
- Ozone Depletion Potential (ODP) measures a substance's ability to destroy ozone; CFCs have high ODP, HCFCs lower, HFCs zero. Global Warming Potential (GWP) measures greenhouse effect relative to CO2 over 100 years.
- EU F-gas Regulation (517/2014) and UK equivalent: phase down HFCs, require leak checks based on CO2 equivalent, mandatory recovery, and certification for personnel and companies.
- Leak check frequency: systems with >5 tonnes CO2 equivalent (tCO2e) must be checked annually; >50 tCO2e every 6 months; >500 tCO2e with leak detection system every 6 months.
- Refrigerant recovery: use approved recovery machine and cylinder; recover to 90% of charge or until vacuum. Recycling cleans refrigerant on-site; reclaiming restores to original purity off-site.
- Natural refrigerants: R-744 (CO2) has GWP=1, used in commercial refrigeration; R-717 (ammonia) is toxic but efficient; R-290 (propane) is flammable (A3) but low GWP.
Must Know
- Calculate CO2 equivalent: mass of refrigerant (kg) × GWP = tCO2e.
- Perform a leak check using an electronic detector and record results.
- Recover refrigerant from a system following safe procedures.
- Identify which refrigerants are banned or phased out under current regulations.
Field and Exam Application
- Field: A supermarket rack system with 100 kg of R-404A (GWP 3922) has 392.2 tCO2e - requires leak detection system and quarterly checks.
- Field: Retrofit an R-22 system to R-407C - check oil compatibility, change filter drier, and adjust TXV.
- Field: Use a recovery machine to remove R-410A from a residential unit - recover to 15 inHg vacuum.
High-Yield Distinctions
- HFCs have zero ODP but high GWP; HFOs have low GWP and zero ODP.
- Leak check thresholds are based on CO2 equivalent, not refrigerant mass alone.
- Recovery cylinders must not be overfilled - maximum 80% fill by volume.
- Ammonia systems require special training due to toxicity; hydrocarbon systems require explosion-proof equipment.
Common Pitfalls
- Venting refrigerant to atmosphere - illegal and harmful.
- Using a recovery machine not approved for the refrigerant type.
- Mixing different refrigerants in the same recovery cylinder.
- Failing to keep records of refrigerant usage and leak checks.
Review Tasks
- Calculate the CO2 equivalent for a system containing 50 kg of R-134a (GWP 1430).
- List the steps for leak checking a system with an electronic detector.
- Explain the difference between recycling and reclaiming refrigerant.
- Describe the safety precautions when working with ammonia or propane.
Heat Pump Technology and Renewable Integration
Syllabus Focus
- Heat pump principles: air-source, ground-source, water-source
- Reversing valve operation and defrost cycles
- Geothermal systems: ground loops, heat exchanger types, efficiency
- Integration with renewable energy: solar thermal, PV, hybrid systems
Key Notes
- Heat pumps move heat from a low-temperature source to a high-temperature sink using a reversing valve. Air-source heat pumps (ASHP) are common; ground-source (GSHP) have higher efficiency due to stable ground temperatures.
- Reversing valve (4-way valve) switches between heating and cooling modes. Defrost cycle reverses to melt ice on outdoor coil; uses electric heat or hot gas bypass.
- GSHP loops: closed-loop (horizontal, vertical, pond) or open-loop (groundwater). Efficiency measured by COP (heating) and EER (cooling). Typical COP 3-5 for GSHP.
- Hybrid systems combine heat pump with gas furnace for cold climates. Solar thermal can preheat water for heat pump; PV can offset electricity consumption.
- Renewable integration: heat pumps can be powered by solar PV, reducing carbon footprint. Thermal storage (e.g., ice storage) shifts load to off-peak hours.
Must Know
- Explain the operation of a reversing valve and defrost cycle.
- Compare COP of ASHP vs GSHP at different outdoor temperatures.
- Size a ground loop for a GSHP based on heating/cooling loads and soil conditions.
- Describe how a heat pump can be integrated with solar PV or thermal systems.
Field and Exam Application
- Field: An ASHP in heating mode has ice buildup on outdoor coil - check defrost thermostat and timer.
- Field: A GSHP has low water flow - check pump, strainer, and loop pressure.
- Field: Design a hybrid system for a cold climate: heat pump for mild temps, gas furnace for extreme cold.
High-Yield Distinctions
- ASHP efficiency drops as outdoor temperature decreases; GSHP efficiency remains relatively constant.
- Defrost cycle typically initiates when coil temperature drops below 32°F and terminates when coil temperature reaches 50°F.
- Ground loop design: vertical loops require less land area but higher drilling cost; horizontal loops need more land.
- Heat pump COP decreases with increasing temperature lift (difference between source and sink).
Common Pitfalls
- Installing an ASHP in a climate with extended sub-freezing temperatures without backup heat.
- Oversizing ground loop - increases cost without benefit; undersizing reduces efficiency.
- Neglecting to insulate refrigerant lines in long runs.
- Setting defrost cycle too frequently - wastes energy.
Review Tasks
- Draw a schematic of a heat pump in heating mode showing refrigerant flow.
- Calculate the COP of a GSHP given entering water temperature and leaving water temperature.
- Explain the difference between a closed-loop and open-loop geothermal system.
- Describe how a heat pump can be integrated with a solar thermal system for domestic hot water.
How To Use These Notes With Practice Questions
Do not jump straight from reading to a full mock. Work by subject first: review the key notes, make a short recall sheet from memory, then answer a focused question set. After each miss, decide whether the problem was missing theory, weak code/source recall, poor measurement setup, calculation error, or a field sequence you did not visualize.
Technical Conquer's question bank, flashcards, mind maps, and spaced review tools are most useful after this instruction layer because they reveal which parts of the notes are not yet retrievable.
Final Review Checklist
- Review all six subjects focusing on key formulas (COP, superheat, subcooling, CO2 equivalent), safety procedures (lockout/tagout, refrigerant recovery), and regulatory requirements (F-gas, leak checks).
- Practice interpreting P-h diagrams and electrical schematics until fluent.
- Memorize common fault diagnosis patterns (e.g., low superheat + low suction = low airflow).
- Understand the differences between refrigerants and their environmental impact.
- Be able to explain heat pump operation and defrost cycles clearly.
- Verify any specific pass mark, eligibility, or exam format with City & Guilds directly.
Official Sources and Further Reading
Use these sources as the final authority for format, eligibility, rules, regulatory limits, and exam updates. Study notes are a preparation layer, not a replacement for official candidate guidance.