Functions and Features of RAC and HP Systems
This element introduces learners to the fundamental components of refrigeration, air conditioning and heat pump systems, their operational principles, and the essential knowledge of refrigerants and oils properties. It also covers the function of system controls and the correct use of industry test instruments, which are critical for safe installation, servicing and maintenance tasks.
Assessment criteria
Topic Overview
The City & Guilds Level 2 Diploma in Refrigeration, Air Conditioning and Heat Pump Systems is a foundational qualification for anyone aspiring to work in the cooling and heating industry. It covers the principles of thermodynamics, refrigeration cycles, system components, and safe handling of refrigerants. This diploma is essential for technicians who install, maintain, and repair systems in commercial, industrial, and domestic settings, aligning with UK regulations like F-Gas and building services standards.
Students will explore vapour-compression refrigeration cycles, heat pump operation, electrical controls, and system diagnostics. The course emphasises practical skills such as brazing, pressure testing, and leak detection, alongside theoretical knowledge of heat transfer and energy efficiency. Understanding these systems is crucial for reducing carbon emissions and meeting net-zero targets, as heat pumps become central to UK heating strategies.
This diploma sits within the broader Construction & Building Services sector, linking to plumbing, electrical installation, and HVAC engineering. It prepares students for roles as refrigeration engineers, air conditioning technicians, or heat pump installers, with progression to Level 3 qualifications or apprenticeships. Mastery of this content ensures compliance with health and safety legislation and competence in handling environmentally sensitive refrigerants.
Key Concepts
Core ideas you must understand for this topic
- →Vapour-compression refrigeration cycle: Understand the four main processes—compression, condensation, expansion, and evaporation—and how they transfer heat from a low-temperature source to a high-temperature sink.
- →Refrigerants and environmental impact: Know the properties of common refrigerants (e.g., R-134a, R-410A), their ozone depletion potential (ODP) and global warming potential (GWP), and the legal requirements for handling under F-Gas regulations.
- →System components: Identify and explain the function of compressors (reciprocating, scroll, rotary), condensers (air-cooled, water-cooled), expansion devices (TXV, capillary tube), and evaporators (direct expansion, flooded).
- →Heat pump operation: Recognise that heat pumps are reversible refrigeration systems, providing heating or cooling by reversing the cycle via a four-way reversing valve, with emphasis on coefficient of performance (COP).
- →Electrical controls and safety: Interpret wiring diagrams for contactors, relays, thermostats, and pressure switches; understand lockout/tagout procedures and safe isolation for maintenance.
Learning Objectives
What you need to know and understand
- Know the function of a range of RAC and HP system components, Know the operating principles for a range of RAC and HP system applications, Know the properties of refrigerants, Know the properties of oils, Know the operating principles of a range of system controls, Know the operation of a range of test instruments used in the RAC industry
Assessment Criteria
Key criteria assessors look for in your portfolio
- Award credit for accurately identifying and describing the function of key system components such as compressors, condensers, evaporators, expansion devices and reversing valves.
- Award credit for explaining the basic vapour compression cycle and how heat transfer occurs in refrigeration, air conditioning and heat pump applications, including the differences between cooling-only and reversible systems.
- Award credit for identifying common refrigerants (e.g., R410A, R32, R134a) and stating their properties such as boiling point, pressure-temperature relationship, environmental impact (GWP, ODP) and flammability classification.
- Award credit for selecting the appropriate type of oil (e.g., mineral, POE, PAG) for a given refrigerant and explaining the reasons, including miscibility and system compatibility.
- Award credit for describing the operating principles of system controls such as thermostats, pressure switches, defrost timers, and electronic expansion valves, and how they maintain system performance and safety.
- Award credit for demonstrating the correct use of test instruments (e.g., manifold gauge sets, thermometers, clamp meters, leak detectors) to measure and interpret system parameters safely.
Assessment Guidance
Guidance for achieving higher grades
- 💡In written assessments, always relate component functions to the refrigeration cycle; use diagrams where possible to support your answer.
- 💡Memorise the key properties (boiling point, GWP, oil type) of common refrigerants as these are frequently tested in multiple-choice and short-answer questions.
- 💡When asked about test instruments, describe not only how to use them but also what safe working practices must be followed (e.g., gauge manifold connection sequence, PPE).
- 💡For questions on controls, explain the effect on the system if the control fails (e.g., low pressure cut-out tripping indicates a leak or blockage).
- 💡Draw clear, labelled diagrams of the refrigeration cycle, showing pressure-enthalpy changes. Examiners award marks for accurate component placement and directional arrows for refrigerant flow.
- 💡Memorise key formulas: COP = Q/W (heating) or Q_c/W (cooling), and superheat/subcooling values. Show all working in calculations, including units, to gain method marks even if the final answer is wrong.
- 💡Link theory to real-world applications: For example, explain how a heat pump extracts heat from outside air at -5°C. Relate to refrigerant boiling points and compressor work—this demonstrates deeper understanding.
Common Mistakes
Common errors to avoid in your coursework
- Confusing the functions of the condenser and evaporator – learners often reverse the heat rejection and heat absorption roles.
- Misinterpreting pressure-temperature charts by using gauge pressure instead of absolute pressure, leading to incorrect superheat or subcooling calculations.
- Selecting incompatible oil for the refrigerant (e.g., using mineral oil with R410A) which can lead to poor lubrication and system failure.
- Overlooking the importance of system controls in fault diagnosis, assuming a component failure rather than a control setting or sensor issue.
- Using test instruments without zeroing or calibrating, resulting in inaccurate readings and misdiagnosis of system performance.
- Misconception: Refrigeration systems 'create cold'. Correction: They remove heat from a space; cold is the absence of heat. The system absorbs heat at the evaporator and rejects it at the condenser.
- Misconception: A bigger compressor always means better cooling. Correction: Oversized compressors cause short cycling, reduced efficiency, and poor humidity control. Correct sizing based on heat load calculation is critical.
- Misconception: Refrigerant can be topped up without fixing leaks. Correction: Leaks must be repaired before recharging; topping up violates F-Gas regulations and wastes energy. Always pressure test and evacuate the system.
Frequently Asked Questions
Common questions students ask about this topic
Pass / Merit / Distinction Evidence Checklist
How your portfolio evidence is graded for CITY & GUILDS LIMITED Functions and Features of RAC and HP Systems
Every vocational unit is marked against named criteria rather than an exam percentage. Your tutor's brief lists the exact codes for this unit — here is what each band is asking you to do.
Demonstrate baseline knowledge, accurate terminology, and core practical application.
Provide detailed analysis, structured explanations, and clear workplace reasoning.
Deliver thorough evaluation, original problem solving, and fully justified recommendations.
Before You Start
Prior knowledge that will help with this topic
- •Basic physics of heat transfer (conduction, convection, radiation) and temperature scales (Celsius, Kelvin).
- •Fundamental electrical principles: voltage, current, resistance, and simple circuit analysis (Ohm's Law).
- •Health and safety awareness, including COSHH regulations and safe handling of pressurised systems.
Coursework AI Review
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Key Terminology
Essential terms to know
- Know the function of a range of RAC and HP system components, Know the operating principles for a range of RAC and HP system applications, Know the properties of refrigerants, Know the properties of oils, Know the operating principles of a range of system controls, Know the operation of a range of test instruments used in the RAC industry
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