NVQ L4 Refrigeration & AC cheat sheet

Workplace exposure limit for R-134a and R-32 is 1000 ppm as an 8-hour time-weighted average; thermal decomposition of refrigerant produces hydrogen fluoride, so brazing requires local exhaust ventilation.

A2L refrigerants (R-32, R-454B) have a lower flammability limit near 0.3 kg/m3; BS EN 378-1 charge limits and ignition-source control apply in occupied spaces.

Confined space entry requires oxygen between 19.5% and 23.5% by volume, a permit to work, continuous gas monitoring and a rescue plan before anyone enters.

Refrigerant vapour is heavier than air, so plant room detectors are mounted at low level and mechanical ventilation is sized against the largest single circuit charge.

Brazing requires an oxygen-free nitrogen purge, flame-retardant clothing, leather gloves and face protection; a fire watch continues for at least 30 minutes after hot work ends.

Lock-off and tag-out before working on compressor or fan circuits; prove dead with an approved voltage indicator to BS 7671 before touching conductors.

Manual handling guideline limits are 25 kg for a single-person lift by a man and 16 kg by a woman; recovery cylinders are moved on trolleys and never carried up ladders.

COSHH assessment is required for refrigerants, lubricating oils, brazing flux and cleaning solvents, and the safety data sheets must be available on site.

Deliberate release of ozone-depleting HCFCs such as R-22 is banned; recovered quantities must be recorded and sent for destruction or reclamation.

Cooling load = fabric gain + solar gain + internal gain + ventilation gain + product load; apply a diversity factor to internal gains and check the total against CIBSE or ASHRAE design data.

Sensible airflow: V (m3/s) = Qs (kW) / (1.2 x 1.005 x dT), using air density 1.2 kg/m3 and specific heat capacity 1.005 kJ/kg.K.

Sensible heat ratio = sensible load / total load; below about 0.75 the latent load dominates and reheat or desiccant dehumidification is required.

Expansion valve design point is 5-8 K superheat at the evaporator outlet; 5-10 K liquid subcooling ensures a solid column of liquid at the valve.

Suction line pressure drop should not exceed 1 K saturation equivalent and liquid line drop 0.5 K; oil return needs 3-4 m/s horizontal and 7-8 m/s vertical riser velocity.

GWP values used in F-gas calculations: R-410A 2088, R-134a 1430, R-32 675, R-454B 466, R-404A 3922, R-290 3.

BS EN 378-1 sets the maximum permitted refrigerant concentration in occupied spaces; A2L charge limits are lower than A1 and may trigger a requirement for fixed leak detection.

Zeotropic blends have temperature glide: R-407C about 5-7 K, R-454B about 1.5 K, R-410A under 0.5 K; glide affects coil circuiting and the charging method.

Critical path is the longest sequence of dependent activities; float = late start minus early start, and every activity on the critical path has zero float.

An HSE F10 notification is required when a project exceeds 30 working days with more than 20 workers on site simultaneously, or exceeds 500 person-days.

Earned value: SPI = EV / PV and CPI = EV / AC; a value below 1.0 means the project is behind schedule or over budget.

Long-lead items such as chillers, air handling units and switchgear run 12-20 weeks, so order dates are back-scheduled from the required on-site date.

CDM 2015 Schedule 2 welfare requires toilets, washing facilities, drinking water, a rest area and changing facilities from the start of the project.

Permits to work are required for hot works, confined space entry, live electrical work and system isolation; each permit is closed out and retained as a record.

Resource levelling moves tasks within their available float to remove peaks; resource smoothing reduces peaks without moving the completion date.

Handover sequence: pressure test, evacuation and charge, commissioning, O&M manuals and training, then a defects liability period of typically 12 months.

BS EN 378-2: strength test at 1.43 x PS with oxygen-free nitrogen, then tightness test at 1.1 x PS; air or oxygen must never be used as the test medium.

Triple evacuation: pull below 500 microns (0.67 mbar) three times, breaking vacuum with OFN between stages; a standing vacuum below 500 microns held for one hour confirms dryness.

Charge by weight only; never charge a blend by pressure alone, and charge as liquid into the liquid line to preserve the designed composition.

Verify performance after charging: 5-8 K superheat, 5-10 K subcooling, and evaporating and condensing approach temperatures within the design tolerance.

Air systems: proportional balancing brings each terminal within plus or minus 10% of design flow, working back from the index terminal.

Water systems: flush and chemically clean before adding inhibitor, then record flow rates, differential pressures and pump head against design figures.

Insulation resistance must be at least 1 MOhm at 500 V dc; verify earth loop impedance and RCD trip times to BS 7671 before energising.

A TM44 air conditioning inspection is required for systems over 12 kW output, and the inspection report is valid for five years.

Handover pack: as-fitted drawings, O&M manuals, F-gas register and leak test records, commissioning data sheets and operator training records.

High superheat + low suction pressure + small evaporator temperature difference = undercharge, liquid line restriction or blocked filter drier.

Low superheat + high suction pressure + flooding back to the compressor = overcharge, expansion valve overfeeding or an oversized valve orifice.

High discharge pressure with high subcooling and bubbles at the sight glass = non-condensables, or a condenser airflow or water flow problem.

Frost or a measurable temperature drop across a filter drier or solenoid valve indicates a restriction; confirm with pressure and temperature readings either side.

Compressor motor check: phase resistances balanced within about 2%, insulation resistance above 1 MOhm at 500 V; a low reading indicates moisture or winding breakdown.

An acid test on the oil confirms burnout; the repair needs an oil change, new liquid line driers and temporary suction line filtration.

Electronic leak detectors to BS EN 14624 sense down to roughly 5 g/yr for HFCs; a positive indication is confirmed by OFN pressure test or UV dye.

Planned maintenance frequencies follow SFG20 tasks plus condition data: oil analysis, vibration trending, thermography of electrical panels and coil cleaning.

Short cycling causes include HP/LP switch settings, low charge, blocked condenser, faulty thermostat and sensor drift; check the control loop before stripping the compressor.

tCO2e = charge (kg) x GWP / 1000; this figure determines the leak check frequency band and whether a fixed leak detection system is mandatory.

Leak checks: 5 tCO2e or more every 12 months; 50 tCO2e or more every 6 months; 500 tCO2e or more every 3 months; intervals halve where a fixed leak detection system is fitted.

Records of charge, recovery, leak checks and repairs must be kept for at least five years and made available to the regulator on request.

A fixed leak detection system is mandatory at 500 tCO2e or more and must itself be checked at least every 12 months.

Only F-gas certified personnel may handle HFCs; companies must hold a company certificate and be registered with the regulator.

Venting is prohibited; recovered refrigerant must be recycled, reclaimed or destroyed, and the quantity is recorded on the transfer note.

R-404A has a GWP of 3922; the HFC phase-down makes high-GWP retrofits non-compliant for many new retail and cold store installations.

The HFC phase-down cuts the quota of virgin HFCs placed on the market in steps, reaching 21% of the 2015 baseline by 2036.

Equipment labels must state the refrigerant type, the charge in kilograms and the tCO2e, and must be updated after any change of refrigerant or charge.

Health and Safety at Work etc. Act 1974: employees must take reasonable care of themselves and others (s.7) and must not misuse anything provided for safety (s.8).

Site supervisors typically hold the five-day SMSTS certificate and working supervisors the two-day SSSTS; both are CITB courses renewed every five years.

Toolbox talks are short documented briefings; the attendance record and content provide evidence of supervision and competence control.

GROW coaching model - Goal, Reality, Options, Will - draws the solution from the technician instead of issuing an instruction.

Herzberg two-factor theory: hygiene factors such as pay and conditions only prevent dissatisfaction, while motivators such as recognition and responsibility raise performance.

Thomas-Kilmann conflict modes are competing, collaborating, compromising, avoiding and accommodating; collaborating suits technical disputes where both parties hold valid data.

Effective delegation matches authority with responsibility and states the limit of authority in writing for safety-critical tasks.

Apprenticeship standards require 20% off-the-job training and an end-point assessment; competence must be recorded in the training log.

Maslow's hierarchy runs physiological, safety, social, esteem and self-actualisation; lower needs are generally met before higher ones motivate.

An Inspection and Test Plan defines hold points, where work stops until signed off, and witness points, where notice is given but work may continue.

KPIs include plant availability, MTBF, MTTR, first-time-fix rate and the planned-to-reactive ratio, with 80:20 planned-to-reactive a common target.

Specific power is kW of input per kW of cooling; a COP of 3.0 equals 0.333 kW/kW, so measured input power is compared directly with the design figure.

Chiller efficiency expressed in kW/TR: 0.6 kW/TR is approximately COP 5.9; record measured input power at the commissioning conditions.

5 Whys and Ishikawa fishbone analysis separate root causes into people, machine, method, material, measurement and environment.

PDCA (Plan-Do-Check-Act) is the Deming improvement cycle; a change is only standardised once the check stage confirms the result.

A non-conformance report records the defect, root cause, containment, corrective action and close-out signature; NCRs are trended monthly.

Life-cycle cost = capital cost + present value of energy, maintenance and disposal; NPV uses the discount rate to compare design options.

Benchmarking uses kWh/m2.yr for buildings and annual maintenance cost as a percentage of asset replacement value, typically 2-4%.