AC Annual Service and Maintenance: Filter Cleaning, Coil Inspection, Refrigerant Check and F-Gas Log Update

Summary

Air conditioning systems that are not serviced regularly lose efficiency, develop faults, and fail prematurely. A split system with a blocked evaporator filter can lose 15-25% of its rated cooling capacity within a single season. A system with a slow refrigerant leak will eventually run on a reduced charge, causing the compressor to work harder, overheat, and fail — at a replacement cost typically five to ten times the cost of the service that would have caught the leak. For commercial customers with legal obligations under the F-Gas Regulation, an unserviced system with an unrecorded leak is a compliance liability.

The annual service is also the AC installer's most reliable recurring revenue stream. A customer who trusts you to service their system every year is more likely to replace it through you, more likely to refer you, and more likely to buy additional units when they extend or refurbish their premises. Service agreements are worth pricing competitively to build this relationship — the long-term value is in the customer relationship, not just the individual service visit.

For commercial premises, a twice-yearly service schedule is the industry standard, typically aligned with the shoulder seasons — April/May before the cooling season and September/October before the heating season. Residential systems can be serviced annually, but filter maintenance between services (every 6-8 weeks during heavy use) should be shown to the customer at commissioning so they can do this themselves. Understanding the difference between a full service (engineer visit) and filter maintenance (customer task) helps set appropriate expectations and reduces unnecessary callouts.

Key Facts

• Annual service — minimum recommended frequency for residential split systems. Twice-yearly for commercial systems.
• Filter maintenance — filters should be cleaned or replaced every 6-8 weeks during heavy use periods. Customer can do this themselves for standard washable mesh filters.
• Evaporator coil — the indoor coil that absorbs heat from room air. Fins can become clogged with dust, mould and biofilm over time, restricting airflow and reducing heat exchange efficiency.
• Condenser coil — the outdoor coil that rejects heat to outside air. Can become blocked with leaves, cottonwood (poplar seed), insect nests and general atmospheric dirt.
• Condensate system — warm humid air passing over the evaporator coil causes condensation, which drains away through the condensate tray and drain line. Blocked drains cause water overflow, property damage and mould growth.
• Condensate pump — where gravity drainage is not possible, a condensate pump lifts the water to the nearest drain. Pump float switch, non-return valve and discharge line must all be checked.
• Refrigerant pressure check — suction pressure (low side) and discharge pressure (high side) must be measured and compared against manufacturer data for the prevailing ambient conditions. Deviations indicate undercharge, overcharge, restriction, or other faults.
• Superheat and subcooling — two derived measurements calculated from pressure and temperature readings that indicate whether the refrigerant charge and expansion valve operation are correct. Published target values are in the manufacturer's service data.
• F-Gas log — mandatory written record for any system containing 5 tonnes CO2e or more of refrigerant. Must record every service visit, refrigerant top-up (quantity weighed in and weighed out), leak checks, engineer certification number, and date.
• Refrigerant weigh-in — any refrigerant added to a system must be weighed using calibrated scales, not estimated. The quantity must be recorded in the F-Gas log.
• REFCOM certification — or equivalent F-Gas Category 1 certification. Required to purchase and handle refrigerants in the UK. Engineer's certificate number must be recorded on the F-Gas log.
• Leak check frequency (F-Gas) — annually for systems 5-50 tonnes CO2e; every 6 months for systems 50-500 tonnes CO2e; every 3 months for systems above 500 tonnes CO2e. Leak detection systems can halve these frequencies.
• CO2e thresholds — R-410A has a GWP of 2,088; 1 kg of R-410A = 2.088 kg CO2e. A system with 2.4 kg of R-410A charge = 5 tonnes CO2e (triggering annual leak check requirement). R-32 GWP = 675; a system needs 7.4 kg of R-32 to reach the 5-tonne threshold.
• Electrical connections — vibration and thermal cycling loosen electrical connections over time. Loose connections cause arcing, component damage and fire risk.
• Performance test — temperature differential between supply and return air should be measured (typically 10-14°C for cooling mode at normal operating conditions). Outside this range indicates a performance issue requiring investigation.

Quick Reference Table

Detailed Guidance

Indoor Unit Service Procedure

Begin with the indoor unit switched off and the isolator locked off. Remove the front panel and take out all filters. On most residential wall-mounted units, the filters are accessible by lifting the front cover — they clip into a frame and pull out easily. Inspect the filters before cleaning: heavy soiling, mould growth, or damage indicates the customer has not been maintaining them between services, which is worth noting in your report.

Washable mesh filters should be cleaned with warm water and a soft brush, then dried thoroughly before refitting. Do not refit damp filters — moisture on the evaporator coil encourages mould growth. Damaged filters should be replaced.

With the filters removed, inspect the evaporator coil. A light layer of dust is normal; heavy dust compaction between the fins, visible mould growth on the coil face, or a musty smell when air flows through the unit indicates a coil that needs cleaning. Use a proprietary coil cleaner (foaming or non-foaming depending on whether rinsing is possible) applied according to the manufacturer's instructions, followed by a water rinse if the installation allows. In wall-mounted units where rinsing risks water ingress to electrical components, a no-rinse foaming cleaner is safer — it breaks down the contamination and allows it to drain away with condensate.

After cleaning the coil, inspect the condensate drain tray beneath it. Algae and slime build-up in the tray is extremely common and will eventually block the drain. Clean the tray thoroughly and flush the drain line with water to verify it is clear. Where a condensate pump is fitted, test the float switch by pouring a small amount of water into the tray and verifying the pump activates and discharges correctly. Check the non-return valve is present and functioning.

Inspect the indoor fan blade (cross-flow fan on most wall-mounted units) for dirt build-up. A coated fan blade reduces airflow and can cause the unit to throw off dirty air when the build-up becomes loose. Clean with a soft brush or appropriate solvent. Inspect the fan motor bearings for noise — a clicking or grinding sound when the fan is rotated by hand indicates bearing wear.

Check all electrical connections within the indoor unit: live, neutral, earth, and communications cable to the outdoor unit. Tighten any loose terminals. Look for discolouration or heat marks indicating previous arcing. Replace any damaged cables or connectors.

Test the unit in all modes (cooling, heating, fan-only, auto) using the remote control. Verify the louvers move correctly in both vertical and horizontal planes. Check that all fan speeds are available and that the unit responds correctly to temperature setpoint changes.

Outdoor Unit Service Procedure

The outdoor unit operates in all weathers and accumulates dirt, debris and biological growth over time. Begin by switching off the isolator and allowing the capacitors in the inverter drive to discharge (at least 5 minutes — follow the manufacturer's lockout/tagout procedure).

Remove the outer panel to access the condenser coil and fan assembly. Inspect the condenser coil fins for blockage with leaves, cottonwood, insects or general dirt. On heavily contaminated coils, flow through the fins may be almost entirely blocked — this is a common cause of high-pressure trips in summer. Clean with a soft brush working from inside the unit outward, followed by a coil cleaner and low-pressure water rinse (not a power washer — high pressure will bend the aluminium fins). Use a fin comb to straighten any bent fins before refitting panels.

Inspect the fan blade for damage, imbalance or dirt build-up. A cracked or chipped blade should be replaced — continued operation risks catastrophic failure. Check the fan motor for bearing noise and secure mounting.

Inspect all electrical connections: compressor terminals, fan motor, inverter drive, and incoming supply. Check the capacitors (discharge and motor run capacitors where applicable — note that inverter-driven units have fewer discrete capacitors than fixed-speed units). Look for bulging, leaking or discoloured capacitors.

Check the refrigerant service valves (Schrader valves or ball valves) for signs of leakage — oily residue around valve cores or connections is a reliable indicator of a refrigerant leak at that point.

Refrigerant Pressure Check and F-Gas Log Update

Measuring refrigerant pressures is the most diagnostically significant part of the service. Using calibrated manifold gauges (or a digital manifold set), connect to the high-side and low-side service ports. Allow pressures to stabilise with the system running in cooling mode before reading.

Compare suction pressure (low side) and discharge pressure (high side) against the manufacturer's data for the current ambient conditions. Most manufacturers publish target pressures as a table or chart in their service manual. Suction pressure significantly below the target range (at a given outdoor temperature and room temperature) typically indicates undercharge or a restriction. High suction pressure with low discharge pressure can indicate a compressor fault. High discharge pressure typically indicates a blocked condenser or overcharge.

Also measure:

• Suction line temperature at the service port — subtract the saturation temperature (derived from suction pressure) to calculate superheat. Typical target: 5-10°C superheat.
• Liquid line temperature — subtract from saturation temperature (at liquid line pressure) to calculate subcooling. Typical target: 3-8°C subcooling.

If pressures and temperatures indicate undercharge, you must check the system thoroughly for leaks before adding refrigerant. Adding refrigerant to a leaking system is not a repair — it is a temporary measure that puts refrigerant into the atmosphere and constitutes a breach of the F-Gas Regulation if the leak is not repaired.

Leak checking methods: electronic leak detector (required for F-Gas compliance), UV dye (use only if already present in the system or if you add UV dye and log it), soap solution (for accessible fittings only as a supplementary check).

If refrigerant top-up is required after repair:

1. Weigh the refrigerant cylinder before and after addition using calibrated scales
2. Record the quantity added (cylinder weight difference) in the F-Gas log
3. Record your F-Gas certification number, the date, the reason for addition, and the system details
4. If the system required more than a trace top-up, investigate the source of the loss — document this in the service report

Performance Test and Service Report Completion

With the service complete and the system running, measure the supply air temperature at the indoor unit and the return air temperature. The difference (temperature differential or Delta T) should be 10-14°C for cooling mode under normal conditions (around 25°C room temperature, typical outdoor conditions). A low Delta T indicates reduced airflow (dirty filters/coil) or reduced refrigerant charge. A high Delta T can indicate very low airflow (again check filters) or oversized unit.

Complete the service report with:

• Date of service
• System make, model and serial numbers
• Indoor and outdoor unit locations
• Refrigerant type and quantity in system (from commissioning records or F-Gas log)
• Refrigerant pressures recorded (suction and discharge)
• Superheat and subcooling values
• Temperature differential test result
• Any refrigerant added (quantity, batch/cylinder reference)
• Leak check result (pass/fail, method used)
• Any defects found and action taken
• Recommended follow-up work
• Engineer name and F-Gas certification number
• Customer signature

A copy of the report should be left with the customer and one retained by the company.

Frequently Asked Questions

How do I know if my customer needs an annual or twice-yearly service?

Residential systems in private homes: annual service is sufficient if the customer maintains filters between visits. Small commercial systems (e.g., a single unit in a retail unit or small office): annual service is usually fine if usage is light. Larger commercial systems, systems in food service environments (grease contamination), systems in industrial premises (dust and particulate contamination), or any system subject to the F-Gas mandatory leak check requirement: twice-yearly is the industry standard. Systems in 24/7 environments (server rooms, pharmacies, cold storage): quarterly maintenance inspections are common.

What is the CO2 equivalent of my system's refrigerant charge, and does it trigger F-Gas obligations?

Multiply the refrigerant charge in kg by the GWP of the refrigerant. Common GWPs: R-410A = 2,088, R-32 = 675, R-22 = 1,810 (banned for top-up since 2015), R-134a = 1,430, R-407C = 1,774. If the result equals or exceeds 5,000 kg CO2e (5 tonnes CO2e), annual leak checks and an F-Gas log are mandatory. For a typical domestic mini-split with 0.6 kg of R-32: 0.6 × 675 = 405 kg CO2e — well below the threshold. For a commercial system with 5 kg of R-410A: 5 × 2,088 = 10,440 kg CO2e — above the threshold, triggering annual leak checks and log requirements.

Can I service a system and top up the refrigerant without an F-Gas certificate?

No. You must hold a relevant F-Gas certificate (Category 1 for recovering, recharging and leak-checking) to purchase refrigerant, add refrigerant to a system, or recover refrigerant from a system. Working on an AC system's refrigerant circuit without the appropriate certification is a criminal offence under the F-Gas Regulation, carrying unlimited fines. You can perform non-refrigerant tasks (filter cleaning, electrical checks, coil cleaning) without certification, but you cannot open or service the refrigerant circuit.

What should I do if I find a significant refrigerant leak?

Stop the service and advise the customer in writing. Do not add refrigerant to a leaking system. Locate and repair the leak (this may involve recovering the charge, pressure testing, repairing the fault, pressure testing again, evacuating, and recharging). Only once the repair is verified leak-free should refrigerant be added. Record everything in the F-Gas log: the leak found, the quantity lost (estimated from pressure deviation or weigh-in), the repair carried out, and the confirmation check. In commercial premises, a significant unrepaired leak may require you to advise the customer that continued operation may not be compliant with their F-Gas obligations.

How long should a service take?

A thorough full service on a single wall-mounted split system typically takes 1.5 to 2.5 hours including travel and administration. Multi-head systems or commercial units with access difficulties will take longer. If filters are badly neglected, coil cleaning takes more time. Building service agreements: price by the number of indoor heads plus a fixed charge for the outdoor unit and F-Gas log administration, not by the hour, so customers are not incentivised to defer service visits.

Regulations & Standards

• F-Gas Regulation (EU) 517/2014 as retained in UK law — sets mandatory leak check frequencies by refrigerant charge size, requires F-Gas logs, and mandates certified engineers for refrigerant handling. [verify current UK statutory instrument number]

• Regulation (EC) 303/2008 (as retained) — minimum requirements and mutual recognition of certificates for companies and personnel recovering certain fluorinated greenhouse gases. Covers F-Gas certification for AC technicians.

• BS EN 378-1 to 378-4 — Safety and environmental requirements for refrigerating systems and heat pumps. Covers maintenance and service procedures.

• REFCOM Register — UK register of F-Gas-certified companies and engineers. Engineers must be registered; verify certification before assigning refrigerant work.

• HSE COSHH Regulations — applicable to refrigerant handling, coil cleaning chemicals, and any hazardous substances used during service.

• Electrical at Work Regulations 1989 — applicable to all electrical inspection work carried out during service visits.

• IET Code of Practice for In-service Inspection and Testing of Electrical Equipment — guidance for checking electrical connections and components during service.

• F-Gas Regulation Guidance — Environment Agency — UK F-Gas obligations and certification requirements

• REFCOM — Register of Companies Competent to Handle Refrigerants — certification body for F-Gas engineers

• BESA TR/19 — Internal Cleanliness of Ventilation Systems — guidance relevant to coil and ductwork cleanliness standards

• CIBSE Guide M — Maintenance Engineering and Management — comprehensive maintenance guidance for HVAC systems

• HSE — Working Safely with Refrigerants — health and safety guidance for refrigerant handling

• f gas regulations guide — full guide to F-Gas certification, log requirements and compliance

• refcom certification — how to obtain and maintain F-Gas certification

• split system installation — installation practices that affect long-term maintenance requirements

• vrf vrv systems overview — VRF-specific maintenance considerations including BCU checks

• hvac commissioning and handover — commissioning documentation used as baseline for service records

• ac condensate drainage — detailed guidance on condensate system design and maintenance