At the Bunhill Energy Centre Phase 2, Islington Council operates a 1 MW ammonia (R717) heat pump that captures waste heat from the London Underground and raises it to district heating supply temperature. This supplies a local school, two leisure centres and over 1000 homes, in addition to selling back to the National Grid. The system, delivered by Islington in partnership with TfL is supported by a comprehensive preventive maintenance regime to safeguard performance, reliability, and compliance.

The strategy includes 24/7 remote monitoring and emergency call-out cover, and a range of scheduled servicing. Twice-monthly engineer site visits are carried out for inspections and cleaning of key heat transfer components, calibrating instrumentation, and verifying that compressors, pumps, and safety systems are operating within specifications. Each visit is logged in a central maintenance logbook with a written report, ensuring full traceability and a documented service history.

Monthly tasks include oil and pressure checks on the Grasso compressors (models V1100 and HP65), vibration and noise monitoring, superheat and discharge temperature checks, shaft seal leak inspections, lubrication of drive motors, verification of variable-speed drives, and belt condition monitoring. Condenser and evaporator surfaces are inspected for fouling, frost, or corrosion; fan motors, adiabatic systems, and strainers are checked for proper operation; refrigerant circuits are visually checked and leak tested; and glycol pumps, pressurisation units, and underfloor heating circuits are inspected. Electrical panels are examined for overheating or deterioration, with safety checks on relief valves and controls carried out on each visit.

Quarterly activities deepen these inspections, with calibration of control and safety sensors, full defrost-cycle monitoring on evaporators, testing of air purgers, and fluid property checks (flow, charge, specific gravity of glycol). Semi-annual tasks include inspection of compressor mounting bolts, end seals, and electrical cubicles is also carried out.

Annually, the regime extends to oil sampling and analysis, oil and filter replacement if required, compressor seam cleaning, and complete plant logging. Condensers are chemically or pressure-jet cleaned, water samples are taken for independent analysis, strainers are replaced or cleaned, and refrigerant levels are recorded across all vessels. Safety checks include emergency stop tests, PRV date checks, and calibration of leak detection and pH monitoring systems. Thermographic scans, automation system tests, and insulation integrity inspections are also performed.

The plant operates on natural refrigerant ammonia (NH₃), which has zero GWP but requires strict adherence to leak detection and safety protocols. Gas leak detectors (TQ8000 and Jumo Aquis systems) are tested and recalibrated annually to maintain compliance with F-gas and ammonia safety standards.

Results: This layered preventive and predictive maintenance approach maximises system uptime, reduces the risk of failures, and preserves the high coefficient of performance required for the scheme to deliver affordable low-carbon heat. Clean heat exchangers, tuned compressors, and calibrated controls ensure the heat pump runs efficiently, lowering the electrical input per unit of heat produced. In turn, this improves affordability for the area’s connected homes and public buildings, while extending asset life and increasing the sustainability and replicability of this district energy system.