Leak Detection Trigger on a Cascade System
A cold store uses a cascade system: a low-temperature R-744 (CO2, GWP 1) circuit and a high-temperature R-404A (GWP 3922) circuit. The R-404A circuit holds 18 kg; the R-744 circuit holds 40 kg. The site has no fixed leak detection system. The owner has heard that 'CO2 is harmless so the CO2 side doesn't count' and asks whether they need leak detection and how often checks are due. The plant room is below ground with limited ventilation and is normally unmanned.
1. Calculate the CO2e of each circuit and determine whether fixed leak detection is required and at what leak-check interval.
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R-404A circuit: 18 kg × 3922 = 70,596 kg CO2e ≈ 70.6 t. R-744 circuit: 40 kg × 1 = 40 kg CO2e ≈ 0.04 t (negligible for threshold purposes).
Threshold analysis: the CO2e that matters is the R-404A circuit at ~70.6 t, which is above the 50 t CO2e band. The CO2 side is effectively irrelevant to the threshold on GWP grounds.
Leak detection: for systems of 50–500 t CO2e, a fixed leak detection system is required (with alarm and regular function checks). Since none is installed, the owner is non-compliant. Until one is fitted, the reduced check interval that applies to systems with detection does not apply.
Check interval: without detection, a system in the 50–500 t band needs checks every 6 months; with detection fitted, the interval relaxes to 12 months. So fitting detection both brings the site into compliance and halves the checking burden.
Correct the owner's assumption: CO2's low GWP does not mean the CO2 side is 'free' - it still needs safe handling - but it is the R-404A charge that drives the regulatory duty.
2. The plant room is below ground and unmanned. What additional safety obligation should you raise alongside the leak detection requirement?
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Raise the confined-space/asphyxiation risk. The R-744 circuit holds 40 kg of CO2 in a below-ground, poorly ventilated, normally unmanned plant room. CO2 is heavier than air and, at sufficient concentration, is an asphyxiant and toxic. A leak could accumulate at low level without anyone noticing.
Obligations to flag:
- The fixed leak detection system should cover the CO2 circuit as well as the R-404A circuit, with an alarm positioned where it will be noticed (and ideally linked to an external alert given the room is unmanned).
- Ensure ventilation (low-level extraction) is adequate and that access is controlled - no lone working in the space without precautions.
- Provide escape/rescue arrangements and signage warning of the asphyxiation risk.
- Include the CO2 circuit in the leak-check and maintenance regime even though its GWP contribution is tiny.
This shows the judgement the exam tests: the environmental threshold and the safety threshold are driven by different refrigerants on the same system.