Find the maximum refrigerant charge for a room, or the minimum floor area for a given charge, for R-32, R-454B and other flammable refrigerants — using the IEC 60335-2-40 formula with charge thresholds and circulation airflow.
Guidance only. The governing requirements are in the edition of IEC 60335-2-40, UL 60335-2-40 or ASHRAE 15 adopted locally and in the equipment manufacturer's installation manual, which may set stricter limits or allow larger charges with leak detection, circulation, ventilation or shut-off valves.
mmax = 2.5 × LFL5/4 × h0 × A1/2
Amin = [M ÷ (2.5 × LFL5/4 × h0)]²
LFL is the refrigerant's lower flammability limit in kg/m³, h0 the release height of the indoor unit — 0.6 m floor, 1.0 m window, 1.8 m wall and 2.2 m ceiling — and A the floor area of the room in m². A leak from a high unit mixes into more air before it reaches the floor, so a ceiling unit is allowed more refrigerant than a floor unit in the same room.
| Refrigerant | Class | LFL (kg/m³) | m1 | Wall unit (1.8 m) | Ceiling unit (2.2 m) |
|---|---|---|---|---|---|
| R-32 | A2L | 0.307 | 1.228 kg | 4.60 kg | 5.62 kg |
| R-454B | A2L | 0.296 | 1.184 kg | 4.39 kg | 5.37 kg |
| R-1234yf | A2L | 0.289 | 1.156 kg | 4.26 kg | 5.21 kg |
| R-1234ze(E) | A2L | 0.303 | 1.212 kg | 4.52 kg | 5.53 kg |
| R-290 (propane) | A3 | 0.038 | 0.152 kg | 0.34 kg | 0.41 kg |
| R-600a (isobutane) | A3 | 0.043 | 0.172 kg | 0.39 kg | 0.48 kg |
A2L refrigerants are mildly flammable; A3 refrigerants such as propane are highly flammable, which is why their limits are so much lower.
Where continuous or detector-activated air circulation is the safety measure, the indoor fan must move enough air to keep a leak diluted below the LFL. Manufacturer manuals commonly give the minimum as Qmin = 60 × mc ÷ LFL (m³/h, charge in kg). The tool shows this value for reference.
R-32 wall-mounted split, 2.4 kg total charge, bedroom of 20 m².
Under IEC 60335-2-40 it is 2.5 × LFL5/4 × h0 × √A. For a wall-mounted R-32 unit (h0 = 1.8 m) that is about 1.03 × √A kg — for example 4.6 kg in a 20 m² room. Systems up to about 1.84 kg have no room-size restriction.
A2L is the ISO 817 / ASHRAE 34 safety class for refrigerants with lower toxicity (A) and lower flammability with a low burning velocity (2L). R-32, R-454B and R-1234yf are A2L; propane (R-290) is A3, highly flammable.
Refrigerant released high in the room mixes with more air on its way down, so the concentration near the floor stays lower. The formula reflects this with the release height h0.
Reduce the charge (shorter pipe runs, smaller system), serve a larger space, or add the safety measures the standard and manufacturer allow — refrigerant leak detection with circulation airflow, mechanical ventilation or safety shut-off valves.
The US uses UL 60335-2-40 and ASHRAE 15, which are based on the same principles but use the releasable charge and refrigerant concentration limit approach with mandatory leak detection above set charges. Use this tool for orientation and the manufacturer's UL-listed instructions for compliance.
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