BS EN 378 charge limits
Maximum charge for an occupied space (concentration limit × room volume)
- where
- maximum refrigerant charge (kg)
- refrigerant concentration limit (kg/m³)
- volume of the smallest occupied room served (m³)
Explain this equation
Works out the largest refrigerant charge that could leak into the smallest room served without exceeding the concentration limit. Use it at design stage for split and VRF systems.
The concentration limit is the most refrigerant per cubic metre of room air that is considered safe for people, from toxicity or oxygen displacement. If the whole charge leaks into the room, its concentration is charge divided by volume. Rearranging gives the maximum charge as limit times volume.
A worst-case mixing assumption: the full charge spread evenly through the room.
Sense check: a 33 m³ room with a limit of 0.39 kg/m³ allows about 12.9 kg.
Watch out: it is the smallest room on the system that governs. Flammable refrigerants have their own, often tighter, limits as well.
Finds the largest refrigerant charge that could leak into the smallest occupied room served without exceeding the BS EN 378 concentration limit.
Enter your values to see the result.
Good to know
Use the smallest occupied room an indoor unit or pipework serves. Values from BS EN 378-1 Annex E (as given by FETA): R410A RCL 0.39, QLMV 0.42 kg/m³; R32 RCL 0.061, QLMV 0.063; R1234yf RCL 0.058; R1234ze RCL 0.061. Older “practical limit” values (EN 378:2008, e.g. R410A 0.44) are superseded. Where the charge exceeds the limit, QLMV or QLAV can be used only with the extra measures EN 378 requires (leak detection, ventilation, shut-off valves). For flammable (A2L, A3) refrigerants also check the flammability limit calculator.
Common mistake
Using the total building volume or a large open-plan area when a small hotel bedroom or office is served by the same system.
Limits
Toxicity/oxygen-deprivation limit only. The full BS EN 378-1 Annex C assessment (occupancy category, location class, system type) decides which limit applies.
Sources
- FETA: An introduction to A2L refrigerants and their use in RACHP applications (EN 378 charge = room volume × RCL/QLMV/QLAV; m_max = 2.5 × LFL^5/4 × h0 × A^1/2; m1, m2, m3 = 4, 26, 130 m³ × LFL; R-410A hotel example)
- BS EN 378-1:2016+A1:2020 Refrigerating systems and heat pumps – Safety and environmental requirements – Part 1: Basic requirements, definitions, classification and selection criteria (Annexes C and E)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.