BS EN 378 charge limits
Flammable refrigerant room charge limit (BS EN 378-1 Annex C)
- where
- maximum charge for the room (kg)
- lower flammability limit (kg/m³)
- installation height factor (m)
- room floor area (m²)
- charge m1, 4 m³ × LFL (kg)
- charge m3, 130 m³ × LFL (kg)
Explain this equation
Works out the maximum charge of a flammable refrigerant, such as R32 or R290, that a unit can hold for the size of room it serves. Use it when fitting split systems with A2L or A3 gases.
LFL is the lowest concentration in air that can burn. A leak spreads out from the unit and pools, so the mounting height h₀ matters: a ceiling unit has more air below it to dilute a leak than a floor unit. A bigger floor area also dilutes more, but only with the square root. m₁ and m₃ are reference charges equal to the LFL times 4 and 130 cubic metres, used as the thresholds in the standard.
The formula is an empirical fit in BS EN 378-1 for how a heavier-than-air leak spreads in a room.
Sense check: R32 (LFL 0.307) in a 20 m² room with a high-wall unit at h₀ = 2.2 gives about 5.6 kg. Quadrupling the floor area only doubles the limit.
Watch out: below 1.5 × m₁ for A2L gases there is no room-size restriction. Above that, also check the caps for the access category and location class.
Works out the largest charge of a flammable (A2L or A3) refrigerant allowed for a room of a given floor area and unit mounting height under BS EN 378-1.
Enter your values to see the result.
Good to know
LFL values come from BS EN 378-1 Annex E (R32 0.307 kg/m³; check the standard for other refrigerants). h0 depends on where a leak would be released: floor-standing 0.6, window 1.0, wall-mounted 1.8, ceiling 2.2. Also check the toxicity limit (RCL × room volume) and use the lower result. Product standards (e.g. BS EN IEC 60335-2-40) may set their own limits for packaged units.
Common mistake
Using room volume in this formula (it uses floor area), or using the height of the room instead of the release-height factor.
Limits
Simplified Annex C formula for human comfort applications; read the standard for the full conditions and for A3 refrigerants.
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)
- ACR Journal: Flammable refrigerants in air conditioning and heat pump systems (EN 378-1: no room restriction up to m1 × 1.5 for class 2L, m1 for class 2 and 3; m1 = 4 m³ × LFL, based on 150 g of R290)
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.