[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A VRF system with R410A serves a floor of offices, including a few small meeting rooms and a server-adjacent store. The total system charge (factory charge plus additional charge for the piping) is around 30 kg. I know EN 378 requires a check that a full leak into the smallest room does not exceed a limit.

• How exactly is the check done, and which limit applies?
• What are the options if a small room fails?
• Is the method the same for R32 and under ASHRAE 15?

[MEP Center Engineering]

Divide the total refrigerant charge of the circuit by the volume of the smallest occupied room it serves; for R410A in a normal occupied space the result must not exceed the EN 378-1 practical limit of 0.44 kg/m³. If it does, you must add a measure such as a larger effective volume, leak detection with shut-off, or a smaller circuit.

Steps

1. Total charge of the circuit: factory charge of the outdoor unit(s) plus additional charge calculated from the liquid pipe lengths and sizes. Use the final as-designed pipe lengths, not an estimate.
2. Smallest room: find the smallest room that contains an indoor unit or refrigerant piping of that circuit. Room volume = floor area x height of the occupied space (the ceiling void counts only if it is not sealed from the room).
3. Classify: EN 378-1 sets the allowed charge from the refrigerant safety class, the access category (a: general, b: supervised, c: authorised only) and the location class of the system. For a direct VRF system with indoor units in rooms used by the public or office staff, the simple rule is: maximum charge = practical limit x room volume.
4. Compare and, if it fails, apply additional measures.

Worked example

Total charge: 11.8 kg factory + 18.4 kg additional = 30.2 kg R410A. Smallest room: meeting room 3.0 x 4.0 x 2.7 m = 32.4 m³.

• Concentration = 30.2 / 32.4 = 0.93 kg/m³ > 0.44 kg/m³ – fails.
• Maximum charge for this room = 0.44 x 32.4 = 14.3 kg.
• Room volume needed for 30.2 kg = 30.2 / 0.44 = 68.6 m³.

Note that simply splitting into two equal circuits of about 15.1 kg still gives 15.1 / 32.4 = 0.47 kg/m³, marginally over the limit. The circuit containing that room must be reduced to 14.3 kg or less, or another measure applied.

Options when a room fails

• Serve the small room from a separate small system (or a split unit) with a low charge.
• Increase the effective volume with permanent openings to an adjacent space, only where the standard’s conditions for combined volumes are met.
• Refrigerant leak detection in the room with alarm and automatic shut-off valves isolating the indoor unit branch, plus mechanical ventilation where required.
• Keep refrigerant joints out of small rooms; route pipes through corridors.

R32 and ASHRAE 15

R32 is classed A2L (mildly flammable). Its EN 378-1 practical limit is about 0.061 kg/m³, and for A2L refrigerants the charge limits of EN 378 and IEC 60335-2-40 depend on floor area and the installation height of the unit, not on volume alone, so use those formulae. In the US, ASHRAE 15 uses the refrigerant concentration limit (RCL) from ASHRAE 34, about 0.42 kg/m³ (26 lb per 1000 ft³) for R-410A, reduced by half for institutional occupancies such as hospitals.

Common mistakes

• Using only the factory charge and forgetting the additional charge.
• Checking the largest room on the floor instead of the smallest.
• Counting a sealed ceiling void or the volume of a closed adjacent room.
• Doing the check at concept stage and not repeating it after pipe routes change.

Related: 10 common VRF design mistakes, VRF heat recovery vs heat pump. For deeper study: VRF/VRV system design course.

Check the edition of the code that applies to your project.

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