GENERAL AIRSTAGE Technical Reference

ARTH45KHTA Return Air Grille Sizing

Recommended return-air sizing and indoor unit construction
For installers, service technicians and system designers working with the ARTH45KHTA ducted indoor unit
Quick answer: The ARTH45KHTA has a nominal/design airflow of approximately 3,250 m³/h (903 L/s). A practical minimum return-air grille size is approximately 1,200 mm × 600 mm for a conventional filtered return-air grille. Where space permits, 1,200 mm × 750 mm is preferred to reduce air velocity, noise and pressure drop.
Indoor unit construction: The ARTH45KHTA indoor unit does not pull apart or separate into smaller sections for installation. Access and installation clearances must therefore accommodate the complete indoor unit.

1. ARTH45KHTA airflow

Based on the fan performance data, the ARTH45KHTA nominal/design airflow is approximately:

Airflow measurementApproximate airflow
Cubic metres per hour3,250 m³/h
Litres per second903 L/s

The technical data does not nominate a single fixed minimum return-air grille dimension. The grille must be selected so that it can pass the required airflow without creating excessive air velocity, noise or pressure drop.

2. Recommended return-air grille size

For return-air grille selection, a useful design target is approximately 1.5 to 2.0 m/s through the effective free area of the grille.

At approximately 903 L/s, this requires around 0.45 to 0.60 m² of actual free area. Because grille blades, frames and filters reduce the usable opening, the nominal face area needs to be larger than the calculated free area.

Nominal grille sizeGross face areaGuidance
1,200 × 600 mm0.72 m²Practical minimum for a conventional filtered return-air grille with suitable free area.
1,200 × 750 mm0.90 m²Preferred where space permits to reduce velocity, noise and pressure loss.
Recommendation: Use approximately 1,200 × 600 mm as the minimum nominal grille size, subject to the grille manufacturer's published free-area data. Where possible, use 1,200 × 750 mm or larger for improved acoustic and airflow performance.

3. Return-air design considerations

  • The nominated dimensions are nominal grille dimensions. Always check the grille manufacturer's effective free area.
  • A return-air filter will add resistance and should be included when assessing pressure drop.
  • Multiple return-air grilles may be used provided their combined effective free area is equivalent to or greater than the required free area.
  • Return duct sizing, bends, transitions, filters and grilles all contribute to the total system resistance.
  • The complete return-air system must remain within the indoor unit's available external static pressure.
  • Larger grille areas generally reduce face velocity and can help minimise return-air noise.
900 × 600 mm is not recommended as the default minimum. At approximately 903 L/s, a typical filtered grille of this size is likely to operate at relatively high effective air velocity once blade and filter restrictions are considered, increasing the risk of additional noise and pressure drop.

4. Does the ARTH45KHTA pull apart?

No. The ARTH45KHTA indoor unit is not designed to be separated into smaller sections for access or installation.

Before installation, confirm that the ceiling access, roof access, plant-room opening or other installation pathway is large enough to move the complete indoor unit into position. The required service and installation clearances should also be maintained.


Technical note: Grille sizing is application-dependent. Final selection should consider the actual grille free-area percentage, filter resistance, duct design and system external static pressure.