ARTH KHTA Ducted Systems - Return Air, Airflow and Plenum Sizing
1. Indoor unit airflow
The design and technical manual specifies the following indoor-unit airflows. The listed values are the same for cooling and heating operation.
| Indoor Model | LOW | MED | HIGH / Rated | External Static Pressure Range |
|---|---|---|---|---|
| ARTH36KHTA | 486 L/s 1,750 m³/h | 589 L/s 2,120 m³/h | 695 L/s 2,500 m³/h | 60-210 Pa |
| ARTH45KHTA | 631 L/s 2,270 m³/h | 767 L/s 2,760 m³/h | 903 L/s 3,250 m³/h | 60-260 Pa |
| ARTH54KHTA | 689 L/s 2,480 m³/h | 836 L/s 3,010 m³/h | 986 L/s 3,550 m³/h | 60-260 Pa |
| ARTH60KHTA | 689 L/s 2,480 m³/h | 836 L/s 3,010 m³/h | 986 L/s 3,550 m³/h | 60-260 Pa |
2. Indoor-unit plenum and return-air connection sizes
The dimensional drawings provide the physical duct connection dimensions at the indoor unit. These dimensions are useful when fabricating the return-air connection and supply plenum, but they are not minimum grille sizes and should not be used on their own to size the complete air-distribution system.
| Indoor Model | Return-Air Connection (rear of unit) | Supply / Plenum Connection (front of unit) | Indoor Unit Overall Size (H x W x D) |
|---|---|---|---|
| ARTH36KHTA | 862 x 324 mm | 851 x 295 mm | 400 x 1,050 x 500 mm |
| ARTH45KHTA | 1,062 x 351 mm | 921 x 304 mm | 425 x 1,250 x 490 mm |
| ARTH54KHTA | 1,062 x 351 mm | 921 x 304 mm | 425 x 1,250 x 490 mm |
| ARTH60KHTA | 1,062 x 351 mm | 921 x 304 mm | 425 x 1,250 x 490 mm |
3. How should the return-air grille be sized?
GENERAL does not nominate a minimum return-air grille width or height for these models because a nominal grille dimension alone does not define how much air it can pass. Different grille and filter products can have significantly different effective free areas and pressure drops even when their outside dimensions are the same.
The installer or system designer should calculate the return-air arrangement using the required system airflow and the selected grille, filter and duct components.
When carrying out the calculation, consider:
- the indoor unit's required airflow at the intended operating condition;
- the effective free area of the return-air grille rather than only its nominal face dimensions;
- pressure drop across filters, grilles and return-air boxes;
- return-air duct length, fittings, transitions and flexible duct;
- air velocity and the potential for objectionable air noise;
- the supply-air duct and plenum resistance;
- the total external static pressure available from the indoor unit; and
- service access and filter removal requirements.
Multiple return-air grilles may be used where appropriate. Where more than one grille is installed, the designer should calculate the combined effective free area and confirm that the airflow is distributed appropriately through the return-air duct system.
4. External static pressure and fan setup
The KHTA indoor units are high-static ducted systems. The technical manual requires the external static pressure setting to be selected according to the resistance of the connected duct system and the applicable fan-performance curve.
| Model | Permitted Static Pressure Range | Factory / Standard Setting |
|---|---|---|
| ARTH36KHTA | 60-210 Pa | 60 Pa |
| ARTH45KHTA / ARTH54KHTA / ARTH60KHTA | 60-260 Pa | 60 Pa |
5. Practical guidance
- Do not select a return-air grille simply because its nominal dimensions are similar to the indoor unit's return-air opening.
- Do not treat the connection dimensions in the technical manual as a recommended minimum grille size.
- Size the return system from the required airflow and the published performance data for the actual grille and filter being installed.
- Use gradual transitions between the return-air duct/plenum and the indoor unit wherever practical.
- Avoid abrupt reductions in duct size immediately before the indoor unit.
- Allow adequate access for filter removal, cleaning and indoor-unit maintenance.
- After installation, confirm actual airflow/static performance where required as part of system commissioning.