FAHU with VRF Outdoor unit - Should we select the Outdoor at 46 C or 34 C ambient temperature

Selecting a VRF Outdoor Unit at 35°C is the Best Choice for FAHU in UAE. In UAE, FAHU operates under two key conditions: 1. High humidity (34°C DB / 32°C WB) – Maximum latent load, requiring the highest cooling capacity. 2. High temperature (46°C DB / 29.4°C WB) – Lower humidity, reducing the cooling coil load by ~20% to 30%. If the VRF condensing unit is selected at 35°C, it provides full capacity for peak humidity conditions. At 46°C DB, its capacity naturally drops by ~20%, which perfectly matches the FAHU’s reduced cooling load.

Dharmesh, I agree with your write up. Honestly most of the FAHU are not properly selected in UAE. It's the right way to select FAHU at 34/32, and for selection of Chiller or VRF units , we should run the system through appropriate heat load calculation software to find out the max desired design load Most of the mechanical engineers work with a philosophy of bigger is better, because except for some really knowledge engineers having fundamentals of psychometrics are just knowledgeable on other MEP because consultancy companies don't hire HVAC specialist. There are so many issues in 95% of buildings in UAE due to the wrong selection resulting in higher power inputs to the building, higher humidity, fungus, respiratory disorders. My one criteria while interacting with most of Arabic MEP engineers used to be to assess their knowledge on psychometrics Well i can write whole lot of issues due to poor design.

Well said

In the case of a Fresh Air Handling Unit (FAHU) with 100% fresh air (evaporator coil), my recommendation is to size the coil for ambient conditions of 34/32, This ensures the evaporator coil can effectively handle the latent heat associated with dehumidifying the fresh air. Designing for 46/29 is not ideal for this region due to the high humidity levels and associated latent load. For the outdoor unit, selecting condensing units rated at 34°C dry bulb will result in undercapacity during peak summer conditions. While the unit may perform adequately for 8–9 months of the year in Middle Eastern climates, at ambient temperatures of 46°C or higher, the unit’s capacity will be derated by at least 20% when compared to nominal conditions. Therefore, the ideal selection for condensing units should be based on 46°C ambient temperature. It is important to note that the cooling capacity required at 46/29 is approximately 20–25% lower than the capacity required at 34/32, primarily due to the significantly higher latent load at the latter condition. As such, there is no justification for selecting outdoor units at 34°C dry bulb, as this does not account for the derating and higher cooling demands during peak conditions.

Generally Ambient condition with 0.70 SHR-0.86 SHR, But in High latent condition we better go for low SHR and high LHR. So instead of choosing Peak load condition, Choose the peak Latent load to design FAHU would be a better option to avoid excessive condensation inside the facilities.