Air diffusion performance index (ADPI) for both cooling and heating

The current guideline of diffuser/ outlet selection for all-air HVAC (heating, ventilating and air conditioning) systems was developed back in the 1970s only for cooling operation. The guideline correlates indoor air distribution with diffuser/ outlet performance to distribute supply air, by using ADPI (Air Diffusion/ Distribution Performance Index) method.

As of when the method was developed, however, new types of diffusers/ outlets have been emerging and the use of all-air HVAC system for the heating purpose is prevalent. In addition, the new type diffusers/ outlets are likely to distribute air in a different manner from existing ones.

Previous ADPI is limited to cooling condition only. However, a designer will probably not allow a separate air distribution/HVAC system for the heating purpose only. It ends up with that the same layout and selection of diffusers will be operated for both cooling (summer) and heating (winter). Remeber, the layout and selection were designed according to cooling ADPI because there was no heating ADPI.

Here is the problem. The airflow for most diffusers is attached to the ceiling to avoid draft on occupants. It is reasonable that cold airflow will travel downward and then mix with air in the occupied zone due to buoyancy. However, warm air from the diffusers would have more difficulties. The indoor space would have higher thermal stratification and poor air quality — air never goes down. Our study shows that air change effectiveness could be as low as 0.4, much smaller than what specified in ASHRAE 62.1.

We developed a new ADPI at heating mode on the basis of thermal comfort theory specified in ASHRAE Standard 55. The new guideline will comply with the vertical stratification requirements of ASHRAE Standard 55, and might allow better input for those seeking to show compliance with Standard 55 and the associated LEED point.

The study developed a new guideline by conducting approximately 700 laboratory measurements, based on the research project of ASHRAE RP-1546: Expansion and updating of the Air Diffusion Performance Index method. The new guideline provides selection guidance for 15 types of diffusers/ outlets operating at both cooling and heating conditions. The research would justify the reduction of the number of diffusers/ outlets required to be supplied into a space under peak cooling or heating load conditions without compromising thermal comfort.

See more details in our publications.


Amai, H., Liu, S. and Novoselac, A. (2017). Experimental study on air change effectiveness: Improving air distribution with all-air heating systems. Building and Environment 125 (15): 515-527.

Liu, S., Clark, J., and Novoselac, A. (2017). Air diffusion performance index (ADPI) of overhead-air-distribution at low cooling loads. Energy and Buildings 134: 271-284.

Liu, S. and A. Novoselac. 2015. Air diffusion performance index (ADPI) of diffusers for heating mode, Building and Environment 87: 215-223.

Liu, S., and Novoselac, A. (2016). The Effect of Deflectors on Air Diffusion Performance Index (ADPI) of Adjustable Diffusers: Cooling Condition (RP-1546). Science and Technology for the Built Environment 22(1):