Experimental study on air change effectiveness: Improving air distribution with all-air heating systems

- Different strategies to improve air change effectiveness were examined.
- Proper adjustment of diffuser improved air change effectiveness up to 30%.
- Lower room-supply air temperature difference improved air change effectiveness.
- Locating return air inlet near floor improved air change effectiveness 70%.
- Diffuser adjustment may narrower the range of airflow rate, impacting ADPI.

A major challenge of all-air heating applications is poor air distribution, which is often associated with a high temperature stratification. The two metrics that are commonly used for design and assessment of supply air distribution in the space are: air distribution performance index (ADPI) and the Air Change Effectiveness (E), respectively. All-air heating systems often produce stagnant air in the occupied part of the room. In this case, E may be very low while relatively uniform temperature in this occupied zone results in acceptable ADPI. Since ventilation design is based on ADPI, many all-air heating systems often produce very low E. This experiment based study identifies situation with very low E and provides simple strategies to improve it. The study provides additional design criteria to the ADPI diffuser selection guide that helps with optimal diffuser selection and adjustments. The results show that additional design criteria significantly improve E as well as temperature distribution, measured by temperature effectiveness (ƐT), with all-air heating systems. Appropriate adjustment of the diffuser may improve E and ƐT up to 30%, while the lower supply-room air temperature difference may increase E and ƐT in average 75% and 45%, respectively. Also, proper return air inlet location significantly improves E and ƐT for all-air heating. However, there are certain trades off as: the diffuser adjustment also may require seasonal adjustment for cooling and heating operation, lower supply air temperature difference requires more fan power, and floor exhaust placement may need more space for duct work.