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VAV vs. Radiant: Side-by-Side Comparison

©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 5, May 2014.

By Guruprakash Sastry and Peter Rumsey, P.E., Fellow ASHRAE

About the Authors
Guruprakash Sastry is a regional manager of Infrastructure and Green Initiatives at Infosys. Peter Rumsey, P.E., is founder of Point Energy Innovations.

Infosys, one of India’s top three software companies, implemented a program in 2011 to find the best way to cool its buildings, while creating lower energy buildings that better suited the needs of its employees. The resulting building, Software Development Block 1 (SDB-1) in Hyderabad, not only became the first radiantly cooled building in India, but also resulted in the world’s largest HVAC side-by-side comparison.
The SDB-1 building includes two cooling systems. Half of the building has an optimized variable air volume (VAV) system. The other half has a radiant cooling with dedicated outdoor air system (DOAS). The building was highly instrumented to measure the impacts of the two systems. After two years of operation, the radiant system has used 34% less energy as compared to the VAV system. The cost of the radiant system was slightly lower than the VAV system and comfort surveys found improved thermal satisfaction with the radiant system.


The Idea

The SDB-1 building was designed with an envelope that minimizes heat and solar loads, while allowing fully daylit office spaces. The result is a building that has insulated walls with thermal breaks as well as an exemplary sun shading system. The directive for the sunshades was straightforward. The sunshades and lightshelves needed to provide for 95% daylight autonomy with no direct sunlight entering the building. Daylight autonomy is the percent of the year that a space can be fully daylit during working hours.

As a consequence, the floor plate of the building needed to be relatively narrow and lightshelves that doubled as sun shades were used throughout. The detailed daylight model verified that no internal shades were needed in the space since glare from direct sunlight was designed out of the building. Overall, the window-to-wall ratio is 30%. However, as a visitor recently observed, the SDB-1 building feels like a building with a much higher window-to-wall ratio because of the lack of blinds and the quality of the building envelope design.

When it came time to design the HVAC system, the design team recommended radiant cooling with a dedicated outdoor air system (DOAS). While the Infosys team was eager to try something that had never been done before, they did want the ability to compare the radiant system to their best in class variable air volume (VAV) system. At the time VAV systems were the standard in all new buildings. Infosys decided to take the 250,000 ft2 (23 226 m2) building and divide it into two halves, one with radiant cooling and the other with the optimized VAV system.


Cooling Systems

The two cooling systems in the building, VAV and radiant, are designed to operate independently and to represent the best possible design of both systems. To have a fair comparison, the SDB-1 building is split symmetrically. Both sides of the building have the same orientation and, therefore, the same solar loads. All parameters such as type of lighting, number of occupants and building envelope are kept the same on both sides.

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