Two-Pipe HVAC Makes a Comeback: An Idea Discarded Decades Ago May Be the Future of School Heating and Cooling

Thomas H. Durkin, Veazey Parrott Durkin & Shoulders
Larry Kinney, E SOURCE/Platts

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Abstract

A 2-pipe HVAC system is one that uses the same piping alternately for hot water heating and chilled water cooling, as opposed to a 4-pipe system that uses separate lines for hot and chilled water. Two-pipe originated 50 or 60 years ago as a cost-effective way to add air conditioning. The premise was a seasonal change-over, where buildings were heated from fall to spring and cooled from spring to fall. Clearly, it saved money not to have to install a second set of pipes, but operationally, the idea never really worked in the cool morning – warm afternoon typical of a spring or fall day.

In 1993, the primary author, a consulting engineer, was asked by a representative of the New Albany, In. school district to design air conditioning for a school on an impossibly small budget. The engineer’s answer reflected conventional wisdom: “the only thing you can afford is 2-pipe, but everybody knows that won’t work.” Nonetheless, the engineer revisited the old system in light of modern technology. The result solved the workability issues and maximized the inherent economy of 2-pipe without compromising indoor air quality, occupant comfort, or humidity control. The result was a remarkable efficient modern 2-pipe system that is proving to be less expensive to build, less expensive to operate, and easier to maintain than any other option for heating and cooling buildings. To date, the new 2-pipe concept has been engineered into 120 buildings. In almost all cases, the resulting utility bills, with air conditioning, have been significantly less than the pre-2-pipe bills without air conditioning.