SS-APG008-EN 11
Hot Gas Bypass
Many years ago, hot gas bypass (HGBP) was successfully added to HVAC systems to correct a
number of operational problems. Hoping to avoid such problems altogether, it eventually became
common practice for designers to specify hot gas bypass in systems. Unfortunately, the practice
often degraded rather than improved reliability.
Hot gas bypass increases the minimum refrigerant charge; it also inflates the first cost of the
system. Besides adding more paths for potential refrigerant leaks, hot gas bypass increases the
likelihood of refrigerant distribution problems. Finally, hot gas bypass uses excessive amounts of
energy by preventing the compressors from cycling with fluctuating loads.
Trane now has several years of successful experience with Evaporator Defrost Control (EDC). Like
hot gas bypass, the EDC system protects the coil from freezing, but it does so by turning off
compressors when a sensor detects the formation of frost on the evaporator coil. The compressor
is released to operate when the coil temperature rises a few degrees above the frost threshold. The
EDC control strategy may reduce the overall energy consumption of the system while maintaining
system control.
Systems should be designed to avoid HGBP whenever possible.
For more information, refer to the Engineers Newsletter, “Hot Gas Bypass – Blessing or a Curse?”
(ADM-APM007-EN).
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