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Bypass factor and contact factor: measuring coil effectiveness
Bypass factor (BF) is the fraction of airflow that passes through a cooling coil without making thermal contact with the fin surface. It is defined as BF = (Tleaving − TADP) / (Tentering − TADP), where TADP is the apparatus dew point, the effective coil surface temperature. A bypass factor of zero means all air reaches the ADP condition (perfect contact). A bypass factor of one means the coil has no effect. Contact factor (CF = 1 − BF) is the complementary term, representing the fraction of airflow that fully contacts the coil and leaves at the ADP condition. Manufacturers typically publish BF at rated face velocity under the standard test procedures set out in AHRI Standard 410; if you reduce face velocity, BF decreases, and the coil achieves a leaving condition closer to the ADP.
Row count, face velocity, and bypass factor in Canadian coil selection
BF decreases with more coil rows and lower face velocity. For standard 2.5 m/s (500 fpm) face velocity: 2-row coils have BF around 0.30; 4-row coils around 0.15; 6-row coils around 0.08; 8-row coils can reach 0.04. At 2.0 m/s (400 fpm), BF drops by roughly 15 to 20% from the standard values. When a low SHR is required (humid climates like Toronto or Ottawa), specifying 6-row coils rather than 4-row reduces BF from 0.15 to 0.08, nearly doubling the fraction of air that contacts the cold surface. This matters most for latent removal, since the same chilled water supply temperature delivers significantly better dehumidification with a lower BF coil. Use the apparatus dew point calculator to find the ADP required for your load, then use this tool to specify the BF needed to hit the leaving wet-bulb setpoint. Since BF is so sensitive to face velocity, it's worth cross-checking the coil face area against duct static with the duct velocity pressure calculator before finalizing a row count.
Field verification: measuring BF during commissioning
During HVAC commissioning, bypass factor can be calculated from measured entering and leaving conditions. The simplest field method uses dry-bulb temperatures only: BF = (Tleaving − TADP) / (Tentering − TADP). Use the T-db Only mode above if you have thermocouple readings from the coil entering and leaving planes and the coil's published ADP from the manufacturer's data. A measured BF significantly higher than the design value usually indicates fouled fins, air bypass around the coil edges, or inadequate chilled water differential, all worth flagging on an HVAC commissioning checklist. Measured BF more than 20% above design is worth investigating before accepting the installation. Cross-check BF results with the sensible heat ratio calculator to verify the full psychrometric performance picture.
Frequently Asked Questions
Typical bypass factors for commercial chilled water coils at 2.0 to 2.5 m/s (400 to 500 fpm) face velocity range from 0.05 to 0.20 depending on row count. A 4-row coil typically gives BF 0.14 to 0.18. A 6-row coil gives 0.06 to 0.10. An 8-row coil can reach below 0.05. DX coils with interlaced circuits typically achieve slightly lower BF than chilled water coils of the same row count. A BF above 0.25 usually means insufficient rows, excessive face velocity, or fouled fins. Use this tool to calculate the actual BF from measured entering and leaving conditions during the verification testing that ASHRAE Guideline 0 calls for, then compare to the coil load calculator output to confirm the selection meets both sensible and latent loads.
Bypass factor directly controls moisture removal. The fraction of air that bypasses the coil carries its original moisture into the leaving airstream. A coil with BF 0.20 means 20% of the airflow bypasses completely, limiting moisture removal to 80% of theoretical. For a given ADP, a higher BF results in warmer and more humid leaving air. If leaving humidity is too high, the space won't meet its RH setpoint even with the correct dry-bulb temperature. This is a common cause of comfort complaints in humid Canadian summers. Reducing BF by adding rows or reducing face velocity improves latent performance without changing chilled water temperature, often the most cost-effective solution. Use the ADP calculator to check whether the required ADP is achievable with your chilled water temperature.