How to use this calculator
- Measure entering and leaving dry-bulb. Same instrument, mixed-air side and supply side of the coil.
- Estimate the ADP. From the psych chart (extend the process line to saturation) or the coil selection data.
- Read BF and the approach. BF against the 0.05-0.30 comfort band; approach shows how close the air gets to the coil surface.
- Act on the latent side. High BF with humidity complaints points to face velocity or coil depth — not thermostat settings.
How it works
The bypass model treats a real coil as a perfect coil plus a leak: a
fraction BF of the air slips through untouched while the
remainder leaves saturated at the apparatus dew point. Mixing those two
streams reproduces the measured leaving condition, which is what makes
the three-temperature formula work:
BF = (T_leave − ADP) / (T_enter − ADP) CF = 1 − BF
It is the fastest commissioning check on a cooling coil: three dry-bulb numbers tell you whether the coil is working the air as deeply as its selection promised. Pair it with the CFM/ΔT load calculator for the sensible capacity the same temperatures imply, and the static pressure calculator when the face-velocity questions start.
Worked example
Verified against the live calculator
Air enters at 80 °F (26.7 °C), leaves at
55 °F (12.8 °C), with an ADP of 50 °F (10 °C):
BF = (12.8 − 10) / (26.7 − 10) = 0.168 → CF = 0.832
A healthy mid-band coil: 83% of the air is effectively worked to the coil surface, with a 2.8 °C (5 °F) approach. If the same coil measured BF = 0.35 at commissioning, the usual suspects are excess face velocity (oversized blower, dirty filter bypassing through gaps) or an ADP estimated too cold.
Frequently asked questions
What is coil bypass factor?
The fraction of air that behaves as if it slipped past the coil untouched: BF = (T_leaving − ADP)/(T_entering − ADP), where ADP is the apparatus dew point (the effective coil surface temperature). The rest of the air — the contact factor, 1 − BF — leaves at the ADP. One number summarizes coil depth, fin spacing and face velocity.
What is a typical bypass factor?
Comfort cooling coils commonly run BF ≈ 0.05-0.30: around 0.20-0.30 for shallow 2-3 row coils at brisk face velocity, 0.05-0.10 for deep 6-8 row dehumidification coils. High BF means the air leaves warm and wet for the coil temperature — the classic shallow-coil humidity complaint.
What is the apparatus dew point?
The point where the coil process line, extended, meets the saturation curve on the psychrometric chart — physically, the effective average surface temperature of the wet coil. It is colder than the leaving air (the approach output shows by how much) and is not the supply air temperature or the chilled water temperature.
How does face velocity change bypass factor?
Faster air spends less time in contact with the fins, so BF rises with face velocity and falls as rows and fin density increase. That is why the same coil dehumidifies better at reduced airflow — and why oversized blowers quietly ruin latent performance.
Method & assumptions
- Dry-bulb (sensible) formulation of the bypass model; the latent picture uses the same BF on the psychrometric chart with enthalpies.
- ADP from chart construction or coil selection data — it is a model parameter, not a direct measurement.
- Steady-state, fully wetted coil assumed; partially dry coils and transient pull-down behave differently.
- Equipment selection still belongs to the manufacturer's expanded performance data (the Manual S step) — this is a field screen, not a selection tool.