Two numbers that sound like they measure the same thing. One describes the space's load. The other describes the coil's physical design. Confusing them is how a thermostat gets satisfied while the room stays humid.
It's an easy mix-up: both Bypass Factor (BF) and Sensible Heat Ratio (SHR) get described as "how much of this is sensible vs. latent," so it's tempting to treat them as interchangeable. They aren't measuring the same thing at all. SHR is a property of the space — driven by occupancy, ventilation rate, and building envelope, it says what fraction of the room's actual cooling load is sensible versus latent, with no reference to any specific piece of equipment. Bypass Factor is a property of the coil — driven by fin spacing, face velocity, and row count, it says what fraction of the air passing through that particular coil skips past its surface without full contact, leaving less conditioned than the air that did. One is about what the room needs. The other is about what a piece of hardware physically does to the air moving through it.
A cooling coil doesn't condition air uniformly. Picture the entering airstream as splitting into two paths through the coil. A contact stream weaves through the fins closely enough, and long enough, to approach the coil's apparatus dew point (ADP) — the theoretical saturated condition the coil surface itself sits at. A bypass stream slips through wider gaps between fins, around the edges of the coil face, or past it too quickly to fully equilibrate — leaving nearly as warm and moist as it entered. The two streams recombine downstream into a single mixed condition: the coil's actual leaving (supply) air state. Bypass Factor is simply the fraction of air that took the bypass path— and it places the leaving air state on the straight line between the entering condition and the coil's ADP, at a distance from ADP set directly by BF.
Sensible Heat Ratio lives entirely on the other side of the wall from the coil. Every space generates a cooling load made of two components: a sensible component (heat that raises dry-bulb temperature — solar gain, lighting, equipment, envelope conduction) and a latent component (moisture that has to be removed — people breathing and sweating, outdoor air ventilation, process moisture). SHR = Q_sensible ÷ Q_totaldescribes that split. A packed, poorly-ventilated conference room has a low SHR — lots of latent load from occupants breathing and little sensible gain. A sun-facing office with few people has a high SHR — mostly sensible, almost no latent. None of this involves a cooling coil at all; it's a property of the room's occupancy, ventilation rate, and envelope, calculated during load calc long before any equipment gets selected.
Selection software and Manual S-style procedures don't just size for total tons — they check that the equipment's SHR at design conditions matches the space's calculated SHR. Because the leaving air state sits on the line between entering air and the coil's ADP at a distance set by BF, a lower-BF coil selection pulls the leaving condition further toward ADP — more dehumidification, more latent capacity, lower resulting SHR for the equipment. A higher-BF coil selection leaves more air only lightly conditioned — higher resulting equipment SHR, less latent capacity. That's the whole point of specifying rows, fin spacing, and face velocity deliberately: a low-bypass-factor coil is chosen specifically becausethe space's load has a low SHR and needs the extra dehumidification, not because low BF is inherently "better." A high-SHR space with mostly sensible load is often better served by a higher-BF coil running more airflow — forcing unnecessary dehumidification there just adds reheat energy and cost for no comfort benefit.
No — they describe two different sides of the same problem, and treating them as synonyms is exactly what leads to a coil that's wrong for the room it serves. SHR is a property of the space's load: it's set by occupancy, ventilation rate, and envelope, calculated in the load calc, and it exists whether or not any equipment has been chosen yet. Bypass Factor is a property of the coil's physical construction: it's set by fin spacing, row count, and face velocity, and it exists whether the coil is installed in a humid gymnasium or a dry, low-occupancy server room. A coil doesn't "have an SHR" on its own the way it has a Bypass Factor — its resultingsensible/total split when serving a specific airstream is what selection software reports, and that number only becomes meaningful once it's compared against the space's own SHR requirement. Correct equipment selection is the act of choosing a coil BF low enough (or high enough) to make the coil's resulting split match what the space actually needs — not assuming the two numbers were ever measuring the same thing to begin with.
Explains the difference between Coil Bypass Factor (BF), a physical property of a cooling coil driven by fin spacing, row count, and face velocity that describes what fraction of air skips past the coil surface without full contact, and Sensible Heat Ratio (SHR), a property of a space's cooling load driven by occupancy, ventilation, and envelope that describes what fraction of the load is sensible versus latent — and why correct equipment selection requires deliberately matching a coil's BF to a space's target SHR rather than treating the two numbers as interchangeable.
Both Bypass Factor and Sensible Heat Ratio get summarized in the field as "how sensible vs. latent this is," which invites treating them as two labels for one concept. In reality they sit on opposite sides of the equipment-selection process: SHR is calculated for the space during the load calculation, before any equipment is chosen, purely from occupancy schedules, ventilation air quantity, and envelope/solar gains. Bypass Factor is a physical characteristic of a specific coil's construction — its fin spacing, number of rows, and design face velocity — and exists independently of whatever space it might eventually serve. Conflating the two leads to selecting equipment on total capacity or a rough sensible/latent split without checking whether the coil's actual bypass factor can deliver the dehumidification the space's SHR demands.
A cooling coil's leaving air condition is a mass-weighted mixture of two limiting streams: air that fully contacts the coil surface, approaching the coil's apparatus dew point (ADP — the theoretical saturated condition of the coil surface itself), and air that bypasses the coil with little contact, leaving nearly at the entering condition. Bypass Factor (BF) is the fraction of air following the bypass path; the resulting leaving-air state lies on the straight line connecting the entering condition to the ADP, positioned a fraction BF of the way back from ADP toward the entering point. Lower BF (achieved with more rows, tighter fin spacing, and lower face velocity) forces more air into contact, pulling the leaving condition closer to ADP and increasing latent capacity for the same total tons. SHR, by contrast, is simply Q_sensible ÷ Q_total for the space's load — governed by heat transfer mechanisms (conduction, solar radiation, internal gains) and moisture generation (occupant metabolism, ventilation air moisture content) that have nothing to do with any coil's geometry.
Equipment selection software and Manual S-style procedures check a coil's resulting sensible/total split against the space's calculated SHR at design conditions — not just total tonnage. A space with a low SHR (heavy latent load, such as a densely occupied auditorium or a space with high outdoor-air ventilation rates) needs a coil selected with a low enough Bypass Factor to deliver adequate latent capacity; selecting on total capacity alone can pass paper calculations while leaving the space humid. Conversely, forcing an unnecessarily low-BF coil onto a high-SHR (mostly sensible) space over-dehumidifies the air, often requiring wasteful reheat to avoid overcooling — a real energy penalty for no comfort benefit. Getting BF and SHR matched, rather than conflated, is the difference between equipment that satisfies both the thermostat and the hygrometer.
Bypass Factor (BF) is the fraction of air passing through a cooling coil that doesn't make enough contact with the coil surface to be conditioned down to the coil's apparatus dew point. It's driven by the coil's physical design — fin spacing, number of rows, and face velocity — not by the space it serves. A well-selected comfort-cooling coil typically runs a BF around 0.10–0.20; a coil with fewer rows or higher face velocity runs a higher BF.
SHR is the fraction of a space's total cooling load that is sensible (temperature-only) versus latent (moisture), calculated from occupancy, ventilation rate, and envelope/solar gains — entirely independent of any coil. Bypass Factor is a physical property of the coil's construction. They describe different sides of the same selection problem: SHR tells you what the space needs; BF tells you how effectively a specific coil can deliver it.
No. Lower BF means more dehumidification capacity for the same total tons, which is exactly what a low-SHR (high-latent-load) space needs. But forcing an unnecessarily low-BF coil onto a high-SHR (mostly sensible) space over-dehumidifies the air, often requiring reheat to avoid overcooling the space — wasting energy for no comfort benefit. The correct BF is whichever value lets the coil's resulting split match the space's actual SHR target, not the lowest possible number.
The coil's leaving air state lies on the straight line connecting the entering air condition to the coil's apparatus dew point (ADP), positioned a fraction equal to BF of the distance back from the ADP toward the entering condition. A BF of 0.20 means the leaving state sits 20% of the way from ADP back toward entering air — closer to fully conditioned than a coil with BF 0.40 would achieve.
Yes — this is exactly the mechanism that connects to Sensible Heat Ratio. A coil with fewer rows, wider fin spacing, or higher face velocity can carry the same total capacity as a coil with more rows and tighter fins, but with a higher Bypass Factor, meaning proportionally less of that capacity goes toward latent removal. That's why total tonnage alone can't tell you whether a coil will dehumidify adequately for a given space's SHR.
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