A cooling coil dehumidifies by cooling air to its dew point; to hold a room's humidity without over-cooling it, the air must be warmed again afterwards. Face-and-bypass does that without a heater: a damper sends part of the air across the coil face and the rest around it, and the two streams mix back to the supply temperature the room needs. The bypassed air is warm and dry enough; the coil air is cold and dry; the mix lands on both targets.

It is the arrangement a cleanroom or a museum handler has, because the reheat is free. In a building simulation a split ahead of a cooling coil that re-merges behind it — the fan's outlet wired to the coil and to the plenum, the coil's outlet to the plenum — is recognised as one damper, and a humidity ceiling on the room is held by bisecting the share of air over the coil — the heater stays off. On the example cleanroom it takes the reheat from 273 MWh a year to none.

Open the builder — the examples under ＋ Open include a machine of this kind, and a building tab runs it against a year of weather.

Other terms

DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year · Superheat · Subcooling · Sensible heat ratio (SHR) · Bypass factor · Wet-bulb temperature · Dew point · COP and EER · Compressor map · Evaporating and condensing temperature · Range and approach · Heat-recovery (three-pipe) VRF · Typical meteorological year (TMY)