Humidity ceiling and floor. A room's upper and lower bounds on moisture; the ceiling makes the machine over-cool and reheat (or bypass), the floor steers a humidifier. Humidity: ceilings and floors.
A room's comfort has a moisture side. ASHRAE 55 bounds the humidity ratio at 0.012 kg/kg — a limit on moisture, not on relative humidity, which moves with temperature — and an operating suite or an archive holds a floor too. A ceiling makes the machine over-cool its supply to a lower dew point and warm it back, by a heater or a bypass damper; a floor makes a humidifier add steam to the supply.
In a building run each is a second requirement the controller judges every hour beside the temperature: the moisture balance of the room is inverted into the supply humidity that lands it on the bound, and the machine is modulated on both reads. Two rooms on one machine can ask for different things; the driest ask wins the ceiling, the wettest the floor. Rooms without a bound simply float, and the report counts the hours above 0.012 kg/kg.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year