DOAS — dedicated outside-air system. A machine whose only job is the ventilation air: 100 % outside air, recovered against the exhaust, dried once at the door and delivered neutral, the rooms' sensible load carried by other machines. Kinds of systems → Recovery unit and DOAS.
A DOAS separates the two jobs an air handler usually does at once. The ventilation air — the outside air a building must bring in for its occupants — is treated by one machine sized for exactly that flow: recovered against the exhaust in a plate core or a wheel, dried once at the door (often with a wrap-around heat pipe so the drying needs no reheat), and delivered at a neutral temperature. The rooms' own sensible loads — people, lights, sun — are carried by other machines: fan coils, VRF, a rooftop, radiant panels.
The reason it pays is latent load. Outside air on a humid day carries most of a building's moisture, and a machine that only has to dry that stream can be small, run at a steady state and hold a dew point precisely; a mixed-air unit has to over-cool its whole supply to get the same moisture out, then reheat it. In a building simulation a DOAS is the machine you give priority on the rooms it serves, so it carries the latent load and the machine behind it only what is left.
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.
SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year