Direct Expansion-Dedicated Outdoor Air Systems


Direct expansion-dedicated outdoor air systems (DX-DOAS), a category of commercial package air conditioning and heating equipment, are defined by their capability to provide ventilation and conditioning of 100% outdoor air. Often, a DX-DOAS is installed to provide dehumidification (latent heat removal) and a separate cooling unit, such as a commercial unitary air conditioner (CUAC), performs sensible cooling. DX-DOAS includes air-cooled ACs, air-source heat pumps, water-cooled ACs (with a cooling tower or chilled water), and water-source heat pumps (ground-source, ground-water source, and water-source)—each with and without ventilation energy recovery systems (VERS). VERS precondition the (outdoor) air entering the DX-DOAS through sensible heat transfer and/or moisture exchange with the building exhaust air. 

DX-DOAS are a category of ASHRAE equipment. 


The publication of the 2016 version of ASHRAE 90.1 triggered DOE’s obligation to set standards for DX-DOASes. DOE established standards for DX-DOAS in a final rule issued in late 2022. Compliance with the standards will be required beginning May 1, 2024. The standard levels are identical to the levels in the 2019 version of ASHRAE 90.1, which are expressed in integrated seasonal moisture removal efficiency (ISMRE) and integrated seasonal coefficient of performance (ISCOP), which DOE translated to updated metrics, ISMRE2 and ISCOP2. 

*The cooling energy efficiency metric, integrated seasonal moisture removal efficiency 2 (ISMRE2) is a weighted calculation of moisture removal efficiencies (moisture removal capacity divided by total power input) at different outdoor air conditions. It is expressed in pounds of moisture per kWh. 

The heating energy efficiency metric, the integrated seasonal coefficient of performance 2 (ISCOP2), is a weighted calculation of the ratio of the heating capacity in watts to the power input values in watts at 47 °F and 17 °F. It is expressed in W/W.


Technology options for improving the efficiency of DX-DOASes include variable-capacity compressors and larger condenser heat exchangers (or more fins).


Federal Date States

Timeline reflects state standards from 2001 to present; federal standards from inception to present.