Dedicated outdoor air system (DOAS), is defined by the ASHRAE Journal as a system which uses separate equipment to condition all outdoor air (OA) brought into a building for ventilation, delivered to each occupied space, either directly or in conjunction with local space or central (zoned) HVAC units serving those same spaces. The local or central HVAC units are used to maintain space temperature setpoint requirements.
In this blog post, we will take a look at the benefits of using DOAS when compared to the conventional variable air volume (VAV) system.
Here are the main benefits discussed:
- DOAS provides better ventilation
- Decoupling sensible and latent cooling functions of air handling systems
- More freedom when selecting local units
- Reduction in energy use
- Enhanced indoor environmental quality (IEQ)
One of the main challenges with achieving efficient ventilation with a central HVAC system like a VAV system which serves multiple zones is that the sensible loads in each zone don’t necessarily vary with the ventilation requirements.
The net effect here is a system-wide increase in ventilation intake in order to comply with ASHRAE Standard 62.1. The benefit of using DOAS to supply ventilation air is that OA needs to be direct to each occupied space or in conjunction with local HVAC units.
Over-ventilation is another common problem with a VAV system. With a centralized ventilation system, a single air handler supplies the OA and recirculated air to multiple zones. This mixture is calculated according to the requirements of the critical zones which results in proper ventilation to these zones but in turn, means noncritical zones are over-ventilated. With DOAS, however, each zone is supplied with ventilation air directly, effectively eliminating this problem. The DOAS can be sized according to the needs of each zone ensuring the most efficient and effective ventilation for every zone.
Humidity control is not the primary objective of a DOAS system but certainly is an added benefit. ASHRAE Standard 62 increased earlier OA requirements in 1989, which means that since then proper indoor humidity control is harder to achieve with some types of HVAC systems. Relative indoor humidity can be increased by introducing more OA into a building in non-arid climates. This also brings in more moisture into the system since OA accounts for a large portion of the overall latent load in many commercial buildings.
DOAS uses two systems to control temperature and humidity individually. The DOAS unit, which controls the conditioning of the ventilation air, can be sized to supply the exact amount of ventilation air required at a low enough dew point to combat the indoor latent load, effectively controlling the indoor relative humidity level without the need of an HVAC unit.
During part-load operations, engineers often reduce the amount of air delivered to space in order to bolster energy efficiency. This can, however, pose a problem when the loads change with a loss of control of the indoor humidity. This can occur even if the temperature remains at an acceptable level. DOAS circumvents this problem by running local units at reduced capacity to meet the reduced sensible loads under part-load conditions. The DOAS unit continues to supply air at a low enough dew point to maintain acceptable indoor humidity levels.
More freedom when selecting local units
By using a DOAS unit to control the indoor humidity and supply OA to the occupied spaces, designers and specifiers have the flexibility to choose from a wider range of local units. If all of the latent loads are controlled by the DOAS, it is possible to use a highly efficient sensible-only HVAC unit. This will certainly increase the overall energy efficiency of the system.
Reduce Energy Use
With a DOAS, there is an increased opportunity to reduce the energy used for ventilation, air conditioning and fan operations as the system does not need to condition as much OA as is the case with a VAV system.
In certain instances, like that of part-load conditions, the local unit may be switched off to save energy when the DOAS delivers ventilation air directly to the occupied zones.
Jeong reported a simulation study comparing the energy and cost performance of a DOAS with parallel ceiling radiant panels to a conventional VAV system with an economizer for a 300 m2 office space in an educational building in Pennsylvania.
They reported a 42 % reduction in annual energy use for the DOAS system with substantial savings in both fan and chiller energy use.
Enhanced indoor environmental quality (IEQ)
It is generally viewed that by decoupling temperature control and ventilation it is possible to achieve better IEQ. In addition to that, DOAS helps reduce the number of pollutants transferred between spaces because of the predictable pressure differences between adjoining spaces.
Fischer and Bayer conducted a study in 2003 of the indoor air quality (IAQ) and humidity control performance in 10 schools in Georgia. The study included five schools with conventional systems and five with DOAS based on the dual wheel energy recovery combination described by Mumma (i.e., an enthalpy wheel, a cooling coil and a sensible wheel). They reported that the schools with conventional systems suffered from poor IAQ (as indicated by higher average levels of total volatile organic compounds (TVOCs), formaldehyde and CO2) because the systems provided far less than the ASHRAE Standard 62-1999 required level of outdoor air. The reason for this reduction was that the conventional systems could not adequately maintain space humidities at the higher ventilation level. In contrast, the five schools with DOAS were reported to have lower levels of measured contaminants as the systems could provide acceptable humidity conditions while supplying 7.5 L/s per person of ventilation.
The schools with DOAS were also reported to have improved comfort and perceived IAQ with lower average absenteeism.