4 Important Benefits of a DOAS

What's a DOAS?

Simple put a DOAS is a "Dedicated Outdoor Air System". What it doas, i mean does, is it manages outdoor air quality. Outdoor air is a necessary component of any well-designed HVAC system. In many climates however the air outside is either so full of moisture or so contaminated that bringing it into a building can create problems for a building’s occupants and contents. Many engineers today have concluded that the best way to handle these issues is to use an HVAC unit that specifically solves these problems.

Benefits:

1. Prevents "sick-building illnesses"

Sick-building illnesses can most often be traced to a lack of effective ventilation air in the occupied spaces. Did you know that US companies lose as much as $48 Billion annually to cover medical expenses and $160 Billion annually in lost productivity as a result of sick-building illnesses? Wow! For these reasons ASHRAE has developed the ventilation requirements in Standard 62.1. Those gases caused by construction materials, fumes from office equipment, and normal build-ups of CO2 from human occupants, all need to be exhausted outside.

2. Better Humidity Control

Human occupants and certain machines in a building will generate moisture that must be controlled, but the largest element of moisture in a building comes from the outside. If this is unabated, mold and mildew could build-up in the conditioned space. Bear in mind that a good comfort index is determined by a combination of the dry bulb temperature, local air velocity, and moisture content of the air (even if you neglect the issue of molds). All three factors must be addressed in a well-designed building.

3. Limitations of conventional equipment

Let us assume that most commercial buildings use rooftop packaged equipment. Some of their limitations are as follows:

• Conventional rooftop units are designed to provide 350 to 450 cfm/ton, but effective dehumidification typically requires only 200 to 300 cfm/ton. ASHRAE 62.1 (section 5.10) limits RH to 65% or less at design dewpoint.
• Dehumidification and drain pans. Coils in conventional rooftop units are typically only 3 to 4 rows deep with a drain pan that extends just past the end of the coil. Effective dehumidification often requires coils up to 8 rows deep. It is also important to provide additional drain pan depth and IAQ-style designs in order to properly handle the amount of moisture that will be removed. ASHRAE 62.1 (section 5.11) requires that drain pan lengths be at least ½ of coil height or designed to limit moisture carryover to .0044 oz.per coil sq.ft. per hour at peak dewpoint condition. On most units this means that the drain pan must be at least 18” long and could approach 30” in larger tonnage units. This will almost never be found in a conventional packaged rooftop unit.
• You can have better filtration with a DOAS.
• Better operational flexibility especially when the outside conditions are beyond stable.

4. LEED Certification

One added bonus of employing the use of DOAS is LEED-NC2.2. But in order to comply with requirement, the criteria for indoor air quality must be met in accordance with ASHRAE 62.1. The application of a DOAS system generates up to 80% of the points required for a LEED certification.

Well there you have it folks. I will discuss some points further on the DOAS in my succeeding blogs. Be sure to subscribe. Thanks for reading.