IAQ, energy efficiency, and LEED
How to balance IAQ and energy efficiency while earning LEED points.
The LEED program considers minimum IAQ a prerequisite in the pursuit of project LEED certification, and requires compliance with ASHRAE standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality, sections 4 through 7 . The required documentation for this prerequisite includes completing the LEED submittal template, providing a description of the ventilation system, and submitting a calculation used to establish the ventilation rate of the HVAC system.
Completing the submittal template is very straightforward and requires no more than 10 minutes. Included is a short narrative describing the configuration of the HVAC system and associated operating parameters, written up as a single paragraph. At the same time, the ventilation rate calculation, a standard effort executed by all HVAC engineers, is a portion of the submittal that can be a little more intimidating if you’re not familiar with the procedures. We’ll discuss this calculation procedure in further detail, identifying its significance in energy-conscious design.
ASHRAE Standard 62.1-2004 provides the engineer with a procedure and a spreadsheet template for calculating the required ventilation air for each HVAC zone. A completed calculation, addressing all appropriate spaces, will indicate the minimum allowable percent of ventilation air to be provided by the HVAC system. Essentially, ventilation air is required to remove carbon dioxide and room contaminants from the indoor air, and this standard provides two methods for establishing the amount of outside air for an HVAC system.
The IAQ procedure, which is a performance-based analysis, focuses on controlling concentrations of containments to achieve acceptable IAQ, and requires knowledge of space and outdoor air contaminants. The other approach, the ventilation rate procedure, is a prescriptive method combining the occupant related ventilation expressed in cfm/person with the building related ventilation expressed in cfm/sq. ft for each room occupancy and room function.
Continuing with the ventilation rate procedure, determining the minimum percentage of ventilation air delivered by a HVAC system, which serves multiple spaces with varying occupancy densities, requires the identification of each room type, the zone air distribution effectiveness factor, the room supply air rate, the room minimum supply air rate, and room occupancy all entered into the ASHRAE 62.1 spreadsheet , which calculates the overall system ventilation efficiency and the associated minimum outside air rate of the air handling system. The variable among these parameters is the zone air distribution factor, which is a measure of the system’s ability to effectively mix supply and room air, and changes as a result of our selection of supply and return diffuser positions and the supply air temperature gradient above space temperature in the heating mode for perimeter spaces. We recognize that the higher the zone air distribution effectiveness factor, the lower the percentage of outside air required to provide adequate ventilation to the variety of spaces.
The calculation template takes into account the multiple space equation, which credits the over-ventilation of less densely occupied adjacent spaces diluting the return air with ventilation air, thereby requiring an overall lower percentage of outside air by the air handling unit. The ventilation rate procedure should be performed for each zone with a different occupant density, supply air rate per unit area, zone air distribution effectiveness, ventilation rate per unit area, and ventilation rate per occupant. Further review of the standard will identify system air distribution configurations, which will result in zone air distribution effectiveness ranging from 0.5 to 1.2. This wide range of zone air distribution effectiveness illustrates the significant impact that the air distribution configuration has on the ventilation rate of a HVAC system and the energy usage associated with the resulting ventilation rate.
Consider a simple single zone system, which includes a 1,000 sq. ft office, with five occupants and a room supply air rate of 1,000 cfm. Per the ASHRAE Standard, each occupant requires a minimum ventilation rate of 5 cfm and an area ventilation rate of 0.06 cfm/sq. ft. The resultant ventilation rate would equal 85 cfm or 17 cfm/person and the air handling system would deliver at least 8.5% outdoor air.
Now, let’s say this room is equipped with ceiling supply and return diffusers and no limit to the heating supply air temperature which results in a zone air distribution effectiveness of 0.8, which means 80% of the supply air reaches the breathing zone of the conditioned space. As a result, our air handling unit will be required to provide a minimum of 10.6% ventilation air, instead of 8.5%, which is 25% more.
If we rethink the application and our system is designed to control the heating supply air temperature to a maximum of 15 F above room temperature, and the diffuser selection satisfying specified terminal velocities promoting air mixing, we would be able to achieve a zone air distribution effectiveness of 1.0 and lower the air handing system ventilation rate to 8.5%. The reduction in ventilation air will reduce our energy consumption associated with conditioning a lower outside air rate.
We can then take this effort a step further and consider pursuing the LEED credit for Increased Ventilation EQc2, which requires increasing the breathing zone ventilation rate by 30%. In satisfying this credit, our HVAC system would be required to provide 11% ventilation air. This increased ventilation rate would improve the overall IAQ within the building, but would use more energy. Offsetting this increased energy consumption would warrant considering energy recovery devices in the air handling system%%MDASSML%%a topic for another day.
Continuing the LEED submittal process
This discussion illustrates that acceptable IAQ can be achieved through proper planning and documentation. We have satisfied the calculation documentation associated with the prerequisite for IAQ and one credit for increased ventilation by simply completing a single calculation exercise on required ventilation. The calculation procedures dictated by ASHRAE Standard 62.1-2004 are standard tools in establishing the minimum ventilation rate of an HVAC system and are accepted as the standard of care in our industry and therefore require no extra effort above our normal practices.
It should be noted that this discussion of ASHRAE Standard 62.1-2004 is on establishing the ventilation rate of an air handling system and is not a replacement to reading the standard. The standard addresses several additional factors to IAQ, including a section on outside air quality, indicating the maximum concentration of outdoor air contaminants, systems, and equipment. It also provides an overview of system components that can effect air quality including intake/exhaust separation requirements and construction and system start-up, all ensuring the quality of the outside air used for ventilation.
The standard also provides data regarding indoor air contaminant concentrations for use in the IAQ procedure calculation and identifies various building types and their respective occupancy categories, and provides recommended ventilation rates in cfm/person and cfm/sq. ft for these occupancies. The latter is used in the ventilation rate procedure spreadsheet template, which is available from ASHRAE. The governing building code may require higher ventilation rates than prescribed by ASHRAE Standard 62.1-2004, and if so, should establish project requirements.
LEED-NC for New Construction, Reference Guide Version 2.2
ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality
Q cfm = 5 cfm/person x 5 people + 0.06 cfm/sf x 1000 sf = 85 cfm
Q % = 85 cfm / 0.8 zade
Q % = 85 cfm / 1.0 zade
Q % = 85 cfm / 1.0 zade