ASHRAE 62 is the worldwide standard used to calculate outside air requirements in commercial buildings. Within ASHRAE 62 there are two methods for determining outside air requirements; Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP).
The VRP is the most commonly used method for determining outside air requirements. The outside air is calculated based on the zone use, the maximum number of occupants, the square footage of the zone and the ventilation effectiveness. The amount of outside air per person is provided in Table 6.1 in the ASHRAE Standard. The amount of outside air required per square foot is also found in Table 6.1. The ventilation effectiveness can be found in Table 6.2 and is based upon the supply and return duct locations as well as the temperature difference between the supply air and the space temperature.
Using a classroom as an example with students ages 9+, the outside air required is 10 CFM per person and 0.12 CFM per square foot. If the ventilation effectiveness is 0.8, the outside air required to the space would be as follows for 30 students and an 800 sqft space:
(10 X 30 + 0.12 X 800) / 0.8 = 495 CFM of Outside Air
The cooling and heating equipment would have to be sized to handle 495CFM of outside air.
In an effort to save energy using the VRP, ASRHAE 62 will allow the outside air to be reduced based on an occupancy sensing technique such as carbon dioxide (C02) Demand Control Ventilation (DCV). This allows the outside air to be “reset” based on the C02 levels. Most controls systems are set up to maintain a difference of 700PPM differential of C02 between inside and outside. If the outside level of C02 is 300PPM, and considering a 700 PPM differential, the C02 DCV setpoint would be 1,000 PPM. If only a few people were in the space, the outside air would be reduced to maintain the 1,000PPM setpoint and much energy would be saved, assuming the HVAC equipment was designed to work efficiently at part load. In some situations, DCV with the VRP can lead to higher humidity conditions because standard packaged rooftop units cannot off-load sufficiently and that leads to large temperature and humidity swings as a large compressor is energizing to cool/dehumidify a much smaller load. The designer must take precaution as to how the selected equipment will operate during part-load conditions.
The IAQP is used less frequently than the VRP. Why you may ask? The primary reason is that the IAQP usually requires more time. In addition, engineers are most often trained only on the VRP. The IAQP has the potential to save the most equipment cost on new and retrofit projects plus provide much higher energy savings due to the ability to reduce outside air by adding air purification and cleaning the air in the building, thus allowing the air to be recirculated and thereby reducing the need for higher outside air quantities. Unlike the VRP, the IAQP does not look at the amount of outside air per person or per square foot, instead it requires reviewing the contaminants of concern and running mass balance equations to determine the steady state conditions of contaminants based on a filtration effectiveness for the chemicals modeled.
In an effort to reduce the consulting engineers time, GPS has developed an IAQ spreadsheet based on ASHRAE 62. The spreadsheet calculates both the VRP outside air requirements and the IAQP requirements simultaneously. All of the mass balance equations and the effectiveness of GPS’ products are already included in the spreadsheet for each chemical modeled. In a matter of a few minutes, one can see how low you can go with the outside air using the IAQP. Generally, the pressurization requirements of the building is the limiting factor as to how low you can go with the outside air. This method applies to all commercial, non-healthcare, applications. Over 600 projects have been designed using GPS’ IAQ spreadsheet.
The Tampa Bay Times forum saved over 700 tons in chiller capacity, which resulted in over $1 million in first cost savings and over $100,000 per year in energy! K-12 applications with 100,000 square feet usually save $350,000 to $500,000 in first cost and over $0.30/sft per year!
In addition to the reduced outside air, DCV can be used with the IAQP. This combination will provide for the most energy efficient building possible while providing exceptional IAQ. Since the cooling/dehumidification equipment is downsized during design, part load conditions do not cause the same “hunting/cycling” that occurs when using the VRP.
Considering how polluted outside air can be, it makes complete sense ($cents$) to reduce outside air, recirculate the indoor air that’s already conditioned and simply purify the indoor air. Here is a link to a website that shows how polluted the outside air may be at your location.
Frequently Asked Questions About Reducing Outside Air
- Will the indoor levels of C02 be higher? Yes. When reducing outside air, C02 levels will be higher.
- Are higher C02 levels harmful? No. When using the IAQP, C02 is only an indicator of occupancy, not the indoor air quality. As you can see from the chart below, when using the IAQP, as C02 levels increase, the other contaminants of concern (VOC’s) are controlled to a minimum steady state level due to GPS’ cold plasma technology.
- How can you be sure higher levels of C02 isn’t harmful? When using the VRP, higher levels of C02 will indicate poor air quality since the contaminants of concern will not be removed due to no air purification being applied. When using air purification to remove the contaminants, like on a Navy submarine, the C02 levels can run higher. The ASHRAE 62 limit is 5,000 PPM when using the IAQP, but at the minimum outside air required for building pressure requirements (usually 3-5 CFM per person of outside air), the C02 will be around 1,500 PPM to 2,000 PPM, well below the limit. To further prove the safety of higher C02 levels when using air purification, the National Research Council was commissioned by the US Navy to prove that personnel subjected to the usual 8,000 to 10,000 PPM of C02 on submarines would cause no issue. The study found that C02 levels of 8,000 PPM, when combined with air cleaning, had no affect on humans, whatsoever. Below is a summary from that study.