An air-handling system uses outside air as its primary source of makeup air. The industry is currently using a number of different HVAC coils to accomplish the heating and cooling requirements of buildings.
These choices can have an effect on the amount of fresh air brought into the space as some HVAC coils offer higher static pressures that result in greater airflow into a building. A lower efficiency coil may not be able to provide this extra outside or return air needed to be metered into a building which could create a serious IAQ problem down the road if not discovered by either a maintenance supervisor, architect, engineer, or end-user.
What is an Outside Air Load (OAL)?
An outside air load, or OAL, is the total minimum volume of outdoor makeup air required for any given space. The base amount of OAL equals the minimum volume of outdoor makeup air needed to dilute contaminants in a space within predetermined limits such as:
- ASHRAE Standard 62.1-2007
- Minimum Volume Based on Building Use
- Mechanical Ventilation System Capacity Controlled by IAQ Management Plan
- Other Conditions Requiring Outdoor Air Supply Measurements
Why Do OALs Matter in HVAC Systems?
OALs matter for commercial and industrial HVAC system design and operation for many reasons. Indoor air quality is a high priority in green building design and OALs are critical for establishing effective ventilation requirements for HVAC systems, especially in the healthcare industry.
According to ASHRAE Standard 62-1999 (Ventilation for Acceptable Indoor Air Quality), “The primary goal of natural or mechanical ventilation is to provide acceptable indoor air quality at reasonable energy costs while avoiding the inherent problems of overventilation. To accomplish this goal, HVAC systems must provide an adequate outdoor ventilation rate.”
ASHRAE 62-1999 also provides minimum ventilation rates for acceptable IAQ based on occupancy type and space use. For example, under normal circumstances (nonhazardous activities, normal office space), at least 15 cfm/person must be supplied to a space for each occupant in the space, assuming a work activity rate of 50 cfm.
This requirement is only the minimum. In order to achieve acceptable IAQ and ensure that occupants are not exposed to pollutants or an excessive concentration of contaminants from recirculated building air, higher ventilation rates are typically specified.
Determining the Best Coil Selection
In the real world, there are all sorts of conditions that can affect a coil, and it’s very difficult to account for every scenario. The best way to determine the best selection is a simulation program that allows you to set your criteria and test results based on those criteria. For example:
- If your outside air load is under 0.5 cfm/sqft, you might consider an internally economized coil as part of your design
- If you have R-8 walls or more, consider two separate zones with one system serving each zone from their respective supply/return registers
- When considering different systems, ask yourself ‘what is my main objective?’ If it’s first cost, then externally managed coils will be more economical, but if it’s life cycle cost then internally managed coils will be more economical.
- For things like comfort, create your design based on your main concern and change the other system requirements to fit within that design criteria.
Factors to Consider When Selecting Coils:
- Airflow: For each CFM of outdoor air, you’ll need an additional .5-1.0″ WC of cooling and heating capacity
- Return temperature: The coil’s ability to remove moisture is directly proportional to the temperature rise across the coil and inversely proportional to outside air quantity and total system airflow
- Air Quality Loadings: Outside air can produce some undesirable conditions such as musty odors or fogging problems if not properly treated
- Capacity for extra fans: If your design requires more than one supply fan it will be important to select a coil with enough capacity so as not to limit system airflow capacity
Factors That Should NOT Affect Your Decision on Coils You Choose:
- Condenser noise: A coil selection should never be made based on noise level alone. If the unit is to operate in a noisy environment, install silencers or add refrigerant to achieve an acceptable operating range
- Operating sound levels: Most manufacturers will provide sound data at both high and low capacities; however, these design conditions may not be representative of your application. The sound level produced by a system is affected by supply-air temperature, duct configuration, type of fan operation, fans running at different speeds, and other factors. The only way to determine the sound levels produced by a given installation is to conduct field tests
- Room size: Room size should not be used as the sole selection criteria for any cooling or heating coil. Good airflow management can help reduce room air distribution problems caused by the under-sizing of COOLING coils. Similarly, careful control of airflow distribution will minimize overheating in certain areas of larger rooms served with HEATING coils.
Types of Coils Available for Use
There are a few different types of coils available in applications where there is an outdoor air load. Each type of coil offers its own advantages and disadvantages.
Consult an HVAC/R Coil Manufacturer
Rahn Industries provides a comprehensive line of HVAC/R coils for use in residential and commercial applications. Our team has extensive experience with the latest coil technologies, which means we can help you select the best coil solution for your needs. Whether it’s air-cooled or water-cooled; single-row or multi-row; round, square, oval, rectangular shapes…we have what you need. Give us a call at (800) 421-7070 to speak with one of our knowledgeable representatives about how our products are designed to optimize efficiency costs without sacrificing durability.