By Ellis G. Guiles, Jr. P.E.
Figure 1. ASHRAE's climate zones across the U.S.
Airside economizers have been an integral part of building HVAC systems for more than five decades, yet in many ways they're still misunderstood, misapplied, and — in many cases — operating poorly. We install them because we believe they provide a level of energy reduction and perhaps improved indoor air quality. However, how confident are we they're actually accomplishing these goals?
An airside economizer is defined by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) as, "A device that, on proper variable sensing, initiates control signals or actions to conserve energy." ASHRAE Standard 90.1-2004 states an airside economizer is a "duct-and-damper arrangement and automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather." As you can see from these definitions, the primary function of an airside economizer is to conserve energy. Yet, how many of us have witnessed installations where controls have been disabled, dampers disconnected, or had their openings blocked with wood or cardboard?
It's time for a review of the codes, guideline requirements, and performance of airside economizers.
The Standard of Care
Energy codes and guidelines, such as ASHRAE Standard 90.1, can be extremely confusing when it comes to the issue of airside economizers. While not a code, ASHRAE 90.1 is considered to be "the standard of care" professional engineers and contractors must employ in their daily practices. It includes lengthy comments and guidelines covering the use and application of airside economizers. Section 6.5.1 of the standard indicates airside economizers "must" be used in all cooling systems with fans. However, this same section includes nine exceptions to the "must" requirement.
Let's start with the requirements. Figure 1 represents the various climate zones in the U.S. Climate zones 5 , 6, and "Moist" require economizers for systems of more than 11 tons. Climate zones 2, 3, 4, 5, 6, and "Dry" require economizers when the system is at or above 5 tons. Economizers are not required in climate zones 1, 2, 3, 4, and "Warm/ Humid." In addition to the climate zone requirements, there are seven other applications for system designers and contractors spelled out in the ASHRAE standard when it comes to airside economizers:
- We must provide the ability for the system to provide 100% of the design airflow via outdoor air. This allows the economizer to deliver 100% of the cooling load under the proper outdoor and indoor conditions.
- A control signal must be used to sequence between mechanical cooling and economizer cooling, and the signal shall not be controlled only by mixed air temperatures. There is one exception to this, and we'll discuss it shortly.
- A high limit shut-off must be used and it will stop economizer operation when cooling loads can't be reduced by using outdoor air.
- Dampers must have shut-off controls and leakage rates as shown in Table 188.8.131.52.4 of ASHRAE 90.1 (to view this table, visit www.contractingbusiness.com/airside). This could be especially challenging to determine compliance because few manufacturers list the leakage rates of economizer dampers used in packaged equipment such as rooftop units.
- Relief of excess outdoor air must be provided to prevent building over-pressurization. Building over- or under-pressurization can cause indoor air quality problems and increase operating expenses.
- Economizer controls shall be integrated with mechanical cooling. Integration means the controls will allow simultaneous operation of mechanical and economizer cooling. Integrated controls allow the partial use of the economizer to meet the load while mechanical cooling is also occurring.
- An economizer shall not increase the heating energy required by the building during normal operation. How this is achieved is as much a matter of proper implementation of the controls as regular maintenance of the system. Think of the number of economizers we've all seen with broken linkage or actuators, dampers propped in the open position, etc. Obviously, all of these situations will result in an increase in building heating energy.
Exceptions to the Rules
While these seven minimum requirements all seem to be fairly reasonable to most HVAC professionals, the nine exceptions aren't always obvious and can make the decision to use an economizer far more difficult than we might think it should be.
The first exception is based upon climate zone and equipment size. Table 6.5.1 from ASHRAE 90.1 shows the various climate zones and the equipment sizes that do not require economizers (to view this table, visit www.contractingbusiness.com/airside).
The following applications also do not require an economizer:
- Equipment or systems using gas-phase outdoor air cleaning to meet ASHRAE Standard 62
- Equipment or systems delivering more than 25% of the supply air to spaces, with humidity levels above 35% for process applications
- Equipment or systems with condenser heat recovery
- Residential spaces with capacities less than five times the limits stated in ASHRAE Standard 62 t Equipment or systems serving spaces with sensible loads (excluding transmission or infiltration loads) equal to or less than the transmission or infiltration loads at 60F
- Equipment or systems that are expected to operate less than 20 hours per week t Supermarket systems where outdoor air might affect the operation of refrigerated cases t Equipment with high mechanical cooling efficiencies, as outlined in Table 6.3.2 of ASHRAE Standard 90.1 (to view this table, visit www.contractingbusiness.com/airside).
Time to Take Control
Once we have worked our way through these requirements and exceptions, and have determined we need to use an airside economizer, our next step is to determine which control option to implement. Earlier, I indicated integrated controls were "required." However, there are exceptions allowed in ASHRAE 90.1, they are:
- DX system with controls which reduce outdoor air to prevent coils from frosting at the lowest compressor operating condition, and the lowest part-load condition is not more than 25% of the total capacity
- DX system with capacities less than 65 mbh with controls that do not allow integrated operation
- systems in climate zones 1, 2, 3a, 4a, 5a, 5b, 6, 7, and 8.
I also stated earlier that mixed air temperature control could not be used. However, there's an exception allowed in ASHRAE 90.1 that allows the use of mixed air temperature controls for single zone systems controlled by space temperature.
Choosing a control option really comes down to choosing the optimal high limit option for any given climate zone. Table 184.108.40.206.3A of ASHRAE 90.1 provides us with a list of allowed and prohibited high limits. Table 220.127.116.11.3B provides us with the control settings we are to employ, by climate zone, for the various high limit options (to view these tables, visit www.contractingbusiness.com/airside).
Airside AssumptionsASHRAE 90.1 makes certain assumptions about the use of economizers. The first is that mechanical systems require more energy to condition 100% outdoor air than to condition mixed air when the outdoor air is above the high limit set point. The second assumption is that mechanical systems will use less energy to condition 100% outdoor air when outdoor air is below the high limit. These assumptions provide us with several challenges.
For example, a dry, marine, or very cold climate might have problems with fixed enthalpy high limit controls, because outdoor temperatures could be dry but very warm. Or, economizers with fixed enthalpy high limits in cold climates might disable during cool, rainy weather, resulting in missed hours of cooling potential.
Simulations of various high limit controls were recently performed by the Trane Company, using Columbus, OH because it represents a part of the country where airside economizers should make sense to use. The results are shown in Figure 2. You'll notice there is not substantial energy savings between fixed dry bulb and differential enthalpy if the fixed dry bulb high limit is set at 70F. Yet fixed dry bulb will obviously be more cost-effective to implement and maintain than a differential enthalpy high limit.
Comparing hours when the space relative humidity is above 58%, we find there is little difference between the various high limit controls.
What Can We Conclude?What can we conclude about airside economizers given all of the requirements, exceptions, and real world implementation challenges?
Frankly, we probably don't put as much thought into the selection and implementation of an airside economizer as we do other parts of the HVAC system; we simply assume they'll save us money. In reality, economizers can lower energy costs, BUT we must select controls to optimize performance, maintain those controls, and use the system the way it was designed and intended to be used. Economizers can also improve indoor air quality, BUT there will be times when use of an economizer will sacrifice indoor air quality to achieve lower energy costs, or will improve indoor air quality and raise energy costs.
Finally, it's important all members of a construction team — the building owner, design engineer, HVAC contractor, and building operator — understand how all of the various components of the HVAC system are intended to work together to achieve the end goal: a comfortable building using the least amount of energy possible.
Ellis G. Guiles, Jr. P.E., is director of sales and marketing, TAG Mechanical Systems, Inc., Syracuse, NY. He can be reached at 315/463-4455.
Figure 2. As these results from Columbus, OH, illustrate, little difference can exist between the various high limit controls used on airside economizers.
This article is based on the presentation, Airside Economizers: Are They Really Saving Money or Improving IAQ? that Ellis Guiles gave at the 2006 Commercial Contracting Roundtable, held in Atlanta, Oct. 25-26. The Commercial Contracting Roundtable, which also incorporates the Design/Build Seminar, is co-sponsored by the Air Conditioning Contractors of America (ACCA) and Contracting Business magazine.
Last year's roundtable featured 15 business management and technical sessions specifically tailored for commercial HVAC and Design/Build contractors.
For more information about the 2007 Commercial Contracting Roundtable, contact Richard Ware at ACCA, 703/824-8843, or visit www.contractingroundtable.com.