Air economizers are a vital component of modern HVAC systems, designed to reduce mechanical cooling loads by using outdoor air for free cooling whenever conditions allow. High limit strategies determine when outdoor air should not be used, typically to avoid introducing excessively hot or humid air that could increase cooling loads or compromise humidity control.
Below, we’ll examine three common air economizer high limit strategies—Fixed Dry Bulb, Differential Dry Bulb, and Fixed Enthalpy—with examples of each and the issues they may encounter. In this lesson, we’re also going to look at a surprising example. Outdoor air can be warmer than the return air yet still be the better choice for cooling. Stick with me until the end, because we’ll put these conditions on a psychrometric chart to really see how it works.
Fixed Dry Bulb Strategy
The Fixed Dry Bulb strategy is one of the simplest and most cost-effective high limit controls for air economizers. It operates by measuring the outdoor air dry bulb temperature (the air temperature without considering humidity) and comparing it to a predetermined fixed setpoint. If the outdoor temperature exceeds this setpoint, the economizer disables, closing the outdoor air dampers and relying on mechanical cooling. This method requires only a single temperature sensor in the outdoor airstream, making it low-maintenance and inexpensive to install.
Example
Consider a commercial building in a moderate climate, such as Climate Zone 4A (e.g., parts of the U.S. Mid-Atlantic region). The fixed dry bulb set point might be set at 69°F (20.5°C). On a day when the outdoor temperature is 68°F (20°C), the economizer enables, allowing outdoor air to mix with return air to cool the space. However, if the temperature rises to 71°F (21.6°C), the economizer shuts off to prevent warmer air from entering, shifting the load to the mechanical cooling system. If you’re in a dryer area like zones 1B through 5B the fixed dry bulb might be set at 73°F (22.7°C).
Issues and Limitations
While straightforward, the Fixed Dry Bulb strategy can encounter errors in certain conditions. For instance, it may incorrectly enable the economizer when outdoor air is cool but very humid, leading to increased latent loads (humidity) that require additional dehumidification energy. Conversely, it might disable the economizer prematurely in dry, warm conditions where outdoor air could still provide sensible cooling benefits. Sensor inaccuracies, typically around ±2°F, can exacerbate these issues, resulting in higher energy use. In humid climates like Zone 1A (e.g., Miami), this could lead to unacceptable indoor humidity levels if not carefully set. Additionally, energy standards like ASHRAE 90.1 restrict its use in some warm, moist climates where it might increase mechanical cooling demands. Optimal set points vary by climate: 69°F (20.5°C) for humid zones like 1A-5A, up to 75°F (24°C) for drier zones like 3C, 6B or 8.
Differential Dry Bulb Strategy
The Differential Dry Bulb strategy improves upon the fixed approach by comparing the outdoor air dry bulb temperature to the return air temperature from the building. The economizer disables only when the outdoor air is warmer than the return air, allowing for more dynamic operation based on actual indoor conditions. This requires two temperature sensors: one for outdoor air and one for return air.
Example
In a data center in a dry climate like Zone 5B (e.g., Denver), the return air might be at 75°F (24°C) due to internal heat loads. If the outdoor temperature drops to 70°F (21°C), the economizer activates, using the cooler outdoor air. However, if outdoor air reaches 76°F (24.4°C), it disables to avoid introducing warmer air. This strategy shines in scenarios with variable indoor temperatures, such as during partial occupancy, where it can extend economizer hours beyond a fixed setpoint.
Issues and Limitations
A key drawback is its potential to err in humid environments. It may enable the economizer when outdoor air is cooler than return air but highly humid, increasing latent loads and potentially raising indoor relative humidity to uncomfortable or damaging levels (e.g., above 60%). In humid climates like Atlanta (Zone 3A), this can result in significant energy penalties from extra dehumidification. Sensor error is higher (±4°F) due to the dual sensors, amplifying inaccuracies. Standards prohibit this strategy in moist, warm climates (e.g., Zones 1A-6A) because it can lead to excessive hours of operation with damp air, boosting mechanical cooling needs and risking mold growth. It’s better suited for drier climates like Zones 1B-8, where humidity is less of a concern.
Fixed Enthalpy Strategy
The first thing we’ll need to do is swap out the dry bulb temperature sensor with a combination dry bulb temperature and humidity sensor. The Fixed Enthalpy strategy accounts for both temperature and humidity by measuring the outdoor air’s enthalpy (total heat content) and comparing it to a fixed enthalpy set point, typically around 28 Btu per pound (65 kJ/kg), which corresponds to conditions like 75°F (24°C) at 50% relative humidity. If outdoor enthalpy exceeds this value, the economizer disables. This requires enthalpy sensors or a combination of temperature and humidity sensors to calculate enthalpy.
Example
In a hospital in a humid climate like Chicago (Zone 5A), the set point is 28 Btu/Lb (65 kj/kg). On a mild day with outdoor conditions at 70°F (21°C) and 40% RH (enthalpy ~25 Btu/Lb.)(58 kj/kg), the economizer enables to leverage the lower energy content of the air. But if humidity spikes to 80% RH at the same temperature (enthalpy ~32 Btu/Lb.)(74 kj/kg), it disables to prevent excess moisture entry.
Issues and Limitations
Despite considering humidity, Fixed Enthalpy has notable flaws. It can err by disabling the economizer in cool, rainy weather (high humidity but low temperature), missing cooling opportunities in cold climates. In dry climates, it might enable during warm, dry conditions, increasing sensible loads unnecessarily. Sensor inaccuracies are significant (plus or minus 2 Btu per pound), especially with humidity sensors that drift over time and require frequent calibration, leading to higher maintenance costs. Analyses show it’s often not cost-effective compared to dry bulb methods, with errors more pronounced in dry climates like Albuquerque (Zone 4B), where it can waste energy. Standards restrict it in dry, marine, or very cold climates and it’s generally not recommended due to added complexity without proportional benefits.
Example of Outdoor Air Warmer but Lower Enthalpy
Let’s walk through an example that shows why looking only at temperature can be misleading when deciding whether to bring in outdoor air for economizer cooling.
First, imagine the return air from the building is at 75°F (24°C) and 50 percent relative humidity. On the psychrometric chart, that condition works out to about 28 Btu per pound of dry air of total heat (65 kj/kg), which is a combination of sensible and latent heat.
Now, compare that to the outdoor air on the same day. The outdoor air is slightly warmer — 77°F (25°C) — but it’s much drier at only 30 percent relative humidity. Even though it’s two degrees warmer, its total heat content, or enthalpy, is only about 25 Btu/Lb. of dry air (58 kj/kg).
So what does that mean? If we used a simple dry-bulb control strategy, we’d reject the outdoor air because it’s warmer than the return air. But in reality, the outdoor air contains less total heat and would actually take less energy to cool down to supply air temperature.
This is exactly the kind of situation where enthalpy-based economizer control makes the right decision, while a dry-bulb-only strategy could make the wrong one. And these conditions happen often in dry or semi-arid climates, where the air can be warm but still very dry.
This example is a good reminder that both temperature and humidity play a role in determining the best air source for cooling efficiency.
Conclusion
Selecting the right high limit strategy for an air economizer depends on climate, building type, and energy goals. Fixed Dry Bulb offers simplicity and is widely recommended across climates with optimized set points and comes at a lower initial cost then some of the other strategies. Differential Dry Bulb provides better adaptability but falters in humid areas. Fixed Enthalpy addresses humidity but suffers from sensor reliability and is often outperformed by simpler methods. Engineers should conduct site-specific analyses, considering standards like ASHRAE 90.1, to balance first costs, maintenance, and energy savings while ensuring indoor air quality.
