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Cooling Tower Fan Speed Control

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Cooling Tower Fan Speed Control

Cooling towers are critical to HVAC and process cooling plants. But how we control the speed of those massive fans can make the difference between an efficient system—or wasted energy. Because fan power scales roughly with the cube of speed, small reductions in rpm can produce outsized kW savings. This article breaks down your control options—constant speed, two-speed, dual-motor arrangements, and variable frequency drives (VFDs)—then compares efficiency when staging multiple towers or cells.

The Physics in One Minute (Fan Affinity Laws)

For axial fans used on most towers:

  • Airflow (Q) ≈ speed (N)
  • Static pressure (ΔP) ≈ N²
  • Fan power (P) ≈ N³

So, if you run a fan at 50% speed, airflow drops to ~50%, but power drops to ~12.5% (0.5³).

Control Options

A. Constant Speed (On/Off)

How it works: These fans are either on or off. It’s the simplest and lowest-cost method, but it comes with drawbacks—coarse temperature control, higher average power, and more wear from frequent starts and stops.

Motor runs at synchronous slip speed via across-the-line starter or soft-starter. Capacity is controlled by cycling the fan on and off and/or using basin bypass or waterflow modulation.

Pros

  • Lowest first cost, simple controls
  • Robust and familiar

Cons

  • Coarse control, temperature “hunting”
  • Highest average kW to meet setpoint
  • Frequent starts increase mechanical/electrical stress (unless mitigated with a soft-starter)
  • Noise fluctuates during cycling

When to use

  • Small towers with permissive temperature deadbands
  • Facilities with tight budget and low run hours

B. Two-Speed Motor (Pole-Changing, e.g., 1200/600 rpm)

How it works: One motor with two synchronous speeds via pole switching (two windings or Dahlander). Control steps: OFF → LOW → HIGH.

Pros

  • Low/medium first cost
  • Meaningful energy reduction at LOW (power ≈ (N_low/N_high)³)
  • Fewer starts than pure on/off

Cons

  • Only two capacity steps; still coarse
  • Requires interlocks and proper sequencing to avoid switching under load
  • Less precise approach control than VFD

When to use

  • Moderate load variability where three steps suffice
  • Retrofit where VFDs are impractical

C. Dual Motors / Dual Fans per Cell

How it works: One tower cell with two smaller fan-motor assemblies instead of one large unit (or two cells run in parallel). Control by staging motors: 0, 1, or 2 fans (and possibly with two-speed/VFD on each).

Pros

  • Redundancy: one fan can be down while the other maintains partial capacity
  • Finer staging than single constant-speed fan
  • Can combine with VFDs for very fine turndown

Cons

  • Higher mechanical complexity
  • More drives/starters and controls
  • Slightly higher static/system effects at multiple inlets/outlets depending on geometry

When to use

  • Mission-critical plants (data centers, hospitals)
  • Plants needing N+1 redundancy at the cell level

D. Variable Frequency Drive (VFD)

How it works: Electronic speed control with continuous rpm modulation based on condenser-water (CW) leaving temperature or approach to wet-bulb.

Pros

  • Best energy performance (precisely exploit the cube law)
  • Smooth ramping: reduced inrush, less mechanical stress
  • Tight temperature control and quieter operation at part load
  • Supports advanced strategies (low approach, plume control, nighttime setbacks)

Cons

  • Higher first cost (drive + filters/harmonic mitigation as needed)
  • Requires attention to motor insulation (inverter duty), cable length, and minimum speed limits for gear/motor cooling
  • Potential for VFD harmonics—consider line reactors/filters and coordination with the utility

When to use

  • Almost always the lifecycle-cost winner for medium/large towers with variable loads
  • Facilities with demand charges, long operating hours, or noise constraints

Efficiency Comparison: One Fan at Full vs. Two Fans at Half

A simple illustration using the cube law:

  • Assume each fan at 100% speed draws 50 kW.
  • Option 1: One fan at 100%, the other OFF → Total 50 kW.
  • Option 2: Two fans at 50% speed each → Power per fan = 50 × (0.5³) = 6.25 kW → Total 12.5 kW.

For roughly the same net airflow (0.5 + 0.5 = 1.0 “unit”), two at half speed can use ~75% less power than one at full speed.

Why it works in towers (often even better than in ducts):

  • Distributing water over more fill area at a lower air velocity often improves heat transfer effectiveness (more contact time, better wetting), so you may achieve the same or better leaving CW temperature at even lower fan speeds.
  • Noise drops dramatically at lower speeds.
  • Caveats: confirm minimum motor/gear speeds, bearing lubrication needs, and avoid water maldistribution at very low air velocities.

Rule of thumb for multi-cell towers:

Run the maximum number of cells you can at the lowest possible fan speed to meet setpoint, subject to water distribution limits, freeze/plume management, and pump energy trade-offs.

4) Multi-Tower / Multi-Cell Control Strategies

A. Common Sequencing Priorities

  1. Meet LWT setpoint (e.g., 85°F / 29.4°C) with a small deadband.
  2. Maximize active cells, then modulate all fans down together (with VFDs).
  3. Respect minimum fan speed (e.g., 20–25%) for motor/gear cooling and to maintain water distribution.
  4. If you hit minimum speed on all active cells and are still below load → deactivate one cell (to keep others above their minimum and maintain water distribution quality).
  5. In cold/wet conditions, include plume and icing logic (bypass, basin heaters, intermittent reverse jog if manufacturer allows).

B. Pump & System Interactions

  • If pumps are constant speed and head doesn’t change much when enabling extra cells, the fan-energy benefit typically dominates.
  • If enabling more cells adds significant hydraulic head (uncommon), re-evaluate the fan vs. pump energy trade-off.
  • With variable-flow condenser pumps, coordinate VFD setpoints: unnecessary high waterflow can offset fan savings.

C. Practical Limits

  • Minimum waterflow per cell: stay within manufacturer’s turndown for proper fill wetting.
  • Freezing risk: winter operation may require cycling fans off, bypassing fill, or minimum speeds to prevent ice.
  • Water treatment/plume: more cells at low speed can increase plume risk in certain ambient conditions—use plume abatement strategies if required.

Option-by-Option Energy & Control Summary

OptionEnergy at Part LoadControl ResolutionReliability/StressFirst CostBest Use Case
Constant SpeedPoor (cycling)Coarse (on/off)More starts; simpleLowSmall/simple towers, low run hours
Two-SpeedFairMedium (low/high)Fewer starts; still steppedLow–MediumModerate variability; simple upgrades
Dual Motors/FansGood (with staging)Medium–High (0/1/2 fans)Redundancy; more componentsMedium–HighMission-critical; N+1 needs
VFDExcellent (∝ N³)High (continuous)Soft starts; least wearMedium–HighMost variable-load plants

6) Control Set Points & Tuning Tips

  • Primary loop variable: Leaving CW temperature (or approach to ambient wet-bulb).
  • Setpoint strategy: Fixed setpoint (e.g., 85°F) or reset based on chiller efficiency (some chillers prefer warmer CW at light loads to reduce lift; always coordinate tower and chiller curves).
  • PID tuning: With VFDs, use slow integral action to avoid oscillation; apply a small deadband (e.g., ±0.5–1.0°F).
  • Starts per hour: Enforce maximum starts if any staged (non-VFD) fans remain.
  • Minimum speed: Honor manufacturer minimum (often 20–30%) for gear/motor cooling and ensure adequate airflow through the motor.
  • Safety interlocks: High/low basin level, vibration switch, gear oil pressure/temp (if applicable), fan contactor/VFD status, freeze protection, and motor space heaters.

7) Reliability, Maintenance, and Noise

  • VFD benefits: Soft starts reduce mechanical shock on gears, couplings, and blades; lower average speed reduces wear and noise.
  • Two-speed motors: Check contactor and interlock sequencing; avoid switching between speeds under load.
  • Dual-fan cells: Plan for access, vibration monitoring per fan, and balanced staging to equalize wear.
  • Noise: Since acoustic power falls sharply with rpm, low-speed multi-cell operation is typically the quietest strategy.

8) Quick Worked Example (Energy)

Goal: Deliver “1.0 unit” of airflow.

  • One fan at 100%: 1.0 airflow → 1.0³ = 1.0 power unit (e.g., 50 kW).
  • Two fans at 50% each: 0.5 + 0.5 = 1.0 airflow → 2 × (0.5³) = 0.25 power units (e.g., 12.5 kW).

Savings: 75% fan power reduction, often with better heat transfer due to more wetted fill area at lower face velocity.

9) Commissioning Checklist (Field-Ready)

  • Verify rotation, tip clearance, blade pitch, and vibration cutouts.
  • Confirm minimum VFD speed and motor/gear cooling requirements.
  • Calibrate LWT sensor; confirm wet-bulb source if using approach control.
  • Test multi-cell sequence: enable extra cells before increasing speed, and shed cells last.
  • Validate freeze protection logic (bypass, heaters, reverse-jog if specified).
  • Trend fan kW, LWT, ambient WB; verify stable control and expected cube-law savings.

10) Bottom Line

  • If you can only pick one upgrade, choose VFDs—they offer the largest, most controllable energy savings and better temperature stability.
  • In multi-cell towers, operate more cells at lower speeds rather than one cell at full speed, subject to manufacturer turndown, plume, and freeze constraints.
  • For critical facilities, consider dual-fan/cell redundancy, ideally each on a VFD, to combine reliability with ultra-low kW/ton of heat rejection.

Process Cooling vs. Comfort Cooling

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Process Cooling vs. Comfort Cooling

Cooling systems play a crucial role in various applications, but not all cooling is the same. Two major categories—process cooling and comfort cooling—serve distinct purposes, operate under different design principles, and follow separate regulatory requirements. Understanding their differences and similarities is essential for HVAC engineers, mechanical contractors, and facility managers when designing and implementing cooling systems.

What is Process Cooling?

Process cooling refers to the removal of heat from industrial and manufacturing processes to maintain product quality, equipment efficiency, and operational safety. Unlike comfort cooling, which prioritizes human comfort, process cooling is primarily focused on cooling machinery, materials, and processes.

Applications of Process Cooling:

Manufacturing & Industrial Processes: Cooling in plastic injection molding, metalworking, and chemical production.

Data Centers: Preventing overheating of servers and networking equipment.

Food & Beverage Industry: Refrigeration of perishable items during production and storage.

Medical & Pharmaceutical Applications: Cooling MRI machines, laboratory equipment, and drug manufacturing processes.

Power Plants: Cooling turbines and condensers in thermal power generation.

What is Comfort Cooling?

Comfort cooling is designed to maintain a comfortable indoor temperature and humidity level for occupants in residential, commercial, and institutional buildings. This type of cooling is commonly associated with air conditioning (HVAC) systems in offices, homes, and public spaces.

Applications of Comfort Cooling:

Commercial Buildings: Office spaces, shopping malls, and restaurants.

Residential Buildings: Apartments and single-family homes.

Healthcare Facilities: Hospitals, nursing homes, and clinics.

Educational Institutions: Schools and universities.

Similarities Between Process Cooling and Comfort Cooling

  1. Heat Transfer Principles: Both systems rely on heat transfer mechanisms such as conduction, convection, and phase change (evaporation and condensation) to remove heat from a space or process.
  2. Refrigeration Cycles: Both use vapor-compression or absorption refrigeration cycles for heat removal.
  3. Cooling Equipment: Chillers, cooling towers, heat exchangers, and air-handling units can be found in both applications, though their configurations may differ.
  4. Efficiency Considerations: Both require efficient energy management and optimization for cost-effectiveness and sustainability.

Codes and Standards Governing Process Cooling vs. Comfort Cooling

Both types of cooling systems must adhere to industry codes and standards, though their regulatory frameworks differ.

Process Cooling Standards & Regulations

ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings): Provides guidelines for energy efficiency in industrial processes.

ASHRAE 15 (Safety Standard for Refrigeration Systems): Governs refrigeration safety in process cooling applications.

NFPA (National Fire Protection Association): Certain process cooling applications (e.g., chemical processing) require compliance with NFPA codes.

OSHA (Occupational Safety and Health Administration): Regulates workplace safety, including cooling systems in industrial settings.

EPA (Environmental Protection Agency) Regulations: Includes refrigerant management requirements under the Clean Air Act.

Comfort Cooling Standards & Regulations

ASHRAE 55 (Thermal Environmental Conditions for Human Occupancy): Specifies acceptable thermal conditions for human comfort.

ASHRAE 62.1 (Ventilation for Acceptable Indoor Air Quality): Regulates indoor air quality in comfort cooling applications.

IECC (International Energy Conservation Code): Establishes energy efficiency requirements for HVAC systems in buildings.

Title 24 (California Energy Code): Mandates efficiency standards for air conditioning and comfort cooling.

EPA Energy Star & LEED (Leadership in Energy and Environmental Design): Encourage high-efficiency comfort cooling systems.

Energy and Cooling Load Calculation Differences

One of the most significant differences between process cooling and comfort cooling lies in how energy and cooling loads are calculated.

Process Cooling Load Calculations

Highly specific to the industrial application.

Factors include heat generated by equipment, chemical reactions, material phase changes, and production schedules.

Often calculated in tons of refrigeration (TR) or kilowatts (kW), with a focus on peak load scenarios.

Redundancy and fail-safe cooling are often included in calculations to prevent process interruptions.

Comfort Cooling Load Calculations

Typically based on building envelope characteristics, occupancy levels, lighting, and ventilation.

Uses Manual J (for residential) and ASHRAE Cooling Load Calculation Method (for commercial).

Factors include solar heat gain, outdoor temperature variations, and human metabolic heat.

Cooling load is calculated in British Thermal Units per Hour (BTU/hr) or tons of refrigeration (TR).

Energy efficiency is often optimized for seasonal variations and part-load performance.

Regulatory Submission Requirements

The submission process for regulatory approval differs for process and comfort cooling systems.

Process Cooling Regulatory Submissions

  1. Industrial cooling projects often require environmental impact assessments.
  2. OSHA compliance reports may be needed for workplace safety.
  3. Specialized permits for hazardous materials (e.g., refrigerants in chemical processing).
  4. Utility companies may require detailed power consumption reports.

Comfort Cooling Regulatory Submissions

  1. Building permits and mechanical system approval are required by local authorities.
  2. Compliance with ASHRAE 90.1, IECC, and other energy codes.
  3. Load calculations and equipment specifications are submitted for energy efficiency validation.
  4. For large projects, sustainability certifications such as LEED documentation may be required.

Summary

While process cooling and comfort cooling share some similarities in principles and equipment, they differ significantly in design objectives, regulatory requirements, and energy calculations. Engineers and contractors must consider these distinctions when designing, installing, and maintaining cooling systems to ensure compliance, efficiency, and reliability.

Understanding the unique requirements of each cooling type helps in selecting the right equipment, optimizing energy use, and meeting industry standards for safety and performance. Whether cooling a manufacturing line or an office building, applying the correct design approach is critical to achieving the desired outcomes.

Air Economizer High Limit Strategies

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Air Economizer High Limit Strategies

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.

Differential Dry Bulb Economizer Control Strategy
Differential Dry Bulb Economizer Control Strategy

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.

Fixed Enthalpy for Air Economizer Control
Fixed Enthalpy for Air Economizer Control

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.

Differential Enthalpy on Psychrometric Chart
Differential Enthalpy on Psychrometric Chart

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.

How to Calculate Mixed Air Temperature

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How to Calculate Mixed Air Temperature

In HVAC systems, particularly in air handlers and packaged air conditioners, outside air (OA) and return air (RA) are blended before passing through the cooling or heating coils. The resulting mixed air temperature (MAT) is critical for coil load calculations, economizer operation, and overall system efficiency.

This article explains the concept, provides the calculation formula, and shows examples for different percentages of outside air and return air, and provides a free downloadable mixed air temperature calculator.

Why Mixed Air Temperature Matters

The MAT determines the entering conditions at the cooling or heating coil. For example:

  • Cooling Season: A higher MAT increases coil load and energy use.
  • Heating Season: A lower MAT increases heating demand.
  • Economizers: When outdoor conditions are favorable, more outside air is used to reduce mechanical cooling but they must be setup correctly to avoid wasting energy.

Accurately calculating MAT ensures coils are sized properly, economizer strategies are effective, and energy costs are minimized. This formula is based on dry bulb temperature.

The Formula for Mixed Air Temperature

The MAT is a weighted average based on the proportion of outside air and return air:

MAT = (%OA × OAT) + (%RA × RAT)

Where:

  • MAT​ = Mixed Air Temperature (°F or °C)
  • OAT​ = Outside Air Temperature (°F or °C)
  • RAT​ = Return Air Temperature (°F or °C)
  • %OA = Percentage of outside air
  • %RA = Percentage of return air (100 – %OA)

Example Mixed Air Temperature Calculations

Example 1: 20% Outside Air

  • Outside Air = 95°F
  • Return Air = 75°F
  • OA = 20%, RA = 80%

MAT = (0.20 × 95) + (0.80 × 75)

MAT = 19 + 60 = 79°F

Result: The coil sees 79°F entering air.

In this case since the outside air is way above the high limit for any economizer, the outside air would be set to the code minimum per ASHRAE 62.1. It makes no sense to waste energy on cooling outdoor air at 95°F when the return air is at 75°F, not considering relative humidity.

Example 2: 50% Outside Air

  • Outside Air = 40°F
  • Return Air = 72°F
  • OA = 50%, RA = 50%

MAT = (0.50 × 40) + (0.50 × 72)

MAT = 20 + 36 = 56°F

Result: The coil sees 56°F entering air.

Example 3: 75% Outside Air (Economizer Mode)

  • Outside Air = 70°F
  • Return Air = 75°F
  • OA = 75%, RA = 25%

MAT = (0.75 × 70) + (0.25 × 75) = 71°F

Result: The coil sees 71°F entering air. This may or may not be a good setup depending on the relative humidity of the outside air. If the outside air carries a higher latent load than the return air, then this could add significantly to the energy consumption as the coil will need to remove excessive moisture in a wet coil situation.

Example 4: 100% Outside Air (100% Economizer Mode)

  • Outside Air = 60°F
  • Return Air = 75°F
  • OA = 100%, RA = 0%

MAT = (1.0 × 60) + (0.0 × 75) = 60°F

Result: In full economizer mode, MAT = Outside Air Temp. This is where the economizer high limit strategy will come into consideration. See our next video on economizer control high limit strategies.

Practical Considerations

Energy Efficiency: Reducing OA percentage during extreme conditions lowers energy cost, while increasing OA during mild weather can reduce mechanical cooling.

Humidity (Enthalpy Method): While temperature-only calculations are common, true load calculations should also consider humidity. Mixed air enthalpy can be calculated similarly using a weighted average.

Sensor Placement: MAT sensors should be located downstream of the mixing box to avoid stratification errors.

Minimum Ventilation: Codes (ASHRAE 62.1, IMC) require minimum outside air for ventilation, typically ranging from 10–30% based on occupancy type and number of occupants.

Latent Heat Considerations

The mixed air temperature calculation shown here is based on a dry-bulb temperature average, which assumes the cooling coil is operating in a dry (sensible-only) condition. In climates or operating conditions where the coil is wet, this method can be misleading because it ignores the additional latent load of dehumidification. In those cases, a more accurate approach is to use differential enthalpy and differential dry-bulb control, which evaluates both sensible and latent energy. This ensures that economizer operation does not increase cooling energy use in humid conditions.

Get a free copy of our Mixed Air Temperature Calculator in the link below.

Free Mixed Air Temperature Calculator

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