How to Calculate Makeup Air Unit Heating Load: HVAC Engineering Guide

Originally published at https://calcengineer.com/hvac/makeup-air-unit-sizing

Introduction

A commercial kitchen in Minneapolis exhausts 4,000 CFM of grease-laden air. In winter, with outdoor temperatures plunging to -10°F, that massive volume of cold air must be replaced. Without a properly sized makeup air unit (MAU), negative pressure will cause backdrafting of combustion appliances, doors will become difficult to open, and the kitchen will be unbearably cold. For HVAC engineers, accurately sizing the MAU's heating capacity is not just about comfort—it's a critical calculation that balances energy efficiency, building safety, and operational performance. Oversizing wastes capital and operating costs; undersizing creates functional failures. This guide provides the professional methodology for this essential mechanical design task.

What Is Makeup Air Unit Sizing?

Makeup Air Unit (MAU) sizing is the engineering process of determining the required thermal capacity—specifically sensible heating load—to condition outdoor air introduced to replace mechanically exhausted air. The primary goal is to calculate the necessary energy input (in kW or BTU/hr) to raise the incoming outdoor air temperature to a target supply temperature, offsetting the building's air volume deficit and preventing negative pressurization.

Engineers perform this calculation during the design phase of any facility with significant exhaust systems. Key applications include:

Commercial Kitchens: Where hood exhaust rates of 2,000–10,000 CFM are common, per NFPA 96 and International Mechanical Code (IMC) requirements.
Laboratories & Cleanrooms: To maintain precise pressure relationships and offset fume hood exhaust.
Industrial Processes: Such as paint spray booths, welding stations, or chemical processing areas with high local exhaust ventilation (LEV).

The Engineering Formula

The core calculation is based on the sensible heat equation for air. The fundamental formula is:

Q_sensible = 1.08 × CFM × ΔT

Where:

Q_sensible = Sensible heating load (BTU/hr)
1.08 = The product of air density (0.075 lb/ft³), specific heat (0.24 BTU/lb·°F), and minutes per hour (60). This constant is valid for standard air at sea level.
CFM = Makeup air volume flow rate (Cubic Feet per Minute). This must equal or slightly exceed the total exhaust airflow.
ΔT = Temperature difference between the supply air temperature and the effective outdoor air temperature after any energy recovery (°F).

When energy recovery ventilation (ERV) is used, the effective outdoor temperature is calculated first:

T_effective = T_outdoor + (T_exhaust − T_outdoor) × (ERV_effectiveness / 100)

This adjusted temperature is then used in the ΔT for the main sensible load equation. The formula assumes dry air and neglects latent load, which is a valid simplification for heating-dominated climates where the primary concern is sensible heating.

Key Factors Affecting Results

Makeup Airflow Rate (CFM)

This is the single most influential variable. The CFM must be determined from a careful analysis of all exhaust systems, including kitchen hoods, bathroom fans, and process exhausts. ASHRAE Standard 62.1 provides ventilation rates for occupancy, but the MAU CFM is driven by exhaust, not occupancy. A common mistake is using the hood's nameplate CFM instead of the calculated exhaust based on hood type and capture velocity.

Design Outdoor Temperature (T_outdoor)

This is not the average winter temperature but the 99% or 99.6% design dry-bulb temperature from the ASHRAE Handbook—Fundamentals. Using an overly conservative (colder) temperature leads to significant oversizing; using an average temperature risks undersizing. For example, the 99% design temperature for Chicago is 5°F, while the average January low is 17°F.

Energy Recovery Effectiveness (ERV_%)

Incorporating an air-to-air energy recovery ventilator (ERV) or heat recovery ventilator (HRV) dramatically reduces the heating load. Sensible effectiveness for plate heat exchangers or heat pipes typically ranges from 50% to 80%. The exhaust air temperature (T_exhaust) is critical here—using the space temperature (e.g., 70°F) is standard, but if the exhaust is from a hot process like a kitchen hood, the temperature could be higher, improving recovery potential.

Reference Values

Kitchen Hood Exhaust: 300–600 CFM per linear foot of hood (Type I). A 10-foot hood may require 3,000–6,000 CFM makeup air.
Typical ERV Effectiveness: 60–75% sensible effectiveness for a well-selected plate heat exchanger.
Target Supply Air Temperature: Often 65–70°F for general makeup to avoid drafts. For direct supply into a space, it may be higher.
ASHRAE 99% Winter Design Temperatures: Representative values: Atlanta (21°F), Denver (1°F), Minneapolis (-10°F).
Heating Load Constant: 1.08 BTU/hr per CFM per °F (IP units). In SI, the formula is Q = 1.23 × L/s × ΔT (kW).

Step-by-Step Calculation Guide

Determine Total Exhaust Airflow: Sum the CFM of all exhaust fans, hoods, and processes. Apply IMC safety factors if required. This sum is your required makeup airflow.
Establish Design Conditions: Obtain the winter design outdoor dry-bulb temperature for your location from ASHRAE data. Define the desired supply air temperature entering the occupied space.
Calculate Effective Outdoor Temperature: If using energy recovery, apply the formula T_effective = T_outdoor + (T_exhaust − T_outdoor) × (ERV_effectiveness/100). Use the space temperature for T_exhaust unless specific process exhaust data exists.
Compute Temperature Rise: ΔT = Target Supply Air Temperature − T_effective (or T_outdoor if no ERV).
Calculate Required Heating Capacity: Apply the sensible heat formula: Q = 1.08 × CFM × ΔT. For a quick and accurate calculation, use the free Makeup Air Unit Sizing Calculator.

Pro Tip: Always add a 10–15% safety factor to the final heating capacity for equipment selection to account for filter loading, slight duct heat loss, and control variance. Document your source for each input parameter (e.g., "ASHRAE Handbook 2021, Chapter 14, Chicago 99.6% DB = 5°F").

Conclusion

Use this manual calculation or the dedicated online calculator during schematic design and early design development phases. It provides a reliable, code-compliant basis for equipment selection and initial energy modeling. The formula's simplicity makes it ideal for quick checks, feasibility studies, and educating clients on the magnitude of the heating requirement.

This method has clear limitations. It calculates sensible load only. In humid climates, or if the MAU will also cool and dehumidify, a full psychrometric analysis and manual J calculation are required. For complex systems with multiple zones, variable air volume (VAV) makeup air, or where latent load is significant, advanced HVAC load simulation software (like Carrier HAP or Trane Trace) is necessary.

Professional best practices mandate that the calculation be clearly documented in the mechanical design narrative. Include a summary table showing all inputs, the formula used, the calculated load, and the selected equipment capacity with its safety factor. Reference all applicable standards, such as ASHRAE 62.1 for ventilation, the IMC for exhaust rates, and ASHRAE 90.1 for energy recovery requirements.

Accurate MAU sizing is a cornerstone of efficient, safe, and comfortable building operation in high-exhaust environments. A properly sized unit ensures building pressure stability, occupant comfort, and minimized energy consumption, making it a critical deliverable in the mechanical engineer's design package.

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