Makeup Air Unit Sizing Calculator

Calculate

Total makeup air volume flow rate — must equal exhaust airflow (CFM)

Winter design outdoor air temperature for your location (°F)

Desired supply air temperature entering the building (°F)

Sensible effectiveness of energy recovery ventilator (0% = no recovery, 50–80% typical)

Temperature of exhaust air stream available for energy recovery (°F)

Fraction of makeup air supplied directly into the exhaust hood (0–80%). Does not reduce heating requirement — shown for reference only.

Overview

A Makeup Air Unit Sizing Calculator estimates the sensible heating capacity required for a makeup air unit (MAU) that replaces air exhausted from a building. Every cubic foot of air exhausted from a commercial kitchen, laboratory, paint booth, or industrial process must be replaced with an equal volume of outdoor air — and in cold weather, that outdoor air must be heated to an acceptable supply temperature before entering the building.

Makeup air heating is often the single largest energy cost in buildings with high exhaust volumes. A commercial kitchen exhausting 3,000 CFM in a cold climate can require 150,000+ BTU/hr of makeup air heating capacity — more than the entire space heating load of the building. Accurate MAU sizing ensures the heating system can maintain indoor comfort without oversizing equipment or wasting energy.

This page uses the standard HVAC sensible air heating equation to calculate MAU heating capacity from makeup air airflow, design outdoor temperature, target supply air temperature, and optional energy recovery effectiveness. The result supports MAU equipment selection, gas-fired heater sizing, and early mechanical system planning for any building with significant exhaust airflow.

How to Use This Calculator

  1. Enter makeup air airflow — in m³/h or CFM.

  2. Enter design outdoor temperature — in °C or °F.

  3. Enter target supply air temperature — in °C or °F.

  4. Enter energy recovery effectiveness — in %.

  5. Enter exhaust air temperature — in °C or °F.

  6. Enter short-circuit fraction — in %.

  7. Click "Calculate" — get required MAU heating capacity, effective outdoor temperature after recovery, and adjusted temperature rise.

Size the MAU to this heating capacity and to the exhaust airflow; evaluate energy recovery in cold climates and add a summer cooling check separately.

Inputs & Outputs

Inputs

  • •Makeup Air Airflow (m³/h / CFM)
  • •Design Outdoor Temperature (°C / °F)
  • •Target Supply Air Temperature (°C / °F)
  • •Energy Recovery Effectiveness (%)
  • •Exhaust Air Temperature (°C / °F)
  • •Short-Circuit Fraction (%)

Outputs

  • •Required MAU Heating Capacity (W / BTU/hr)
  • •Effective Outdoor Temperature (after recovery) (°C / °F)
  • •Adjusted Temperature Rise (°C / °F)
  • •Makeup Air Airflow (CFM)
  • •Short-Circuit Airflow (m³/h / CFM)

Formula

Calculator Formula

This page uses the standard HVAC sensible air heating equation to calculate makeup air unit heating capacity.

Step 1: Effective outdoor temperature after energy recovery

T_effective = T_outdoor + (T_exhaust − T_outdoor) × (ERV% / 100)

Where:

  • T_effective = effective outdoor air temperature after energy recovery (°C or °F)
  • T_outdoor = design outdoor air temperature (°C or °F)
  • T_exhaust = exhaust air temperature available for recovery (°C or °F)
  • ERV% = energy recovery ventilator sensible effectiveness (0–100%)

If no energy recovery is used (ERV% = 0), T_effective = T_outdoor.

Step 2: Adjusted temperature rise

ΔT_adjusted = max(T_supply − T_effective, 0)

Where:

  • ΔT_adjusted = temperature rise the MAU burner must provide (°C or °F)
  • T_supply = target supply air temperature (°C or °F)

If energy recovery pre-heats the air above the target supply temperature, ΔT_adjusted = 0 and no burner heating is required.

Step 3: MAU heating capacity

Imperial:

Q_heating = 1.08 × CFM × ΔT_adjusted  (BTU/hr)

Metric:

Q_heating = cp × ρ × q × ΔT_adjusted  (W)
       = 1.005 × 1.202 × (m³/h ÷ 3600) × ΔT × 1000

Where:

  • Q_heating = required MAU heating capacity (BTU/hr or W)
  • 1.08 = standard air sensible heat factor (BTU/hr per CFM per °F)
  • CFM = makeup air volume flow rate (ft³/min)
  • cp = specific heat of air = 1.005 kJ/(kg·K)
  • ρ = air density at standard conditions = 1.202 kg/m³
  • q = volume flow rate (m³/s)

The constant 1.08 in the imperial formula is derived from: 0.075 lb/ft³ × 0.24 BTU/(lb·°F) × 60 min/hr = 1.08.

Short-Circuit Fraction

Short-circuit makeup air is supplied directly into the exhaust hood face and is immediately captured by the exhaust without entering the occupied zone. However, short-circuit air still requires full tempering to prevent condensation, cold drafts at the cook line, and thermal shock to cooking equipment. This calculator applies heating to 100% of the makeup air airflow regardless of short-circuit fraction.

Calculator Variables

Variable Meaning Units
CFM Makeup air volume flow rate ft³/min
m³/h Makeup air volume flow rate m³/h
T_outdoor Design outdoor temperature °C / °F
T_supply Target supply air temperature °C / °F
T_exhaust Exhaust air temperature °C / °F
ERV% Energy recovery effectiveness %
T_effective Effective outdoor temp after recovery °C / °F
ΔT_adjusted Temperature rise after recovery °C / °F
Q_heating Required MAU heating capacity W / BTU/hr / kW

What is Makeup Air Unit Sizing?

Makeup air unit sizing is the process of determining the heating capacity required for a mechanical unit that heats outdoor air to replace air exhausted from a building. Every building with a significant exhaust system — commercial kitchens, laboratories, paint booths, welding shops, industrial processes — must replace the exhausted air with an equal volume of outdoor air to maintain neutral building pressure. In cold climates, that outdoor air must be heated before entering the building to prevent freezing conditions, occupant discomfort, and equipment damage.

Why Makeup Air Heating Matters

Makeup air heating is often the single largest energy cost in buildings with high exhaust volumes. A commercial kitchen exhausting 3,000 CFM in a northern climate with a design outdoor temperature of 10°F (−12°C) requires approximately 146,000 BTU/hr of makeup air heating capacity — equivalent to a large residential furnace dedicated solely to heating outdoor air. Without adequate makeup air, the building operates at severe negative pressure: exhaust hood capture deteriorates, combustion appliances back-draft, doors become difficult to open, and unconditioned infiltration air enters through every gap in the building envelope.

Direct-Fired vs Indirect-Fired MAUs

A direct-fired MAU burns gas with the combustion products mixed directly into the supply airstream, achieving near 100% thermal efficiency. A small amount of CO and combustion byproducts enters the space, so direct-fired units are approved for industrial and commercial kitchen applications but not for occupied residential spaces. An indirect-fired MAU uses a heat exchanger to separate combustion products from supply air, achieving 80–85% efficiency but with no combustion product contamination risk. Indirect-fired units are required in applications where combustion products in the airstream are not acceptable.

Energy Recovery Impact

Energy recovery ventilators (ERV or HRV) transfer heat from warm exhaust air to cold incoming outdoor air before it reaches the MAU burner. A 70% effective ERV in a kitchen exhausting 75°F (24°C) air into a 10°F (−12°C) outdoor condition pre-heats incoming air to approximately 55.5°F (13°C) — nearly eliminating the MAU heating requirement at design conditions.

ERV reduces MAU burner size, gas consumption, and operating cost. In cold climates with large exhaust volumes, ERV payback periods of 2–5 years are common. Many energy codes now require energy recovery on exhaust systems above defined airflow thresholds.

HVAC Unit Conversions

Unit Equivalent
1 CFM 1.699 m³/h
1 m³/h 0.5886 CFM
1 W 3.412 BTU/hr
1 kW 3,412 BTU/hr
°F to °C (°F − 32) × 5/9
°C to °F °C × 9/5 + 32

Practical Tips

Set MAU airflow equal to exhaust airflow — it is fixed by the exhaust system, not a design variable. Temper 100% of makeup air regardless of short-circuit fraction; short-circuit air still requires full conditioning to prevent cold drafts and equipment damage at the cook line. Evaluate energy recovery for any system above 1,500 CFM (2,550 m³/h) in cold climates — ERV payback of 2–5 years is common for large kitchen exhaust systems. Run a separate summer cooling check using the same airflow and formula with summer design temperatures.

Key Facts

  • Makeup air volume must equal exhaust airflow — the MAU airflow is not a design variable, it is fixed by the exhaust system.
  • The standard sensible heat factor 1.08 (BTU/hr per CFM per °F) is derived from standard air density (0.075 lb/ft³) × specific heat (0.24 BTU/lb·°F) × 60 min/hr.
  • A 3,000 CFM MAU in a cold climate (10°F design outdoor) requires approximately 146,000 BTU/hr of heating capacity without energy recovery.
  • Energy recovery at 70% effectiveness can reduce MAU heating capacity by 70% or more, often eliminating the heating requirement entirely at moderate conditions.
  • Direct-fired MAUs achieve near 100% thermal efficiency; indirect-fired MAUs achieve 80–85% but produce no combustion products in the supply air.
  • ASHRAE Standard 90.1 and many energy codes require energy recovery on exhaust systems above defined airflow thresholds.
  • Short-circuit makeup air still requires full tempering — cold unconditioned air at the cook line causes condensation, drafts, and thermal shock to equipment.
  • Demand-controlled kitchen ventilation (DCKV) reduces MAU operating cost by 30–50% by modulating airflow at part load, but does not change the maximum capacity requirement.

Applications

  • Commercial kitchen makeup air unit sizing and selection
  • Restaurant HVAC system design — makeup air heating load
  • Laboratory exhaust makeup air heating capacity estimation
  • Paint booth and spray booth makeup air system sizing
  • Industrial process exhaust makeup air heating
  • Welding shop and fabrication facility ventilation makeup air
  • Ghost kitchen and dark kitchen makeup air planning
  • Institutional kitchen MAU sizing (schools, hospitals, correctional facilities)
  • Energy recovery feasibility screening for high-exhaust buildings
  • Demand-controlled kitchen ventilation (DCKV) baseline MAU capacity determination

Example Calculation

Imperial Example

Given:

  • Makeup Air Airflow = 3,000 CFM
  • Design Outdoor Temperature = 10°F
  • Target Supply Temperature = 55°F
  • Energy Recovery Effectiveness = 0%
  • Short-Circuit Fraction = 0%

Calculation:

Step 1: Effective outdoor temperature (no recovery)
  T_effective = 10°F

Step 2: Adjusted ΔT
  ΔT_adjusted = 55 − 10 = 45°F

Step 3: Heating capacity
  Q_heating = 1.08 × 3,000 × 45 = 145,800 BTU/hr

Result: HIGH MAKEUP AIR REQUIREMENT A gas-fired MAU rated at approximately 146,000 BTU/hr and 3,000 CFM is required for this condition.

Imperial Example with Energy Recovery

Given:

  • Same inputs with 70% ERV effectiveness
  • Exhaust temperature = 75°F

Calculation:

Step 1: Effective outdoor temp after recovery
  ΔT_total = 75 − 10 = 65°F
  T_effective = 10 + (65 × 0.70) = 55.5°F

Step 2: Adjusted ΔT
  ΔT_adjusted = 55 − 55.5 = −0.5°F → 0°F

Step 3: Heating capacity
  Q_heating = 1.08 × 3,000 × 0 = 0 BTU/hr

Result: Energy recovery eliminates the heating requirement at this design condition.

Metric Example

Given:

  • Makeup Air Airflow = 5,100 m³/h
  • Design Outdoor Temperature = −12°C
  • Target Supply Temperature = 13°C
  • Energy Recovery Effectiveness = 0%
  • Short-Circuit Fraction = 0%

Calculation:

Step 1: Effective outdoor temperature (no recovery)
  T_effective = −12°C

Step 2: Adjusted ΔT
  ΔT_adjusted = 13 − (−12) = 25°C

Step 3: Heating capacity
  q = 5,100 / 3,600 = 1.417 m³/s
  Q_heating = 1.005 × 1.202 × 1.417 × 25 × 1000
            = 42,800 W = 42.8 kW

Result: MODERATE MAKEUP AIR REQUIREMENT A gas-fired MAU rated at approximately 43 kW and 5,100 m³/h is required for this condition.

Standards & References

  • ASHRAE Handbook — Fundamentals, Ch. 18 — ventilation air sensible load calculations and psychrometric analysis for outdoor air heating
  • ASHRAE Standard 62.1 — minimum outdoor air requirements for occupied spaces including kitchens and industrial areas
  • NFPA 96 — makeup air requirements specifically for commercial kitchen exhaust hood systems
  • ASHRAE Standard 90.1 — energy code requirements for energy recovery on high-exhaust systems above defined airflow thresholds
  • IMC Section 507 — International Mechanical Code provisions for commercial kitchen makeup air

Limitations

  • This calculator is a first-pass screening tool for sensible heating capacity at winter design conditions, not a full psychrometric MAU selection.
  • Does not calculate latent load from outdoor air humidity or dehumidification requirements.
  • Does not include summer cooling coil sizing — use the same formula with summer design temperature for a separate cooling estimate.
  • Does not calculate duct static pressure and fan selection requirements.
  • Does not model energy recovery unit pressure drop and fan power impact.
  • Does not differentiate direct-fired vs indirect-fired MAU efficiency — direct-fired achieves ~100%, indirect-fired achieves 80–85%.
  • Does not include gas supply pressure and burner sizing details.
  • Does not model freeze protection and low-limit controls for energy recovery equipment.
  • Does not calculate modulating burner turndown ratio requirements.
  • Does not include sound attenuation requirements for MAU installations.
  • Final MAU specification requires a full psychrometric analysis and manufacturer performance review.

Common Mistakes to Avoid

  • Sizing the MAU airflow as a design variable rather than treating it as fixed by the exhaust system. The makeup air volume is not optional — it must equal exhaust airflow to maintain space pressure. Attempting to reduce MAU size below exhaust volume results in persistent negative pressure, poor hood capture, and back-drafting of combustion appliances.
  • Ignoring energy recovery in cold climates. A 3,000 CFM kitchen in a cold climate can spend more on MAU gas heating than on all other kitchen energy combined. A 70% effective ERV unit paying back in 2–3 years is often the single highest-return energy investment in a restaurant mechanical system, yet it is routinely omitted from early design budgets.
  • Sizing MAU heating capacity only for winter design conditions without checking summer cooling requirements. In hot climates, the MAU may need a cooling coil or evaporative cooler to prevent hot outdoor air from overwhelming the kitchen space cooling system.
  • Assuming short-circuit makeup air requires no conditioning because it exits through the exhaust hood without entering the occupied zone. In practice, short-circuit air still requires tempering to prevent condensation on cooking equipment surfaces, cold drafts at the cook line, and thermal shock to hot equipment. Always temper 100% of makeup air airflow regardless of short-circuit fraction.

Frequently Asked Questions

How do you size a makeup air unit?
Makeup air unit sizing requires two calculations. Airflow is set equal to exhaust airflow — the MAU must replace every CFM exhausted from the space. Heating capacity is calculated from airflow and the required temperature rise: Q_heating = 1.08 × CFM × ΔT (Imperial) or Q_heating = cp × ρ × q × ΔT (Metric), where ΔT is the difference between the design outdoor temperature and the target supply air temperature. Energy recovery effectiveness reduces the required ΔT and heating capacity proportionally.
How many BTU does a makeup air unit need?
MAU heating capacity depends on airflow and climate. As a practical example: a 3,000 CFM MAU in a climate with a design outdoor temperature of 10°F and a target supply temperature of 55°F requires 1.08 × 3,000 × 45 = 145,800 BTU/hr. The same unit in a mild climate with a design outdoor temperature of 35°F would require only 1.08 × 3,000 × 20 = 64,800 BTU/hr. Climate is as important as airflow for MAU capacity sizing.
What is the difference between direct-fired and indirect-fired makeup air units?
Direct-fired MAUs mix combustion products directly into the supply airstream, achieving near 100% thermal efficiency. They are approved for commercial kitchens and industrial spaces where combustion byproducts in the airstream are acceptable. Indirect-fired units use a heat exchanger to keep combustion products separate from supply air, achieving 80–85% efficiency and are required in applications where clean supply air is critical, such as occupied office or residential spaces.
Why must makeup air equal exhaust airflow exactly?
Any imbalance between exhaust and makeup air creates a pressure differential between the exhausted space and adjacent areas. If exhaust exceeds makeup air, the space goes negative — hood capture deteriorates, combustion appliances back-draft, doors become difficult to open, and unconditioned infiltration air enters through all envelope gaps. If makeup air exceeds exhaust, the space goes positive — unconditioned air spills into adjacent corridors and dining areas. Neutral or very slightly negative pressure (relative to dining area) is the correct target for commercial kitchen design.
How does energy recovery reduce MAU heating capacity?
An energy recovery ventilator (ERV or HRV) uses a heat exchanger to transfer heat from warm exhaust air to cold incoming outdoor air before it reaches the MAU burner. A 70% effective ERV in a kitchen exhausting 75°F air into a 10°F outdoor condition pre-heats incoming outdoor air to approximately 55.5°F — nearly eliminating the MAU heating requirement at design condition. ERV reduces MAU burner size, gas consumption, and operating cost. In cold climates with large exhaust volumes, ERV payback periods of 2–5 years are common.
What is short-circuit makeup air?
Short-circuit makeup air is supplied directly into the exhaust hood face through a dedicated plenum or perforated panel, where it is immediately captured by the exhaust without entering the conditioned kitchen space. Because short-circuit air bypasses the room, it does not add to the room sensible cooling load in summer. However, short-circuit air still requires full tempering in winter to prevent cold drafts at the cook line, condensation on hot equipment surfaces, and thermal shock to cooking appliances. Short-circuit fractions of 70–80% are common in energy-efficient kitchen ventilation designs.
When is demand-controlled kitchen ventilation (DCKV) worth considering for MAU sizing?
DCKV uses sensors to reduce exhaust and makeup air fan speed at part load — during idle periods between meal services, exhaust and makeup air volume can be reduced to 30–50% of design flow. Since MAU heating energy scales with airflow, DCKV reduces MAU operating cost by 30–50% in most kitchens. DCKV is particularly cost-effective for kitchens with long operating hours and significant idle periods. Many energy codes now require DCKV on kitchen exhaust systems above defined airflow thresholds. Size the MAU for the full design exhaust flow — DCKV reduces operating cost but does not change the maximum capacity requirement.
Does this calculator include summer cooling capacity?
No. This page calculates sensible heating capacity for winter design conditions only. Summer makeup air cooling sizing requires a separate calculation — the same sensible air equation applies but ΔT is the outdoor-to-supply temperature drop rather than rise. In hot climates, MAU cooling coil or evaporative cooler sizing may be as critical as winter heating capacity. Use the same formula with the summer design outdoor temperature and target supply temperature to estimate summer cooling capacity as a separate calculation.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Commercial Kitchen Energy Recovery

Return Air Ratio Calculator

Supply Air Temperature Calculator

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