Direct-Fired Makeup Air Unit (MAU) Calculator

Q: Why can direct-fired burners never be used with recirculated return air?

Direct-fired burners introduce trace carbon dioxide and moisture directly into the air. If air were recirculated, CO2 and moisture would continuously build up to hazardous levels. Direct-fired MAUs are code-mandated to operate strictly on 100% outside air.

Q: What happens if the burner turndown ratio is too low (e.g. 4:1)?

On mild 50°F days where only a small 15°F temperature rise is needed, a low turndown burner cannot throttle low enough. It cycles ON and OFF, causing wild temperature swings and cold drafts in the kitchen.

Size dedicated outdoor makeup air units to balance kitchen and factory exhaust hoods, calculate sensible heating MBH, and verify ANSI Z83.4 combustion air purity standards.

Total Hood Exhaust CFM

MAU Replacement Ratio (%) 80-90% keeps kitchen under slight negative

Design Temperatures

Winter Design Outdoor T_in (°F):

Supply Discharge Temp T_sup (°F):

Fuel Gas Type

Burner Turndown Ratio

Fuel Utility Cost ($ / Therm)

Required Supply MAU CFM

5,525 CFM

-975 CFM (Slight Negative OK)

Max Heating Output

358.0 MBH

Gas Flow: 358 CFH (3.58 Therms/hr)

Sensible Heat & Thermal Rise

Temperature Rise (ΔT): 60.0 °F (10°F to 70°F)

Burner Thermal Efficiency: 100% (Direct-Fired In-Airstream)

Minimum Modulating Turndown Heat: 11.9 MBH (No overshooting)

ANSI Z83.4 Combustion Air Quality Safety

Carbon Monoxide in Airstream: < 1.0 ppm (< 5.0 ppm ANSI max)

Carbon Dioxide in Airstream: < 1,200 ppm (< 4,000 ppm limit)

Peak Hourly Operating Cost: $4.30 / hour full fire

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Direct-Fired Gas Makeup Air Unit (MAU) Engineering

Commercial kitchens and factories exhaust massive volumes of grease, smoke, and chemical fumes. If this exhausted air is not actively replaced, the building plunges into severe negative pressure, causing backdrafting of water heater flues, slamming doors, and reduced hood capture velocity.

1. Building Pressurization & The 85% Rule

Kitchen makeup air units are typically engineered to supply 80% to 90% of total exhaust hood CFM:

$$\text{CFM}_{\text{MAU}} = 0.85 \times \text{CFM}_{\text{exhaust}}$$

The remaining 10% to 15% is drawn from the dining room as transfer air, creating a slight negative pressure zone in the kitchen that prevents cooking odors from migrating into customer seating.

2. 100% Thermal Efficiency of Direct-Fired Burners

Unlike indirect-fired units with stainless steel heat exchangers and chimney stacks (which waste 20% of heat out the flue), direct-fired burners place the flame profile directly into the incoming fresh airstream. 100% of fuel heating value is transferred to the air:

$$Q_{\text{BTU/hr}} = 1.08 \cdot \text{CFM} \cdot \Delta T$$

3. ANSI Z83.4 Safety Standards

Because combustion products enter the occupied space, direct-fired units must strictly comply with ANSI Z83.4 / CSA 3.7: carbon monoxide must remain below 5.0 ppm, carbon dioxide below 4,000 ppm, and nitrogen dioxide below 0.5 ppm.

Frequently Asked Questions

Why can direct-fired burners never be used with recirculated return air?

Direct-fired burners introduce trace carbon dioxide and moisture directly into the air. If air were recirculated, CO2 and moisture would continuously build up to hazardous levels. Direct-fired MAUs are code-mandated to operate strictly on 100% outside air.

What happens if the burner turndown ratio is too low (e.g. 4:1)?

On mild 50°F days where only a small 15°F temperature rise is needed, a low turndown burner cannot throttle low enough. It cycles ON and OFF, causing wild temperature swings and cold drafts in the kitchen.