NOTE A makeup air unit (MAU) is a dedicated air handler that conditions and delivers 100% outdoor air to replace air removed by exhaust systems, maintaining a neutral-to-slightly-positive building pressure. Unlike a comfort air handler that recirculates room air, a MAU treats the full outdoor airstream from the design winter (and often summer) condition to a deliverable discharge temperature. Its sizing is driven by the total exhaust it offsets, not by a thermal zone load. This distinction governs every downstream decision in this standard — heat capacity, fan arrangement, casing, and controls all follow from the replacement-air mission. (1.1)

NOTE This standard governs packaged and field-assembled MAUs serving replacement-air functions, whether stand-alone or interlocked with kitchen hoods, laboratory exhaust, or general industrial exhaust. (1.2)

NOTE Equipment covered includes the heat section (direct gas-fired, indirect gas-fired, hot-water, steam, or electric), optional cooling section (evaporative, DX, or chilled-water), supply fan assembly, integral mixing and recirculating dampers, filtration, and the unit control package. (1.3)

NOTE Kitchen hood canopies, grease duct, and hood fire-suppression systems are outside this standard and are governed by Kitchen Exhaust Systems; this standard covers only the makeup air unit that serves them. (1.6)

NOTE Dedicated outdoor air systems that provide full sensible-and-latent conditioning for occupant ventilation loads - rather than replacement air for exhaust - are governed by Dedicated Outdoor Air Systems. (1.7)

NOTE Energy- and heat-recovery ventilators used as exhaust-to-supply transfer devices are governed by Energy Recovery Ventilators; an energy-recovery wheel furnished as an integral MAU section remains within this standard. (1.8)

NOTE Zone-level supply distribution, VAV terminal units, and discharge sound attenuation are governed by Variable Air Volume Terminals and Hvac Sound Attenuators respectively. (1.9)

NOTE Where a referenced standard has been superseded by the authority having jurisdiction, the currently adopted edition shall apply. (2.3)

NOTE UL 1995 was officially superseded by UL 60335-2-40 effective January 1, 2025. Both marks remain valid on equipment already in service, and many field-installed MAUs will carry the older mark for years. New purchasing should reference the current standard so that the as-furnished unit is listed under an active certification. (4.4)

NOTE The installing contractor shall provide a factory-trained manufacturer's representative to perform or witness startup. Direct and indirect gas-fired heat sections involve a combustion safety chain — proof of airflow, flame supervision, and high-limit cutout — whose initial setup determines safe operation. A manufacturer representative at startup verifies the burner sequence, modulation, and safety interlocks under live conditions rather than relying solely on factory test data. (4.8)

NOTE Incorrectly located intakes recirculate exhaust contaminants directly back into the supply airstream and defeat the purpose of a dedicated outdoor air unit. Rooftop layouts frequently place a MAU intake near a grease or general exhaust fan because both serve the same space. Verify the 10 ft horizontal separation — and greater separation where the exhaust is hazardous or high-velocity — on the coordination drawing before the rooftop equipment arrangement is finalized. (5.2)

NOTE A unit that cycles off in winter while exposed to outdoor air is at risk of freezing a wet coil, because outdoor air can reach the coil through the de-energized fan section. Chilled-water and DX coils that see outdoor air require an explicit freeze strategy: drain-down or glycol on hydronic coils, and low-ambient controls or a positive-closure intake damper that closes on shutdown. Specify the strategy here rather than leaving it to the installing contractor to discover during the first cold snap. (5.5)

NOTE Direct gas-fired units introduce all products of combustion into the airstream and approach 100% thermal efficiency, but are prohibited where ignitable concentrations of vapor or dust may be present. Direct gas-fired burners sit in the airstream and transfer nearly all of the fuel's energy to the supply air, which makes them the most efficient and most common choice for 100% outdoor-air kitchen makeup. They are not acceptable for spray-finishing or solvent-laden exhaust applications, where NFPA 33 effectively requires an indirect-fired or electric source because an open flame cannot share an airstream that may carry ignitable concentrations. (6.2)

NOTE Indirect gas-fired units isolate combustion products behind a heat exchanger and vent flue gas separately, at the cost of some efficiency. The indirect heat exchanger keeps flue gas out of the supply air, which is required for spray booths and preferred wherever combustion-product contact is unacceptable. Because the exchanger is a wear item subject to thermal cycling, access for inspection and cleaning is a design requirement, not an option. (6.3)

NOTE A heating-only MAU is the default for kitchen and many industrial replacement-air applications, where the supply air need only be neutral, not cooled. In a commercial kitchen the hood captures the cooking heat and the makeup air simply replaces the exhausted volume; cooling the makeup air below room temperature wastes energy and can disrupt hood capture. Cooling is added where occupant comfort in the served space depends on the supply temperature, or where the climate makes untempered 95 °F+ outdoor air unacceptable. (6.7)

NOTE Kitchen and laboratory makeup air shall be 100% outdoor air (ASHRAE Cycle I); recirculation is permitted only where the served occupancy and code allow return air to be mixed with outdoor air. Cycle I delivers all outdoor air with no recirculation and is the safe default for grease-laden, fume, or process exhaust where return air would carry contaminants. Cycle III modulates an outdoor/return damper to recover energy and is appropriate only for general occupancies where the exhausted air is clean enough to partly recirculate. (6.8)

NOTE Casing panels shall be double-wall construction with a minimum 1 in. insulating core to limit casing heat loss and surface condensation. Double-wall panels protect the insulation from airstream erosion and damage during service, and keep the interior surface above the dew point in conditioned sections. Single-wall casings are acceptable only on small heating-only units in benign indoor environments. (7.3)

NOTE Galvanized casing corrodes rapidly when continuously exposed to grease aerosol and the moisture of a commercial kitchen, leading to early casing failure and rust staining. For food-service makeup air, specify stainless-steel or epoxy/phenolic-coated casing for any surface exposed to grease-laden return paths or wash-down. The premium is small relative to replacing a corroded unit, and it preserves the unit's listing and appearance over its service life. (7.5)

NOTE A 10,000 CFM-class MAU can weigh between roughly 2,000 lb and 8,000 lb depending on heat and cooling sections, which often governs roof framing at the unit location. Coordinate the equipment weight, footprint, and support-point reactions with the structural engineer early. Discovering after order that the roof cannot carry the selected unit forces either a structural retrofit or a unit re-selection, both of which are far more costly than a coordination check during design. (7.7)

NOTE Sizing makeup air below the exhaust it offsets is the most common and most damaging MAU error, producing sustained negative pressure that back-drafts gas appliances and makes doors hard to open. Before sizing, total every exhaust fan that draws on the served space — kitchen hoods, toilet exhaust, general and process exhaust — not just the hood being replaced. Undersizing leaves the building under persistent negative pressure, which back-drafts atmospheric gas appliances, slams or jams doors, whistles at every opening, and generates continuous occupant complaints. For commercial kitchens, NFPA 96 Section 8.3.1 is the controlling provision: replacement air must prevent negative pressure greater than 0.02 in. w.c. at the cooking equipment face, with 80% to 90% of hood exhaust volume a reasonable starting baseline. (8.3)

NOTE A fixed-speed makeup air unit cannot follow a variable-speed exhaust hood, so the building pressure swings out of balance whenever the exhaust modulates. Demand-controlled kitchen ventilation and laboratory exhaust both vary their flow with load. If the MAU is single-speed, partial-load exhaust leaves the space over-pressurized and full-load exhaust leaves it under-pressurized. A VFD with a building static pressure reset keeps supply and exhaust matched across the full operating range, which is also the energy-efficient arrangement ASHRAE 90.1 favors. (8.5)

NOTE Heating capacity shall be computed as Q (BTU/hr) = 1.1 × CFM × ΔT (°F), where ΔT is the rise from the design outdoor temperature to the discharge setpoint. The 1.1 factor bundles the specific heat and density of standard air; at high altitude or extreme temperature the density correction should be applied explicitly. Size to the 99% winter design day so the unit can recover the space during the coldest expected operating hours without continuous high-fire operation. (9.2)

NOTE Unlisted direct gas-fired units shall not discharge air exceeding 150 °F; listed units shall observe their listed maximum discharge temperature. (9.3)

NOTE IMC limits the supply-air temperature differential at the space to within 10 °F of the occupied temperature where the resulting load would otherwise exceed the building HVAC capacity. A MAU discharging air far above or below room temperature dumps a load the zone HVAC must then absorb. Keeping the discharge close to neutral — or tempering toward room temperature — keeps the makeup air from overwhelming the comfort system, which is the IMC concern behind the differential limit. (9.4)

NOTE Without a high-limit cutout, a gas valve that sticks open can discharge dangerously hot air into the occupied space or duct system. The high-limit control is the last line of defense in the combustion safety chain. It is required by the listing standard and the fuel gas code, and its setpoint and manual-reset behavior should be verified at startup rather than assumed from the factory configuration. (9.7)

NOTE NFPA 54 scales the required safety devices with input — larger burners require dual safety shutoff valves and proof-of-closure. The gas train is a listed assembly; field modification voids the listing, so any field-required changes must be made by the manufacturer or its authorized representative. (10.3)

NOTE Large direct-fired units operating at high fire may require elevated gas supply pressure beyond standard service pressure; confirm adequate utility capacity before finalizing the gas train. (10.7)

NOTE Higher-efficiency filtration raises static pressure and is warranted where the served space has an indoor-air-quality requirement. The pre-filter protects the heat exchanger and coil and removes the coarse outdoor load. (11.5)

NOTE A MAU that runs while its exhaust is off pressurizes and back-drafts the space, so the interlock is a code requirement, not a convenience. The interlock ties the makeup air to the exhaust it offsets. Omitting it allows the supply fan to run alone — over-pressurizing the building when hoods are off, or leaving exhaust unbalanced when the MAU trips — which defeats the pressure control the unit exists to provide. Wire the interlock so that loss of either side is annunciated. (13.2)

NOTE Supply air short-circuiting into the hood reduces capture efficiency and can extinguish pilot lights, defeating both the hood and the appliance. The makeup air must reach the space without blowing across the cooking surface or into the hood throat. Coordinate diffuser location, throw, and velocity with the kitchen hood design so the replacement air mixes into the room rather than being captured straight back out, which would short-circuit the entire ventilation system. (13.5)

NOTE Functional performance testing confirms the unit performs as designed in place, not just on the test floor. The interlock and pressure tests are the ones most likely to reveal a coordination gap — a missed exhaust fan in the makeup-air total, a reversed interlock, or a diffuser short-circuit — and should be witnessed. (14.5)

NOTE Negative-pressure draw-through cooling sections require a deep-seal condensate trap, or the negative pressure holds water in the pan until it overflows. A draw-through coil sits under negative pressure, which pulls water up the drain leg and prevents free drainage through an unsealed or shallow trap. Provide a trap with a seal depth at least equal to the unit's negative static pressure plus a margin (minimum 1 in. seal), so the pan drains continuously rather than filling until it spills into the unit. (15.6)

NOTE The unit shall be set on vibration isolation appropriate to the fan size and structural support, coordinated with Electric Motors for the fan motor and drive selection. (15.8)