Short answer: A make-up air unit (MUA) supplies conditioned replacement air to a building at the same rate air is being exhausted. Without one, exhaust hoods and fans create negative pressure that pulls 115-degree dust-laden desert air through every crack in the building envelope. In Las Vegas, MUAs are both a code requirement and a practical necessity for any building with significant exhaust capacity.
Why Las Vegas Buildings Have a Pressure Problem
Every commercial kitchen exhaust hood on the Strip pulls 1,500 to 5,000 CFM of air straight out of the building. Every bathroom exhaust fan, dryer vent, and combustion appliance does the same on a smaller scale. That air has to come from somewhere. Without a dedicated make-up air unit replacing what gets exhausted, the building goes negative, and negative pressure in the Mojave Desert means pulling 115-degree dust-laden air through every crack, gap, and door opening in the envelope. That is the core problem make-up air units solve. They are not air conditioners. They are not ventilators in the traditional sense. A make-up air unit (MUA) is a dedicated piece of equipment that supplies conditioned replacement air to a building at the same rate air is being removed. It keeps pressure balanced, prevents backdrafting of combustion gases, stops uncontrolled infiltration, and maintains indoor air quality in buildings that exhaust significant volumes of air. In Las Vegas, where the outdoor design temperature hits 115 degrees and dust storms send PM10 concentrations through the roof, getting make-up air right is not optional. It is a code requirement and a practical necessity.
What Exactly Is a Make-Up Air Unit?
A make-up air unit is a packaged HVAC system designed to bring in outdoor air, filter it, temper it (heat or cool it to a usable temperature), and deliver it into the building. Unlike a standard rooftop unit that recirculates indoor air, an MUA handles 100% outdoor air. Every cubic foot it delivers is fresh air from outside. The basic components include:- Intake hood and dampers: Controls outdoor air entry and prevents rain, sand, and debris intrusion. In Las Vegas, intake hoods face away from prevailing southwest winds to reduce dust loading.
- Filtration section: Typically MERV 8 minimum, with MERV 13 recommended for Las Vegas conditions. Pre-filters catch the coarse desert dust before it reaches the primary filter bank.
- Heating section: Gas-fired burners or electric heat strips that warm incoming air during winter. Las Vegas winter mornings can drop to 28-35 degrees, and delivering unconditioned outdoor air at those temperatures creates comfort and humidity problems.
- Cooling section: Direct expansion (DX) coils or chilled water coils that cool incoming air during summer. This is the expensive part in Las Vegas because you are cooling 100% outdoor air from 110-115 degrees down to 72-78 degrees.
- Supply fan: Moves the conditioned air through ductwork into the building.
- Controls: Interlocks with exhaust fans so the MUA activates when exhaust systems run. Modulating gas valves and staged cooling maintain discharge temperature regardless of outdoor conditions.
Why Buildings Need Make-Up Air
Negative Pressure and Its Consequences
When exhaust systems remove more air than enters a building, the interior pressure drops below atmospheric. This negative pressure creates several problems that directly affect occupant health, safety, and building performance.
Doors become hard to open. Walk into any restaurant with an undersized or missing MUA, and the front door fights you when you pull it open. That resistance is the building trying to suck air in through the only available opening. In commercial kitchens running 3,000-5,000 CFM exhaust hoods without adequate make-up air, the negative pressure can reach -0.05 to -0.10 inches of water column, enough to make entry doors slam shut and create uncomfortable drafts for staff and customers.
Worse, negative pressure pulls unconditioned outdoor air through every unsealed penetration in the building envelope. In Las Vegas, that means 110-degree air carrying desert dust, construction particulate, vehicle exhaust, and whatever else is floating in the valley. This uncontrolled infiltration bypasses all filtration, dumps massive sensible heat loads on the HVAC system, and introduces contaminants directly into occupied spaces.
Backdrafting of Combustion Appliances
This is the safety issue that gets people's attention. Gas-fired water heaters, furnaces, and boilers are designed to vent combustion gases up through a flue using natural draft. Negative building pressure can reverse that draft, pulling carbon monoxide and other combustion byproducts back into the building instead of venting them outside.
In Las Vegas commercial buildings, this is particularly relevant for restaurants with gas ranges, bakeries, laundromats with gas dryers, and any facility with atmospheric-vented water heaters running alongside exhaust fans. The International Mechanical Code requires make-up air for exactly this reason. Section 505 mandates that make-up air be provided for exhaust hood systems, and Section 507 specifies that make-up air cannot reduce the exhaust capacity below what is required for the application.
Exhaust Hood Performance
Kitchen exhaust hoods are rated for a specific capture velocity, typically 100-150 feet per minute at the face of the hood. When the building goes negative and the hood is fighting against pressure resistance, actual capture velocity drops. Grease-laden cooking vapors that should be captured by the hood spill into the kitchen, coating surfaces, degrading air quality, and creating fire hazards.
A properly sized MUA delivering air at 80-90% of the exhaust rate maintains slight negative pressure (which keeps cooking odors from migrating to dining areas) while keeping the hood at rated capture performance. This is a balancing act that requires proper engineering, not guesswork.
How Make-Up Air Units Handle Las Vegas Extreme Heat
Here is where Las Vegas makes the MUA conversation different from every other market in the country. When outdoor air is 112 degrees and you need to deliver 3,000 CFM of replacement air into a kitchen that is already running at 85-90 degrees, you have a massive cooling load dedicated entirely to outdoor air tempering.
Consider the math. Cooling 3,000 CFM from 112 degrees to 78 degrees is roughly 12 tons of cooling capacity needed just for the make-up air alone. That is before the building's regular AC handles the internal gains from cooking equipment, lighting, people, and solar load through the roof. A 3,000 CFM MUA with DX cooling in Las Vegas is a substantial piece of equipment, typically 10-15 tons of packaged cooling capacity.
Energy costs reflect this reality. Running a 15-ton MUA at NV Energy's commercial rate of roughly $0.10-$0.13 per kWh, with the compressors running 12-16 hours a day from May through September, adds $800-$1,500 per month to the electric bill. That is the cost of conditioning 100% outdoor air in the desert. There is no shortcut around the thermodynamics.
Strategies to Reduce MUA Cooling Load
Several design strategies reduce the energy penalty of conditioning 100% outdoor air in extreme heat:
- Energy recovery wheels or plates: Transfer cooling energy from the exhaust airstream to the incoming outdoor air. In a kitchen application, you are exhausting 78-degree air and bringing in 112-degree air. An energy recovery wheel can pre-cool that incoming air by 15-25 degrees, saving 3-5 tons of mechanical cooling. Payback in Las Vegas is typically 2-4 years.
- Indirect evaporative pre-cooling: Las Vegas humidity runs 5-15% in summer. Indirect evaporative coolers can drop outdoor air temperature by 15-20 degrees without adding moisture to the supply air. This reduces the DX cooling load significantly and costs a fraction of mechanical cooling to operate.
- Demand-controlled ventilation: Modulate make-up air volume based on actual exhaust rates rather than running at maximum capacity continuously. Variable-speed fans on the MUA matched to variable exhaust fan speeds cut energy use during partial-load conditions.
- Night pre-cooling: In the shoulder months (March-April, October-November), Las Vegas nights drop to 55-65 degrees. Running the MUA economizer during these hours pre-cools the building mass and reduces next-day cooling load.
Sizing a Make-Up Air Unit
Undersizing is the most common mistake. Building owners see the equipment cost and energy penalty, then spec a unit that covers 60% of the exhaust rate and hope for the best. The result is negative pressure, door problems, backdrafting risk, and an HVAC system fighting uncontrolled infiltration all summer.
The sizing process starts with a complete exhaust air audit:
- Catalog every exhaust source: Kitchen hoods (measured at rated CFM, not nameplate), bathroom exhaust fans, dryer vents, process exhaust, server room exhaust, and any other system removing air from the building.
- Total the exhaust CFM: A Las Vegas restaurant with a Type I hood over the grill line (2,500 CFM), a Type II hood over the dishwasher (800 CFM), two bathroom fans (100 CFM each), and a general exhaust (300 CFM) totals 3,800 CFM of exhaust.
- Apply the make-up air ratio: The MUA should supply 80-90% of total exhaust to maintain slight negative pressure in the kitchen zone while preventing excessive negative pressure building-wide. For the example above, that means 3,040-3,420 CFM of make-up air.
- Calculate cooling and heating loads: Using Las Vegas design conditions (115-degree cooling, 28-degree heating) and the target discharge temperature, determine the required cooling and heating capacity.
- Account for filtration pressure drop: MERV 13 filters add static pressure. The fan must be sized for the filter resistance at dirty conditions, not just clean filter performance.
For residential applications, the calculation is simpler but still matters. A home with a 600 CFM range hood, two bathroom exhaust fans at 80 CFM each, and a dryer vent at 150 CFM exhausts 910 CFM total. Most homes handle this through controlled infiltration and HVAC system return air. But if the home is tight (ACH50 below 3.0, which new Las Vegas construction increasingly achieves), a dedicated residential MUA or fresh air intake on the HVAC system becomes necessary.
Maintenance Requirements in Desert Conditions
Make-up air units in Las Vegas take more abuse than the same equipment in Portland or Charlotte. Every cubic foot of outdoor air carries desert particulate, and the cooling system runs at full capacity for five months straight. Maintenance schedules that work in moderate climates will fail here.
Filter Replacement
Pre-filters on Las Vegas MUA units need replacement every 30 days during dust season (March through October) and every 60 days during winter. Primary MERV 13 filters last 60-90 days depending on local conditions. Units near active construction sites or major roadways may need even more frequent changes. A clogged filter on an MUA does not just reduce airflow. It increases fan energy consumption, reduces the volume of replacement air entering the building, and shifts the building toward negative pressure.
Cooling Coil Cleaning
DX cooling coils on MUA units processing 100% outdoor air in Las Vegas accumulate dust at 3-4 times the rate of a recirculating air handler. Quarterly coil cleaning is minimum. Monthly visual inspection during summer ensures coils are not coating over between scheduled cleanings. A dirty coil reduces cooling capacity, which means hotter supply air entering the building, which means the building's main HVAC system works harder to compensate.
Belt and Fan Inspection
Supply fan belts on MUA units degrade faster in extreme heat. Inspect belts monthly and replace at the first sign of cracking, glazing, or stretch. Belt failure means zero make-up air, which means immediate negative pressure and backdrafting risk. Keep spare belts on site.
Gas Section Service
Annual combustion analysis on gas-fired heating sections confirms proper flame pattern, CO levels in the flue, and heat exchanger integrity. Las Vegas MUA units run heating sections fewer hours per year than northern climates, but the rapid temperature swings during shoulder seasons (40-degree mornings followed by 85-degree afternoons in March) cause thermal stress that can crack heat exchangers over time.
Controls Calibration
Discharge air temperature sensors, outdoor air temperature sensors, and pressure transducers drift over time. Annual calibration ensures the unit is actually delivering air at the target temperature and responding correctly to exhaust fan interlocks. A sensor reading 5 degrees low means the unit is overcooling and wasting energy. A sensor reading 5 degrees high means the unit is undercooling and dumping hot air into the building.
Recommended Maintenance Schedule
- Monthly: Filter inspection and replacement as needed, visual coil inspection, belt check, condensate drain verification
- Quarterly: Coil cleaning, fan bearing lubrication, electrical connection inspection, refrigerant pressure check
- Semi-annually: Full HVAC maintenance inspection including controls calibration, gas section combustion analysis, damper operation verification, and ductwork inspection
- Annually: Complete system performance test measuring actual CFM delivery against design, discharge temperature accuracy, and building pressure verification
Signs Your Building Needs a Make-Up Air Unit
Not every building has one, and many that should don't. Here are the indicators that a building is running without adequate make-up air:
- Doors are hard to open or slam shut on their own. This is the most obvious sign of negative pressure. If your front door requires extra effort to pull open, the building is pulling air in through it.
- Whistling or drafts around windows and doors. Air rushing through gaps in the envelope to satisfy exhaust demand creates audible noise and uncomfortable drafts.
- Kitchen exhaust hoods not capturing smoke or steam. Reduced capture velocity from negative pressure means cooking vapors spill out from under the hood canopy.
- CO detector activations near water heaters or furnaces. Backdrafting combustion appliances push carbon monoxide into occupied spaces. This is a safety emergency, not just a comfort issue.
- Excessive dust accumulation despite regular cleaning. Uncontrolled infiltration through unsealed envelope penetrations bypasses all filtration systems and deposits raw desert dust throughout the building.
- HVAC system running constantly but not holding temperature. If unconditioned 112-degree air is pouring in through cracks and gaps because of negative pressure, no AC system can keep up.
Frequently Asked Questions
How much does a make-up air unit cost in Las Vegas?
Residential MUA units with heating only run $1,500-$3,500 installed. Commercial units with DX cooling range from $8,000 for a 1,000 CFM unit to $35,000-$60,000 for a 5,000-10,000 CFM unit with full cooling and heating. The cooling section is the major cost driver in Las Vegas because of the tonnage required to condition 100% outdoor air from 115 degrees. Installation, ductwork, electrical, and controls typically add 30-50% to the equipment cost.
Can I just open a window instead of installing a make-up air unit?
Opening a window provides unfiltered, unconditioned air. In Las Vegas summer, that means 110-degree air carrying desert dust flooding into the building and overwhelming your AC system. It also provides no control over the volume of replacement air and does not interlock with exhaust systems. Code requires make-up air for commercial exhaust hoods specifically because passive solutions like open windows and doors cannot reliably prevent negative pressure or backdrafting. For residential range hoods under 400 CFM, a passive air inlet may be acceptable per code, but anything larger requires a dedicated make-up air source.
Does my home need a make-up air unit?
If your home has a range hood rated at 400 CFM or higher, the International Residential Code (Section M1503.6) requires a make-up air system. Many Las Vegas homes built after 2015 have tighter envelopes that make natural infiltration insufficient to replace exhausted air. Signs you need one include difficulty lighting a gas fireplace when the range hood is running, the water heater pilot light going out, or doors whistling when exhaust fans operate. A fresh air intake on the HVAC return duct is the most common residential solution.
How much energy does a make-up air unit use in Las Vegas?
A commercial MUA with DX cooling running in Las Vegas summer consumes roughly 1.0-1.5 kW per ton of cooling capacity. A 10-ton unit running 14 hours per day at NV Energy commercial rates adds approximately $1,000-$1,400 per month to your electric bill during peak summer. Units with energy recovery ventilators reduce this by 25-35%. Gas heating costs during winter are minimal in Las Vegas since the heating season is short and mild. Annual operating costs for a mid-size commercial MUA typically run $6,000-$12,000 including electricity, gas, filters, and maintenance.
How often should make-up air units be serviced in Las Vegas?
Monthly filter checks are non-negotiable during the March-October dust season. Quarterly coil cleaning is required because MUA units process 100% outdoor air and accumulate desert particulate faster than recirculating systems. Full professional service should happen twice per year, ideally before cooling season in April and before heating season in October. Units near construction zones, gravel lots, or major roadways need more frequent attention. Skipping maintenance on an MUA does not just reduce efficiency. It reduces airflow, which means less replacement air, which means negative pressure and all the problems that come with it.
Need Make-Up Air Solutions in Las Vegas?
The Cooling Company designs, installs, and maintains make-up air systems for Las Vegas commercial and residential buildings. From restaurant kitchen MUA units to whole-building ventilation solutions, our licensed technicians size systems based on actual exhaust audits and Las Vegas design conditions, not rules of thumb.
We provide complete HVAC services including make-up air unit installation, preventive maintenance plans that keep filters and coils on schedule, and indoor air quality assessments that identify negative pressure problems before they become safety hazards.
Serving Las Vegas, Henderson, North Las Vegas, Summerlin, Green Valley, Centennial Hills, Mountains Edge, Southern Highlands, and all Clark County communities.
Call (702) 567-0707 to schedule a make-up air assessment for your building.