1. Why Dryers Are Really Just Very Hungry Fans

If you stand behind a big commercial dryer while it’s running, you can feel the invisible river of air pouring out of the exhaust duct. Every minute, that machine is pulling in room air, heating it with a gas burner, pushing it through a tumbling load of laundry, and throwing it outdoors along with the moisture it picked up.

From the building’s point of view, a dryer is basically a powerful exhaust fan with heat and a drum attached. A typical clothes dryer moves on the order of 100 to more than 1,000 cubic feet of air per minute (CFM), depending on whether it’s a small commercial unit or a large industrial tumbler. In multi‑dryer laundries and industrial plants, the combined exhaust can easily reach tens of thousands of CFM.

Physics insists on balance: if that much air leaves the building, the same amount has to come back in from somewhere. If you don’t provide a dedicated path for it, the building will pull air in through every crack and opening it can find—around doors, through leaky windows, down chimneys, and even back through other exhaust vents. That replacement air is what engineers call make‑up air.

Make‑up air is simply the fresh air that “makes up” for the air you intentionally exhaust. For gas‑fired appliances like dryers, it’s not just about comfort or drafty doors; it’s about safety, dryer performance, fuel efficiency, and even how long your equipment lasts.

2. Combustion Air vs. Make‑Up Air: Two Different Jobs

It’s easy to blur together three related ideas: combustion air, ventilation air, and make‑up air. In the real world they overlap, but it helps to keep their jobs straight:

• Combustion air is the oxygen the gas burner needs to burn natural gas or propane cleanly. If a dryer doesn’t get enough, the flame can become lazy and yellow, soot can form, and carbon monoxide levels can rise.
• Ventilation (or dilution) air is the general fresh air that keeps indoor pollutants from building up and helps carry away heat and moisture.
• Make‑up air is specifically the air that replaces air exhausted by devices like dryers, range hoods, and other fans. It keeps the building’s pressure from dropping too low.

For small appliances in leaky older buildings, these three often come “for free” through cracks and gaps in the building envelope. But as construction has become tighter and commercial laundry systems have become more powerful, codes and standards have tightened the rules around how we supply outdoor air.

Fuel gas codes such as NFPA 54 (the National Fuel Gas Code) explicitly warn that when exhaust fans and clothes dryers interfere with the operation of fuel‑burning appliances, additional make‑up air must be provided so that all appliances have enough air and can vent safely.

Mechanical codes add another layer: they don’t just care about the burner, they care about the total volume of air you’re expelling. At some threshold—often only a few hundred CFM—designers are required to show that there’s a planned path for replacement air, not just hope that the building leaks are good enough.

3. What the Codes Actually Say (in Plain Language)

Codes vary by country and local jurisdiction, but many North American projects follow some combination of:

• The International Mechanical Code (IMC)
  • The International Residential Code (IRC) for smaller/light‑commercial installations
  • NFPA 54 / ANSI Z223.1 (National Fuel Gas Code)
  • Local mechanical, fuel gas, and energy codes, plus the dryer manufacturer’s installation instructions

One key idea shows up again and again: when you exhaust a significant amount of air, you must provide a deliberate source of make‑up air.

For example, the 2021 International Mechanical Code states that any clothes‑dryer installation exhausting more than 200 CFM must be provided with make‑up air. It also gives minimum opening sizes when a dryer is installed in a small room or closet; a typical requirement is at least 100 square inches (about 0.065 m²) of free area if that’s how you’re supplying air to the space, or an equivalent engineered solution.

Residential‑scale fuel‑gas codes echo the same threshold: if your dryer exhaust system pulls more than 200 CFM and there’s no obvious way for air to get back into the space, you’re supposed to add make‑up air so that dryers and other gas appliances don’t compete with each other for oxygen or back‑draft combustion gases into the room.

NFPA 54 adds a more general rule of thumb for combustion air: when exhaust fans, clothes dryers, or kitchen hoods interfere with gas‑fired equipment, make‑up air must be provided so that each appliance gets the air it needs. In commercial laundries, where multiple high‑BTU dryers may share a space with boilers, water heaters, and makeup‑air units of their own, this becomes a coordination challenge: all the exhausts and all the intakes have to be counted.

The important takeaway is that the code does not dictate a single magic “CFM per dryer” number. Instead, it sets thresholds (like 200 CFM) and basic opening sizes, then pushes designers toward an engineered solution based on the actual exhaust volume and building layout.

And crucially, codes are minimums. A system that barely squeaks by on paper may still perform poorly in the field if dryers are added later, doors are weather‑stripped, or energy retrofits make the building envelope much tighter. Many engineers design beyond the minimums to keep rooms comfortably pressurized and dryers operating consistently under real‑world conditions.

4. How Much Air Does a Commercial Dryer Really Move?

So how big is the “air appetite” of a commercial or industrial dryer?

For residential dryers, published data and energy research place typical exhaust flows around 100–225 CFM. Commercial and industrial dryers are larger and hotter, and their exhaust rates scale up accordingly. A single 30‑pound gas‑fired tumble dryer can easily move hundreds of CFM; a twin 30‑pound tumbler with an input on the order of 200,000 BTU/h may move around 1,100 CFM of air.

Multiply that by a bank of 8, 12, or 20 dryers, and you’re suddenly exhausting several thousand to well over ten thousand CFM whenever the plant is busy. In a laundromat, that might be a peak‑evening condition when most pockets are full. In an industrial uniform plant or hotel laundry, it might be the entire morning shift.

From the building’s perspective, that much exhaust is like throwing open a huge invisible window. If you don’t provide a matching “window” for make‑up air, the building pressure will drop, outside doors will become hard to open, cold air will whistle in around frames, and combustion appliances with conventional chimneys may start back‑drafting their flue gases.

Manufacturers know this, which is why installation manuals for commercial dryers usually include explicit make‑up air requirements. These often specify the minimum square footage of free area openings per dryer or per BTU of input, and may suggest locating the intake openings low on exterior walls, near the dryers, to minimize drafts across the rest of the space.

Because every model is different, engineers typically combine three pieces of information:

1. The manufacturer’s required make‑up air per dryer (either as CFM or as grille area per appliance).
   2. The total dryer‑exhaust fan capacity (from nameplates or duct‑fan selections).
   3. The local code thresholds that trigger make‑up air provisions.

When in doubt, they sum the exhaust flows for all the dryers and design make‑up air to at least match that total, often with a margin of 10% or more to maintain a slight positive pressure so lint and odors don’t leak into adjacent spaces.

5. What Happens If You Don’t Provide Enough Make‑Up Air?

On paper, make‑up air can feel like a dry, technical requirement. In the field, the symptoms are very tangible:

• Slow drying and customer complaints. If the room is starved for air, the dryer’s fan has to work harder to pull air through a nearly “vacuum‑sealed” space. Airflow drops, and wet loads take longer to dry.
• Overheating and nuisance shutdowns. When airflow is restricted, heat builds up in the dryer and exhaust ducts. Modern dryers often have high‑temperature limit switches that shut down the burner (or the entire machine) for safety. Operators see this as random tripping or mysterious “error codes.”
• Back‑drafting and combustion safety problems. In buildings with naturally drafted gas appliances, negative pressure can reverse the flow in chimneys or vents, pulling carbon monoxide and water vapor into the laundry area instead of sending them outside.
• Uncomfortable building pressures. Doors can slam, whistle, or become hard to open. In cold climates, cold air may roar in through any available crack, making the laundry area drafty and uncomfortable for staff.
• Lint migration. When the building is strongly negative, air is pulled from adjacent spaces toward the laundry. Lint‑laden air can move into hallways, lobbies, and guest areas, creating cleanliness and maintenance issues.

None of these are theoretical. Industry groups, code officials, and manufacturers have repeatedly identified lack of make‑up air as one of the most common problems in commercial and industrial laundry facilities. Fortunately, it’s also one of the easiest to fix with good design and commissioning.

6. How Engineers Size Make‑Up Air (Without Getting Lost in Equations)

There are detailed formulas for combustion air, infiltration rates, and duct sizing, but the basic design workflow for make‑up air in a laundry is surprisingly intuitive:

Step 1: Add up all the exhaust
The design team totals the rated CFM of all the dryers and any additional exhaust fans in the room—lint collectors, general exhaust, spot ventilation, and so on. This gives a “worst reasonable case” exhaust load. In a large plant, this might be 15,000–20,000 CFM or more.

Step 2: Decide on a pressure strategy
Most designers aim to keep the laundry space close to neutral or slightly positive pressure relative to outdoors, to prevent uncontrolled infiltration of cold or humid air. To do that, the make‑up air system is usually sized to supply roughly 100–110% of the total exhaust CFM.

Step 3: Choose how to deliver the air
There are two broad options:

• Passive make‑up air: openings (grilles or louvers) that let outdoor air flow in whenever the room goes negative. This can work for small installations with modest exhaust flows, but it offers limited control over drafts and temperature.
• Mechanical make‑up air: a dedicated supply fan (often with heating, and sometimes filtration or cooling) that blows outdoor air into the space. Gas‑fired make‑up‑air units are common in cold climates; they warm incoming air so the space stays comfortable even when large volumes are being replaced.

Manufacturers of make‑up‑air units offer simple sizing guidance: add up all exhaust CFM and size the make‑up unit to match or slightly exceed that value. In more complex plants, designers may split make‑up air between several units located near major exhaust sources.

Step 4: Pay attention to where the air enters
Dumping all the make‑up air in one corner and expecting it to magically find its way to a line of dryers on the far wall is asking for trouble. Good layouts bring make‑up air into the same zone as the dryers, often low on the wall, so the air can sweep across the machines and into the intake openings without creating cold drafts at workstations.

Step 5: Control and verify
Modern laundry exhaust fans are often controlled by duct‑pressure sensors that speed up or slow down the fan as dryers turn on and off. Make‑up‑air units can be tied into the same logic: when exhaust ramps up, the supply increases to match, keeping the pressure balanced.

Commissioning is the last step. Technicians can measure building pressure relative to outside, check stack effect during hot and cold weather, and confirm that doors open easily and combustion appliances vent correctly under worst‑case exhaust conditions.

7. Special Cases: Tight Buildings, Big Industrial Plants, and Cold Climates

Not all laundry rooms are created equal. A few situations demand extra care:

• Very tight envelopes. Newer buildings and deep‑energy retrofits often have very low natural leakage. In these spaces, even a modest bank of dryers can depressurize the room without a mechanical make‑up‑air system.
• Mixed‑fuel mechanical rooms. If your laundry shares a space with boilers, water heaters, or other gas‑fired appliances, all of the combustion and exhaust air needs have to be considered together. Codes that allow simple “rules of thumb” based on room volume quickly break down when multiple high‑BTU appliances share a confined area.
• Large industrial tunnels and batch dryers. In textile, food, or industrial laundries, individual dryers can move thousands of CFM each. Here, make‑up air becomes part of the overall process‑ventilation strategy, and designers may model airflow through the entire plant.
• Cold and hot/humid climates. In cold climates, unconditioned make‑up air can create uncomfortable work conditions, frozen pipes, and condensation. Gas‑fired make‑up‑air units or tempered air from energy‑recovery systems are common solutions. In hot, humid climates, large volumes of unconditioned outdoor air can overload cooling systems and raise indoor humidity, so some degree of conditioning or dehumidification may be needed.

In all of these cases, the basic principle stays the same—replace what you exhaust—but the engineering to do it comfortably and efficiently becomes more involved.

8. A Practical Checklist for Laundry Owners and Designers

For owners, managers, and non‑specialist designers, make‑up air doesn’t have to be mysterious. Here’s a simple checklist:

1. Count your exhaust sources
   List every dryer and exhaust fan in the laundry, along with its rated CFM or BTU input if that’s all you have. If you don’t know the airflow, ask the manufacturer or consult the installation manual.
2. Look up the rules
   Check the local mechanical and fuel gas codes your jurisdiction enforces, paying special attention to sections on clothes dryers, exhaust systems, combustion air, and make‑up air. These may be based on the IMC, IRC, NFPA 54, or local amendments.
3. Read the manuals
   Dryer manufacturers usually specify minimum make‑up air openings per machine, often in square inches of free area or CFM. These are not suggestions—they are part of the listing and must be followed for the warranty and safety approvals to remain valid.
4. Walk the building
   Do a simple “forensic tour” of your existing facility or design:
   • Where does the air you’re exhausting actually come from?
   • Are doors hard to open when many dryers run?
   • Do you see signs of back‑drafting at water‑heater or boiler vents?
   • Are staff complaining about drafts or hot, stuffy conditions?
5. Get help when needed
   If the numbers are large, the building is tight, or multiple gas appliances share the same space, it’s wise to involve a mechanical engineer or qualified HVAC contractor. They can size ducts and make‑up‑air units, check combustion safety, and make sure the design satisfies both code and common sense.
6. Plan for change
   Laundries evolve. Owners add dryers, convert from coin to card, change operating hours, or expand wash–dry–fold volume. A make‑up‑air strategy that worked for 10 dryers may not work for 18. Good designs include some headroom so the system can accommodate growth without a full redesign.

9. The Big Picture: Dryers That Breathe Easy Work Better

At first glance, make‑up air can feel like just another box to check in a stack of building‑code requirements. But for commercial and industrial gas dryers, it’s a foundational part of how the entire system works.

A well‑designed make‑up‑air system helps dryers do their job: moving large amounts of hot air through laundry and sending moisture safely outside. It keeps burners supplied with oxygen, protects against back‑drafting, stabilizes drying times, reduces nuisance shutdowns, and makes the laundry space more comfortable for staff and customers.

In a sense, a laundry is a living, breathing ecosystem. The dryers are the heart, pumping air and heat; the exhaust ducts are the arteries; and the make‑up‑air paths are the veins that complete the circuit. When any part of that loop is starved or blocked, the whole system suffers.

The good news is that the physics are simple and the tools are well‑understood. By paying attention to make‑up air early in the design—and revisiting it whenever equipment or the building envelope changes—owners and designers can keep their dryers breathing freely, their combustion safe, and their operations profitable for years to come.

References (Selected)

• International Code Council (ICC). International Mechanical Code (IMC), various editions – sections on clothes‑dryer exhaust and make‑up air.
  • International Code Council (ICC). International Residential Code (IRC), various editions – fuel‑gas and clothes‑dryer make‑up‑air provisions.
  • NFPA 54 / ANSI Z223.1. National Fuel Gas Code – combustion air and appliance interaction with exhaust systems.
  • ASHRAE and industry presentations on dryer exhaust system design and make‑up‑air sizing for commercial laundries.
  • Manufacturer installation manuals for commercial and industrial gas tumble dryers, which typically specify minimum make‑up‑air opening areas or CFM per dryer.
  • Building‑science resources on make‑up air and combustion‑appliance zone safety, including guidance from national laboratories and building‑performance organizations.
  • Trade and engineering articles on venting laundry exhaust and designing mechanical make‑up‑air systems in commercial and multi‑family buildings.