Makeup Hood Air System Math Practice

Understanding How to Calculate Airflow for Makeup Hood Systems

The math behind a makeup hood air system isn't rocket science, but it's where most people cut corners and end up with a system that either barely moves air or blows so hard it ruins the finish. The core question you need to answer is simple: how much air needs to enter the space to replace what's being exhausted? This is where Makeup Hood Air System Math Practice comes in. You start with the hood itself. Measure the open face area in square feet. That's the width of the opening times the height. A typical walk-in spray booth might have a face opening of 8 feet wide by 7 feet tall, giving you 56 square feet. Multiply that by the desired face velocity, which for most automotive refinish operations sits somewhere between 100 and 150 feet per minute. So 56 times 120 gives you roughly 6,720 cubic feet per minute, or CFM, that the exhaust system needs to pull. That number tells you the exhaust fan requirement. Now the part people skip. Makeup air. For every cubic foot of air the exhaust pulls out, roughly one cubic foot needs to come back in, or you're creating negative pressure. Negative pressure is what causes backdrafting, flame impingement on gas heaters, and doors that slam shut on their own. In my experience, the first time I properly sized a makeup system I was working with a shop that had a 12,000 CFM exhaust fan and basically zero intentional makeup air. The booth would depressurize within seconds of firing up the fan. The paint was blowing everywhere because the air was trying to get in through every crack, gap, and open doorway. Not good.

Makeup Hood Air System Math Practice

The practical workflow goes like this. You determine your exhaust CFM first, then size the makeup air handler to match. The makeup air needs to be conditioned — heated, cooled, and filtered — depending on your climate and the sensitivities of the materials you're spraying. A common mistake is thinking you can just vent a large opening to the outside and call it a day. Unconditioned outdoor air in winter at 20°F hitting a freshly prepped surface is going to cause curing issues, and in summer it'll be humid enough to wreck certain coatings. Here's the calculation for the actual duct sizing once you know your CFM. You pick a velocity in the duct itself, usually between 1,500 and 2,500 FPM for main trunks. Take your 6,720 CFM and divide by 1,800 FPM and you get about 3.73 square feet of duct cross-section. That translates roughly to a 24-inch by 20-inch rectangular duct, or a 22-inch round duct. Round numbers are easier to source. Most commercial makeup air handlers come in standard sizes like 20-inch, 24-inch, and 30-inch round, so you're working with whatever's available at your supplier, not custom fabrication. Filter selection matters more than people realize. A MERV 8 filter on the makeup air will catch most dust and pollen but won't protect you from fine particulates that cause fisheyes and contamination. I've seen booths spec MERV 11 or even MERV 13 for the makeup side when they're doing clear coat work consistently. The trade-off is static pressure. A MERV 13 filter on a high-CFM system can add 0.5 to 0.8 inches of water column pressure drop, which your fan needs to overcome. If your fan curve doesn't have enough static pressure capability, you'll be getting significantly less CFM than rated. Always check the fan curve, not just the nameplate CFM.

Balance is where the math gets real. The theoretical calculation assumes perfect conditions. In practice, you'll have leakage, duct friction, and the occasional door left open. The way I handle this is to design the makeup system at about 90 to 95 percent of the exhaust CFM, then use dampers to fine-tune. You want a slight positive pressure in the booth when the fan is running, maybe 0.02 to 0.05 inches of water column. That prevents unfiltered air from sneaking in through the walls and floor instead of through your conditioned intake. A magnehelic gauge across the door seal will tell you this in real time. One edge case that trips people up: high-altitude locations. At 5,000 feet or more, air density drops significantly. Your fan moves the same volume of air but the mass flow is lower, which affects both cooling capacity and combustion air supply for any gas-fired units. The CFM numbers stay the same for ventilation purposes, but your heating and cooling equipment needs derating. I had a job in Colorado where the spec called for 10,000 CFM of conditioned makeup air and the installed unit was undersized by about 15 percent because the contractor didn't account for altitude. We ended up adding a booster fan on the makeup side, which fixed it but added complexity you should avoid by getting the sizing right from the start. The other common failure point is duct layout. Long runs with multiple elbows add up fast in terms of friction loss. Every 90-degree elbow in a 24-inch duct at 1,800 FPM adds roughly the equivalent of 15 to 20 feet of straight duct run in friction loss. So a makeup air system with four elbows and 50 feet of duct isn't the same as 50 feet of straight duct. Use the equivalent length method from the ASHRAE fundamentals or SMACNA tables. It takes maybe five extra minutes and prevents the situation where your calculated CFM never actually makes it to the booth.

Control strategy is another thing worth getting right. Variable frequency drives on both the exhaust and makeup fans let you maintain that slight positive pressure as conditions change. When the booth door opens, the pressure drops, and a proper control loop will ramp up the makeup fan to compensate. Without VFDs you're relying on manual damper adjustments, which means someone has to actually go adjust the damper every time the operational setup changes. That rarely happens consistently, which is why the pressure drifts over time and the system ends up unbalanced. There's also the question of where the makeup air enters the space. Diffuser placement matters. You don't want the conditioned air shooting directly at the wet paint surface. Position the supply diffusers so the air mixes with the room air before reaching the work zone, or use low-velocity linear slot diffusers along the ceiling to distribute the air evenly. The goal is uniform temperature and cleanliness across the entire booth floor, not a laminar flow that looks good on paper but creates turbulent spots where dust collects. If you're working with an existing system that doesn't meet these calculations, don't just add a bigger fan and hope for the best. Run the actual measurements. Set up an anemometer at the hood face and verify the velocity across the entire opening. Check the differential pressure with a manometer. Measure the static pressure at the fan inlet and outlet. Compare those readings against the fan curve for your actual model. What you measure is what you have. What the paperwork says you have is irrelevant once the system is installed.