Five Common Mistakes Engineers Make Sizing Make-Up Air for Industrial Buildings
Every cubic foot a plant exhausts has to come back in somewhere.A Midwest food plant was running at minus 0.15 inches of water column — doors slamming, workers complaining, unconditioned air leaking in through every crack. The fix was one 40,000 CFM direct-gas-fired make-up air unit with a 25,000 CFM fresh-water evaporative cooling module to temper summer air, MERV 8 filtration, and a VFD interlocked with the exhaust so make-up air tracks exhaust in real time and the building stays positively pressurized.
Every cubic foot of air a plant exhausts has to come back in somewhere. If the make-up air system doesn’t bring it back in on purpose, filtered and tempered, it comes in through dock doors, wall cracks, and flues, unfiltered and at whatever temperature the weather is that day. We see the same handful of sizing mistakes on project after project. Here are the five most common. Where the numbers matter, we work them through one example: a 20,000 CFM paint booth in a northern plant.
1. Undersizing for the Exhaust
Start with the exhaust, not the floor area. A single paint booth or welding line can pull 20,000 CFM on its own. If the make-up air unit is sized for less, the building goes negative. Doors become hard to open and won’t close on their own, gas appliances backdraft, and the booth never reaches its design airflow, which shows up as overspray and poor finish quality.
Total every exhaust source that can run at the same time: booths, weld fume extraction, process exhaust, and roof fans. Then size the make-up air to match, typically with a small surplus to keep the space slightly positive. For our example, that means a make-up air unit delivering 20,000 CFM.
2. Ignoring Static Pressure
A unit’s CFM rating means nothing without a static pressure behind it. Make-up air has to be pushed in against the resistance of the intake hood, filters, heating and cooling sections, ductwork, and diffusers, plus any negative pressure the exhaust is creating in the space. If the blower is selected at a nominal static and the real system is higher, the unit slides back on its fan curve and never delivers its rated airflow.
Add up the external static pressure of the actual duct system and select the blower and motor at that point on the curve, with margin for filters loading up between changes.
3. Forgetting Winter
Cooling usually gets the attention, but heating 100 percent outside air is a big load. The sensible heating load is about 1.08 × CFM × temperature rise. For our booth, raising 20,000 CFM from −10 °F to 65 °F is 1.08 × 20,000 × 75, or about 1.6 million BTU/h of output, before burner efficiency.
In summer, cooling the same 20,000 CFM from 95 °F outside air down to a typical 55 °F supply takes roughly 125 to 150 tons of DX, depending on the design wet bulb. That is about 1.5 to 1.8 million BTU/h, so the winter load is right in the same range as the summer load. This is where gas-fired burners and hot water coils get undersized: the heating section is treated as an afterthought instead of a primary design load.
Just as important is how the heat is delivered. A make-up air unit runs at full design load only on the coldest days. In spring and fall it may need only 10 to 30 percent of its capacity. A single large on/off burner will overshoot, short-cycle, and blow uncomfortable air. Specify staged or modulating heat, such as a modulating gas burner with a wide turndown or a hot water coil with a modulating valve, so the unit holds supply temperature across the whole season.
4. Skipping Filtration
Industrial sites pull in dust, road grit, and fumes from neighboring processes. Without proper filtration, all of it goes straight across the coils and into the building. Coils foul, pressure drop climbs, capacity falls, and for a paint booth, dirt ends up in the finish.
Specify filters suited to the process, size the unit for their dirty-filter pressure drop, and make the filter section easy to reach so they actually get changed. A dirty filter sensor is an inexpensive way to make sure that happens.
5. No Controls
A make-up air unit running flat out all day, regardless of what the exhaust is doing, wastes energy and swings the building pressure. Make-up air should track the exhaust: when the booth or weld line shuts down, the make-up air should step down with it.
Interlock the make-up air with the exhaust fans, use a variable frequency drive on the blower where the exhaust varies, and control the heat or cooling to a supply air setpoint. A simple building pressure reading confirms the balance is right.
The Bottom Line
Sizing make-up air correctly comes down to five questions: How much air leaves the building? Against what static pressure? How cold does it get, and how hot? What is in the outside air? How will the unit follow the exhaust as it changes? Answer those up front, and the unit will hold pressure, temperature, and air quality all year.
Bessamaire has been building industrial make-up air and air handling equipment in Cleveland, Ohio since 1958. See the JH Just Heat Series and HC Heating and Cooling units, or read how a 20,000 CFM HC unit supplies make-up air to an industrial paint booth. If you have a project that needs a second set of eyes on the sizing, our engineering team is glad to help.
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