Makeup Air Design Problem

Here is a recent problem that we ran into when helping a customer design a makeup air unit for a paint shop. Hopefully you find this valuable. Let us know what you think by replying to this email. 

What would you recommend in this situation?

The existing space is roughly 120′ X 100′ x 25’ tall; basically just a large warehouse space being used to paint large metal construction beams. It’s important to note that the painters are only using a small fraction of the space.

The only HVAC equipment on-site are two sidewall exhaust fans.

The biggest problem is the strong paint odor for the workers, which the owner would like to remove. 

The owner wants to install a 10,000 cfm makeup air unit, and then relocate a 35,000 cfm exhaust fan to this space (and slow the exhaust fan speed down to roughly 10,000 cfm).

Will this work?

Depends what you mean by “work”. 

Let’s look at some back-of-the-napkin math:

  • 10,000 cfm only provides 2 air changes per hour. For comparison, standard offices have around 3/4 to 1 air changes per hour. An old leaky warehouse building will naturally have 1.5 air changes per hour. 
  • 10,000 cfm is 0.8 cfm per sq feet. That is the same amount of airflow you have in your house.

So back-of-the-napkin math says this system probably won’t work. 

What about the codes?

The IFC requires a minimum of 100 FPM velocity of air through a “paint booth”. In this example that would be — (100*25) x 100 FPM = 250,000 cfm. That’s right, fire code would require 250,000 cfm. 

Since that is probably unrealistic, what is the right answer? 

We would recommend building an actual paint booth room inside this warehouse, with a much lower ceiling, and only building the square footage that the owner needed for the paining operation. 

Side note: if the process needs a baking cycle, make sure to include that in the design!

Be careful when designing make up air units for painting applications – they can be tricky. 

Can we help you on a job like this? Leave us a comment below or call us directly at 816-842-5400.

Brad Telker
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