4 Common Confined Space Ventilation Pitfalls

Ventilating a confined space isn't just about flipping on a fan and calling it a day. There’s an art—and a bit of science—to getting it right. If you don’t, you could be putting yourself and your team at risk. Let's break down four common mistakes people make when ventilating confined spaces: short-circuiting, recirculation, inadequate CFM, and bending ductwork like it’s a contortionist’s routine.

1. Short Circuiting

We’re not talking about blowing a fuse here—this short circuit is all about airflow. Picture this: you set up your ventilation to push fresh air into a confined space, but instead of circulating throughout the area, the air takes a shortcut right out of the portal. The result? Only a fraction of the space is getting ventilated, potentially leaving atmospheric contaminants in the space or workers’ breathing zone.

To avoid this, don’t just place your ventilation gear down and hope for the best. position the intake and exhaust strategically—ideally, on opposite ends of the space. Make sure that fresh air travels through the entire space, hitting every nook and cranny, before it exits. If the space in question only has a single portal, ensure that the ductwork is long enough and configured in a way that allows for ventilation in the area that work is taking place. On a side note, longer ductwork can result in decreased overall CFM – check with your manufacturer for details.

Recirculating air might be great for your car’s A/C, but in confined spaces, it can be a fatal mistake. If the air you’re pulling out of the space ends up getting sucked back in, you’re just circulating the same contaminated air over and over again. While the space may feel like it is being ventilated, the contaminates never get diluted out since they are being reintroduced back into the space with each air change.

The fix? Make sure the intake of your ventilation system is positioned in an area that is pulling clean air into the space and ensure the exhaust air is vented far away from the intake. Keep the intake and exhaust well-separated to avoid creating this vicious cycle.

3. Inadequate Ventilation Flow Rates

CFM, or Cubic Feet per Minute, is ventilation’s way of saying “How much air am I actually moving here?” If you don’t have enough CFM, you’re not pushing out the bad air fast enough, and the space could stay hazardous longer than you’d like, or possibly, not control the hazard at all.

Before you set up, do the math. Figure out how much air you need to move based on the size of the space and the level of contaminants you’re dealing with. Too little CFM, and you’re not doing much good. There are multiple methods to calculate the CFM required for your space (I know, math…) however, there are online calculators that can assist with this if math isn’t your strong suit. The typical formula starts with determining the volume of the confined space in cubic feet and the deciding the number of air changes per hour (ACH) required by your organization. 

"While OSHA and ANSI don't recommend a specific number of air changes per hour, a general rule of thumb is around 7 ACH before beginning work. The American Conference of Governmental Industrial Hygienists (ACGIH) recommends 20 ACH when ventilating confined spaces". 

To calculate the CFM requirements, multiple the confined space volume by the air changes per hour and divide that number by 60.

Example:

CFM =  Volume of space (ft³) X Air Changes Per Hour / 60 minutes

CFM =  2500 ft³ X 20 ACH / 60 min

CFM =  50,000 / 60 min

CFM = 833 ft³/min

After you’ve figured out your minimum CFM requirements, you can determine your minimum purge time prior to entry. You can choose to manually calculate this, use an online calculator, or use a ventilation purge time chart like the one provided here. This chart is calibrated to provide a pre-entry purge time representative of 7 complete air changes.

If you're into doing math, the formula below can be used to calculate your pre-entry purge time.

Example: 

Purge Time = Volume of Space X Pre-Entry Air Changes / CFM

Purge Time =  2500 ft³  X 7 Air Changes / 833 ft³/min

Purge Time =  17,500 / 833 ft³/min

Purge Time = 21 minutes

Ever try to suck a thick milkshake through a crazy straw? That’s what too many bends in your ventilation ducts can do to your airflow. Every bend creates resistance, slowing down the air and reducing the overall effective CFM of your ventilation system.

To avoid this, keep your ducting as straight as possible. If you do need to make a turn, go for gradual curves instead of sharp bends. This keeps the air moving smoothly and ensures that you’re getting the ventilation you need where it’s needed most. As with any piece of equipment, check the manufacturer’s guidelines for specific information. While every manufacturer will have their own guidelines for their products, most manufacturers rate their products as a decrease of around 15% per 90-degree bend with a max of two 90-degree bends. Typically this information will be provided on the device itself as seen in the picture below.

Conclusion

Ventilating a confined space is more than just a box to check—it’s about creating a safe atmosphere to work and breath in, or at least controlling the hazards to a level that’s as low as reasonably practicable. By avoiding these common pitfalls, you’re not just moving air; you’re ensuring that it’s doing its job effectively. So next time you’re setting up ventilation, remember: keep the air flowing where it needs to go, and don’t let short circuits, recirculation, inadequate CFM, or ductwork disasters stand in your way. Proper ventilation is a breath of fresh air – literally…..

ONLINE REFERENCES:

ACGIH: Ventilation

OSHA 1910.146: Permit-Required Confined Spaces

ANSI Z117.1 - 2022: Safety Requirements for Entering Confined Spaces

Chris McGlynn, M.S., CSP is a Certified Safety Professional and Nationally Registered Paramedic who serves as the Director of Safety and VPP Coordinator for Roco Rescue. He currently serves as Director-at-Large on the VPPPA Region VI Board of Directors and Secretary of the American Society of Safety Professionals Region IV Board of Directors. Chris also represents ASSP on the ANSI Z117 Confined Space and Z390 Hydrogen Sulfide Training Standard Development Committees. He is also an active OSHA Special Government Employee within the Voluntary Protection Program and is currently working towards a Ph.D in Occupational Safety & Health through West Virginia University's Statler College of Engineering.