Lab Fume Hoods
A typical CAV hood consumes the same energy as three average houses ($2500-$3000/year).
Why do hoods consume so much energy?
Because they constantly remove conditioned air (heated or cooled) from the lab without the ability to easily turn them off when not needed. It is like heating your home in the winter with the windows and doors open. Fans also contribute to energy consumption but the loss of conditioned air is by far the largest source of energy consumption.
What is a Constant Air Volume (CAV) Hood?
A CAV hood is a fume hood which removes the same volume of conditioned air (heated or cooled) from the lab whether the sash is open or closed. So regardless of whether a hood is in use (sash up) or not (sash down), the flow of air from the lab, into the hood, and exhausted from the building is the same. To prevent air velocity from increasing as the sash is closed (reducing the bottom opening to the hood), a bypass opening widens (typically behind a grill near the top of the hood) as the sash is lowered.
How does a Variable Air Volume (VAV) Hood Save Energy?
In contrast to a CAV hood, a VAV hood reduces the volume of conditioned air being removed from the lab as the sash is lowered. So when a sash is up at the maximum safe working level, the VAV hood acts like a CAV hood removing a similar volume of conditioned air as a CAV hood, but as the sash is lowered, the volume of air being removed from the lab decreases. Unlike a CAV hood, a VAV hood does not have a bypass opening. Instead, to keep the air velocity from increasing as the sash is closed, the exhaust fan slows accordingly to keep a constant air speed.
- Keeping a sash completely closed on a VAV hood can greatly reduce the hood’s energy consumption. Some say consumption can drop by up to 60%, but it really depends upon the conditions in the lab where the hood is located.
- On CU-Boulder's campus, you can tell if your hood is VAV by looking for the “Shut the Sash!” sticker with a black hand (shown on this page).
How many fume hoods does CU-Boulder have?
Approximately 350 CAV and 60 VAV
Are there plans in place to reduce fume hood energy consumption on campus?
Fume hoods are absolutely fundamental to lab safety. Our teaching and research activities at the same time present a major energy conservation opportunity.
Near Term Strategy:
Our goal is to educate and remind our students and lab staff to lower and close sashes when VAV hoods are not in use to save energy and conserve needed make up air in lab buildings (using educational posters and stickers). We will also work with researchers to find out if any teaching or research CAV hoods are no longer needed due to changing research requirements. If that is the case, we will offer a very simple and user friendly program as well as incentive to retire unused CAV hoods, without any compromises to safety.
Long Term Strategy:
Our goal is to create funding opportunities to replace all 350 CAV fume hoods overtime with high performance energy efficient VAV hoods. This is a major undertaking with an extremely high energy conservation opportunity with high upfront investment. While funding opportunities are being reviewed, it makes very good common sense to partner with campus EH&S as well as lab managers and researchers to decide which hoods are connected (exhausting costly conditioned air 24x7) but are not in use. Retire these idle hoods as prerequisite to our long term investment for hoods upgrade.
Kathy Ramirez-Aguilar, Ph.D.
CU Green Labs Program Manager