Selection and Location of Fume Hoods Have a Big Impact on Your Lab’s Electric Bill

A key aspect of laboratory design and layout – and cost – is ventilation, the number of times the conditioned air in a room is replaced with exterior air, typically expressed as the number of air changes per hour (ACH).

[Air conditioned via an HVAC system that has been thermally adjusted to provide temperature and humidity comfort no matter what the season.]

Recommended air exchange rates are typically presented as a range rather than a specific number because the mathematical calculations that produce them are based upon a variety of factors, such as unique concentrations of unhealthy substances [carbon dioxide, carbon monoxide, allergens, etc.] in a defined space, season of the year, outdoor air quality, and regional climate.  

Impact of lab hoods

According to Labs21®(sponsored by U.S. EPA and Dept. of Energy), the recommended exchange range for a lab is 6-10 ACH, which is significantly greater than the typical air exchange rate of ~1 ACH for a typical office.
Ventilation requirements for lab hoods play a large role in sizing HVAC systems that perform air exchanges for laboratories.

Even though there may be variation in local codes, in Laboratories for the 21st Century: An introduction to Low-Energy Design, the EPA and DOE report the exchange rate for some labs has been arbitrarily set as high as 15-25 ACH even if there is not a need for such a high exchange rate.

Over-sizing an HVAC system is an expensive proposition

The cost of an over-sized HVAC system goes beyond its initial price and affects the overall efficiency of the system because oversized air conditioners run for shorter periods of time than those engineered for optimum operation.

In an HVAC sizing guide, says the efficiency of air conditioners is low when they first start and increases gradually, reaching peak efficiency in about 10 minutes.
A too-big system that operates in short cycles never reaches peak efficiency; some areas in the building may be too cool, others too warm.

To worsen matters, the over-sized system may not operate in cycles long enough to remove humidity. Owners of over-sized HVAC systems may find themselves investing in additional equipment, such as room dehumidifiers and space heaters, to attempt to make work areas more comfortable for workers. Energy costs go up.

Choosing appropriate hood types, being cognizant of the ventilation demand of lab hoods, and right-sizing the HVAC system can help keep operating costs in check.

Differences in laboratory hood engineered airflow

Most labs can be optionally equipped with hoods that have differing ventilation requirements (i.e., ducted to a building’s exterior venting system or ductless) but nonetheless maintain the proper airflow and filtration standards required for specific applications.

Some applications may require hoods with dedicated access to the building’s exterior venting system. Ducted hoods of this type typically draw in conditioned room air and exhaust that air into ventilation ducts, creating a need for replacement air (sometimes referred to as make-up air) that has been conditioned by the HVAC system.

Ductless hoods don’t require make-up air

Ductless hoods, those that do not require dedicated access to a ventilation system, maintain cleanliness standards by drawing in room air, filtering it to remove contaminants [spores, bacteria, vapors, particulates, etc.] and exhausting the filtered, conditioned air back into the room.

You can find a method for calculating the air conditioning costs of both ducted and ductless hoods here.

An additional benefit of a ductless hood is portability. In the case of overcrowding or a need to move an application to another room, ductless hoods are not tied to a ventilation duct and can be readily moved.
Many of Sentry Air’s customers are able to use our ductless hoods in typical office workspaces.
 Each application, of course, must be analyzed to determine the appropriate hood and filter media type and ventilation environment.

Although Sentry Air Systems hoods can be vented outside via a building’s exhaust system, most of our hoods are ordered ductless by our customers. In that configuration, the hoods recycle filtered air back into the air-conditioned work area.

Our ductless hoods are used in many scientific applications, including powder containment, chemical fume containment, and biological applications.
Because of their portability, our hoods are sometimes used in field testing applications to create a compact and filtered negative-pressure environment for operator protection during certain research activities.
“I am using the Sentry Air unit to conduct tissue extraction in an outdoor field setting. I work with a very powerful fixative solution that I use when I extract brain tissue, and the unit has proven to be both protective and efficient in containing the chemical within the hood. The system is also portable and easy to move, which has proven to be of great importance as I move between field sites frequently. Assembly and disassembly is very simple, and the unit has proven to fit my needs as a molecular biologist working outdoors under a variety of conditions, be it changing weather or terrain. I would highly recommend this fume hood to any biologist or chemist that requires protection and portability, as it has proven to be both durable and reliable.”
— SarahA. Laredo, Animal Behavior Graduate Student, Universityof California, Davis

Making choices

Choosing appropriate types of lab hoods starts with a thorough hazard analysis. The OH&S website [] has published a guideto choosing laboratory fume hoods that touches upon the role HVAC specialists and industrial hygienists play in appropriate hood selection and HVAC sizing.

The Labs for the 21stCentury website,, offers a variety of tools for energy management in laboratories, including case studies, technical bulletins, and a design process manual.

For more information about our laboratory products, visit the ductless fume hoods section of the Sentry Air Systems website.

For information about our energy-efficient, positive pressure hoods, check out our portable clean rooms, or call us at 800-799-4609or 713-690-2153.