Vented fume hoods (both constant air volume and variable air volume) constantly remove conditioned air (heated or cooled) from the lab without the ability to easily turn them off when not needed. This is like heating your home in the winter with the windows and doors open. Fans inside the fume hoods contribute to energy consumption but the loss of conditioned air is usually the largest source of energy consumption. Vented fume hoods are often the largest energy consumer in the lab.
Vented fume hoods are the most common category of fume hood in laboratories today. However, in select applications, ductless fume hoods are being implemented as a more energy efficient alternative. Among vented fume hoods, there are two types: constant air volume and variable air volume. There are approximately 300 CAV (constant air volume) and 200 VAV (variable air volume) fume hoods at CU Boulder currently. There are just a handful of ductless fume hoods.