Basic Documentation

Choice of Ventilation System
In a facility with a VAV type of ventilation system, individual laboratory room air needs will
vary mainly in accordance with the extent that VAV fume hood sashes are open. If fume hood
users open their sashes only when they actually need access to the inside of their hoods and
only as much as necessary, the amount of air that needs to be supplied to the laboratory
rooms will be minimized. Considering also that the length of time a fume hood really needs to
have its sash fully open will be minimal, and that these instances will be staggered in time
throughout any given day, a large facility with many VAV fume hoods will likely never need to
supply the maximum possible amount of air to every lab at the same time.
With this in mind, it is conceivable to even reduce the overall size and air capacity of VAV
primary air handling units for a new facility so that they only need to be capable of supplying
a nominal expected maximum amount of air rather than the theoretical maximum amount of
air. This practice of “downsizing” VAV primary air handling units is commonly referred to as
applying a VAV diversity factor and nominally allows at least a 40% reduction in equipment
sizing. In other words, the facility’s HVAC systems and associated mechanical equipment
(boilers, chillers, pumps, etc.) could be sized based on being able to provide about 60% (or
less) of the maximum or worst case overall facility demand and still be able to meet actual
facility needs virtually all the time.
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It should be noted that since CAV systems have no
provisions for reducing the room supply air or exhaust air during occupied periods, a CAV
system whether a one or two position type of system, must typically be sized to provide 100%
of the maximum or worst case capacity.
When applying a diversity factor or downsizing VAV systems and equipment, the air
distribution ductwork within the facility can also be included in the list of equipment that can
be considered for downsizing. Downsizing can also be applied to virtually all of the primary
mechanical systems (boilers, chillers, pumps, converters, piping, etc.), since the reduced
ventilation air will also enable a reduction in the capacity of the mechanical systems that
condition the supply air. Thus, in addition to providing a significant reduction in annual
laboratory facility energy costs, VAV systems can also provide lower first costs due to
downsizing of the systems.
Choice of Ventilation System
Any basic type of ventilation system if properly designed can provide the required level of
occupant protection, ambient environment and comfort required for a laboratory facility. The
selection of the specific type of ventilation system (that is, CAV or VAV) should be a matter of
analysis of the first cost, life cycle cost, maintainability, expandability and in general what is
most applicable for the facility. Whereas VAV has the potential for the lowest operating costs,
it will likely have a higher first cost due to the added VAV control equipment and more
extensive startup and commissioning requirements.
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It must be understood that the possibility still does exist that at some point in time the coincidental opening of a very large
percentage of a facility’s fume hoods, or that situation in combination with very extreme weather conditions could cause the
facility needs to actually exceed the system capacity. The consequences of such a possibility, albeit unlikely, must be considered
before the utilization of a diversity factor in sizing VAV systems and equipment.
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