The engineer performs exhaustive calculations on a building before designing an HVAC system. These take into account the lighting and equipment heat sources, the latent and sensible load of people, in addition to contributions from the usual elements of the building envelope: walls, roof, floor, and windows. The result is a precise amount of air that must be delivered to every room served by an air handler.
Many things can happen after the original commissioning of the building’s mechanical systems. New equipment is brought in, task lights are added, staff changes and furnishings are altered. In addition, the ductwork becomes dirty, diffusers get blocked with dust, and leaks develop in the seams and fittings. All of these things affect the ability of the ductwork to deliver the design quantity of air at the optimum temperature to each zone.
Variable air volume systems are less susceptible to such air- distribution problems because the air to each zone is metered. Only when all the zones are fully open is the older, undersized system unable to deliver the full quantity of air to each zone. This situation, which can occur first thing in the morning at startup or at the peak load hour of the day, may result in a few zones that can’t be cooled or heated sufficiently for comfort until some of the other zones are satisfied, and their supply dampers modulate closed. It is rare, however, for all zones to call for maximum cooling simultaneously.
In constant-volume systems, which are quite common in older buildings, and in smaller buildings recently constructed, the same volume of air is conditioned by the air handler at all times. The ductwork is designed to portion out this air among the zones to provide ideal conditions throughout the facility.
If the internal or external loads in the zones haven’t changed appreciably since the original design, the only potential problem may be in the ductwork. A qualified testing and balancing firm can measure the air delivered to each diffuser. A comparison with the original system can indicate problems in the air-distribution system, such as leaking or dirty ductwork. Rebalancing the system to comply with the original design results in more comfortable conditions in each zone. Of particular importance is the zone where the thermostat is located. If the quantity of air supplied to this zone is inadequate, then all the zones served by the unit suffer the consequences.
If extra equipment is in use in many zones, or other fundamental aspects of the zones have changed appreciably, then it may be necessary to rerun the full building HVAC loads analysis, to determine new air- quantity values to suit the current conditions. Once these are calculated, the system can be rebalanced. The HVAC system will then operate at peak performance and according to the original design intent.
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Excerpt from Energy Conservation Projects.