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HVAC BALANCE Air System Survey - Call us at (800) 217-8437We test and adjust blower RPM to design requirements. We check to verify that all start up is complete. We test and record leaving air temperatures - D.B. heating a cooling coils. A multi-zone unit uses a single fan to produce both cold and hot air. Cooling and heating coils are both used to produce either cold air, hot air, or a mixture of both when the space temperature has been satisfied. Actuators are used to move mixing dampers in the discharge of the unit to achieve the correct position to meet space temperature requirements. The unit can maintain a designed discharge air temperature on both the cooling and heating sides of the unit by modulating the chilled and hot water valves. The unit is made up of 2 or more zones serving various spaces. Each zone duct will have a manual balancing damper that MUST be installed to balance out the airflow on each zone duct. Design airflow should be set first by pitot tube traverse through the zones before any grille balancing is started. In VAV systems, the supply-air quantity varies with load, but the minimum required outside-air quantity does not change. Exhausts, such as toilet exhausts, might be constant regardless of load. Design occupancy and the resulting outside-air quantity required for ventilation also might be constant. If the minimum required outside-air quantity remains constant, but the supply-air quantity decreases, the minimum required outside-air quantity increases as a percentage of the supply-air quantity. When a system operates with minimum outside air (and an economizer is not active), a fixed minimum-outside-air damper position will result in smaller amounts of outside air as the VAV supply-air quantity decreases. Builders will soon need to get up to speed on duct testing, since recent code changes will require that all residential duct systems except those that are located entirely within a home thermal envelope will need to be tested for leakage. If some ducts are outside of the thermal envelope, the 2009 IRC will require duct tightness to be verified by either a rough-in test or a post-construction test. Either test requires all register boots to be taped or otherwise sealed during the test. The threshold for the rough-in test is total duct system leakage of 6 cfm per 100 square feet of conditioned floor area (when tested at 25 Pascals). If the air handler is not installed, the total leakage must be less than or equal to 4 cfm per 100 square feet of conditioned floor area. The threshold for the post-construction test is duct system leakage to outdoors of 8 cubic feet per minute (cfm) per 100 square feet of conditioned floor area when tested at 25 Pascals. Alternatively, total duct system leakage must be less than or equal to 12 cfm per 100 square feet of conditioned floor area. There are three types of VAV boxes: traditional, fan-powered and VAV with reheat. The traditional VAV box consists of VAV box with an integral damper and is controlled by thermostat. This is the most basic type of VAV box. The fan-powered VAV is built the same as the traditional, but it also contains a fan which is triggered when the area reaches optimal temperature or the damper is 50% closed. It provides additional air circulation when there is less demand for conditioned air. A VAV with reheat also is built the same as the traditional VAV, but it contains an electric coil designed to reheat the air when the damper reaches a predetermined position. This feature allows heating in the specific zone instead of having to heat the whole building. In many buildings, the air distribution system also includes a return air system so that conditioned supply air is returned to the AHU (return air) where it is mixed with supply air, re-filtered, re-conditioned, and re-circulated throughout the building. This is usually accomplished by drawing air from the occupied space and returning it to the AHU by: (1) ducted returns, wherein air is collected from each room or zone using return air devices in the ceiling or walls that are directly connected by duct work to the air-handling unit; or (2) plenum returns, wherein air is collected from several rooms or zones through return air devices that empty into the negatively pressurized ceiling plenum (the space between the drop ceiling and the real ceiling); the air is then returned to the air-handling unit by duct work or structural conduits. The use of high performance HVAC equipment can result in considerable energy, emissions, and cost savings (10%-40%). Whole building design coupled with an extended comfort zone can produce much greater savings (40%-70%). Extended comfort includes employing concepts such as providing warmer, but drier air using desiccant de-humidification in summer, or cooler air with warmer windows and warmer walls in winter. In addition, high-performance HVAC can provide increased user thermal comfort, and contribute to improved indoor environmental quality (IEQ). Proper performance and energy-efficient operation of HVAC systems can only be ensured through a successful O&M program. The building design team should provide systems that will perform effectively at the level of maintenance that the owner is able to provide. In turn, the owner must understand that different components of the HVAC system will require different degrees of maintenance to perform properly. |
