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Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced each hour.
In cold climates, with a 15 MPH wind, residences often have air exchange rates of 1.0 to 1.5 ACHs, far in excess of the ventilation air needs and are thus called loose construction. It is very easy to reduce infiltration rates to less than 1.0 ACH. Smoke candles and blower-door tests can help identify less-than-obvious leaks.
Room air distribution is characterizing how air is introduced to, flows through, and is removed from spaces. [1] HVAC airflow in spaces generally can be classified by two different types: mixing (or dilution) and displacement .
Conventional overhead mixing systems usually locate both the supply and return air ducts at the ceiling level. Supply air is supplied at velocities higher than typically acceptable for human comfort and the air temperature may be lower, higher, or the same as desired room temperature depending on the cooling/heating load.
During winter months, for example, an air conditioning register can be closed to prevent cold air from being pulled from the room. This allows the hot air to mix more completely with the cold air in the room, improving the efficiency of the HVAC system. [7] (The return should be efficient enough to draw off the cooler air.) [10] [11]
The rating, developed by the American Society of Heating, Refrigerating and Air Conditioning Engineers, reports a filter’s ability to capture larger participles between 0.3 and 10 microns.
Constant air volume (CAV) is a type of heating, ventilating, and air-conditioning system. In a simple CAV system, the supply air flow rate is constant, but the supply air temperature is varied to meet the thermal loads of a space. [1] Most CAV systems are small, and serve a single thermal zone.
Variable refrigerant flow (VRF), also known as variable refrigerant volume (VRV), is an HVAC technology invented by Daikin Industries, Ltd. in 1982. [1] Similar to ductless mini-split systems, VRFs use refrigerant as the primary cooling and heating medium, and are usually less complex than conventional chiller-based systems.
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