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The good news is that poor humidity and temperature levels are likely to be the cause of occupant discomfort. And while not all building occupants will be satisfied by the same thermal conditions, ASHRAE has attempted to identify norms, based on a Predictive Mean Vote, of 80 % satisfaction. Many states are now adopting ANSI/ASHRAE Standards 55-2004 on humidity and 62-2004 on IAQ into their building codes.
ASHRAE Standard 55-2004 sets an upper limit to absolute humidity levels, above which most occupants become uncomfortable, while 62-2001 specifies that "Relative humidity in habitable spaces preferably should be maintained between 30 % and 60 % relative humidity to minimize growth of allergenic or pathogenic organisms." Updated standards are more specific.
When thinking of heating, cooling and humidity controls, equipment selection is extremely important. Equipment should be staged or have some sort of capacity control. On average, 2.5 % of extreme seasonal temperatures are beyond equipment capacity making the equipment undersized during that time. However, oversized equipment increases the risk of moisture related complaints and problems.
Most humidity-related comfort complaints occur at part load conditions when equipment run times are shorter. Less operating time means less moisture removal. Changing from a fixed restrictor metering device to a thermal expansion valve will ensure maximum evaporator capacity at part load conditions. Other alternatives would be a Timed-On-Control device, dehumidifer, and making sure high moisture areas vent to the outside.
From dry bulb temperature and relative humidity measurements, temperature-humidity meters such as the Fluke 971 can calculate wet bulb temperature and dew point temperature, and psychrometric points that are essential for HVAC evaluations and diagnostics.
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