From a psychrometric (the study of air and moisture relationship) perspective, 72 degree air at 55% relative humidity is not much different than the standard HVAC indoor design condition of 75 degrees at 50% relative humidity. Specifically:
Originally Posted by Old Lennox
72°F/55%RH yields a dew point of 54.9 degrees and 64.5 grains of moisture per pound of dry air. Total enthalpy (heat content) of this air is 27.3 btu/lb.
75°F/50%RH yields a dew point of 55.1 degrees and 64.9 grains of moisture per pound of dry air. Total enthalpy of this air is 28.1 btu/lb.
So what's the significant difference between these two conditions, you might ask? The main one is that it will cost you more to achieve a lower temperature in the home, when the slightly higher temperature with a lower humidity might keep you as, if not more, comfortable than an artificially depressed temperature.
In other words, what many people do when their systems can't deliver comfort to them at higher temperatures is move the thermostat setting lower. This is often because such a system is a poor dehumidifier. To make up for higher moisture levels, which slows heat loss from the body via evaporation, the temperature is pushed lower, which speeds heat transfer via convection, conduction, and radiation.
The penalty is that the typical residential thermostat only reads temperature, so if the system has to run longer to achieve a lower temperature against outdoor conditions, that spins the meter and costs money. Although a house in summer will gain both sensible (heat you can feel) and latent (hidden) heat, the thermostat only reads sensible. The greater difference there is between the outdoor and indoor temperature, the harder the system must work to achieve that difference, as the rate of heat transfer into the structure increases with a concurrent increase in the temperature difference (delta T) between inside and outside the house.
All this may be more than you wanted to know, but the aforegoing offers an insight as to why most heat load calculation programs choose 75 degrees/50% relative humidity as a target design condition. It is a condition right in the middle of the ASHRAE comfort zone, providing a measure of economy along with comfort. It is true some folks believe they need their environs a bit cooler than others, but often they don't really have any metric other than temperature to determine when they are comfortable. Since I began measuring indoor air conditions, it's been interesting to learn what conditions make me comfortable...and they're often at higher temperatures than I first thought.
Last edited by Shophound; 02-07-2008 at 10:51 AM.