Originally Posted by
thermojohn
One other factor so desperately needs to be included.... The dry bulb. Key word about humidity, it is 'relative' of the dry bulb temperature.
If the space is not heated to 68 or 70 DB, then the humidity would remain what the ambient is. Heat that air up, it expands, and the grains of moisture per lb of dry air decrease, reducing the humidity.
The result is 'dehumidification' without using mechanical refrigeration to do it. It is a magic trick that I think you are beginning to see. I remember the very day I grasped the concept of dew point and how it relates to the 'thermodynamics' around us. I was free, free, free at last, lol.
Lets slow down a little. The quantity of moisture in the outside air is grains per lb. of air. Dew point is the temperature when the air is 100%RH.
An example is 40^F, with rain outside. 40^F, 90%RH, 38^F dew point, 35 grains of moisture per lb. of air.
Warming the air to 68^F does not change the grains or the dew point. The %RH is reduced to 32%RH. The dew point and grains stay the same.
As we slowly purge a home with this heated dry air, the materials in the home and air adjust. After a couple days without occupants, the air in the home will be settle to 32%. When occupancts enter the home expect the %RH to rise, depending on the amount of moisture they add and the moisture content of the fresh air.
When the outdoor dew points are low, expect drying. When the outdoor dew points rise, expect the indoor %RH to rise.
Usually adequate fresh air results in minor humidification during cold weather and dehumidification during weather with dew points +55^F.
Does this make sense.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"