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Well done Harold

Posted by The Insulator on February 19th, 2001 09:15 PM
In reply to Air pressure by Harold Kestenholz - Hydronic Network on February 19th, 2001 05:56 PM [Go to top of thread]

I agree that, if all things are done properly, there should be no need for adding humidity to a house (The only exception I can think of is a tight house that isn't lived in for an extended period of time).

We need to remember that living in a house produces moisture. Showers, baths, cooking, even breathing put moisture in the air. Then if you have a gas stove, the moisture keeps on climbing. If the house has been built properly (Which in this situation means reasonably tight. One of the most persistant myths of building is that "the house needs to breath" between the living space & the structure.), the problem should be getting rid of the moisture, not adding more moisture to the house.

Recently my central exhaust system failed (Bad bearings in the motor), taking away my main house ventilation system. Through our cold northern Wisconsin winter I kept the dehumidistat set at 35-40% RH. Without my usual ventilation, my indoor RH went up to about 50% in 2 days, creating more condensation on the windows. this in a house with only 2 people that are gone a lot of the time. I have never considered a humidifier.

Other factors that can amplify leakage issues and dry a house is an improperly balanced forced air system. If the supply is greater than the return in a given room, this room has a positive pressure, thereby pushing air, and the moisture it is carrying into any leak. If the return is greater than the supply, a negative pressure is created, drawing air in through any leak, dilluting the moisture still in the house. If either of these situation exist, a basic law of physics comes in play (for every action there is an equal & opposite reaction), resulting in every bit of air that comes in, an equal amount of air must go out, and visa versa. Every forced air system has these issues. Just check your bathroom Bathrooms are supply only rooms for, well, obvious reasons).

Another issue I've run into is caused by fresh air intakes attached to the cold air return (In some places this is code, if no other air exchange system is unstalled.) the problem is that when the air handler is running, the cold air return has a negative pressure, outside air is drawn in, effectively pressurizing the whole house, forcing air our through other areas. the net effect is dehumidification.

Understand, please, that my promotion of a tight house is not in without taking into consideration the need for sufficient ventilation. It's just that living space ventilation is not an insulation or house tightness issue. It is it's own issue and needs to approached as such.

The Insulator

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