Latest Discussions : HVAC


12:21AM | 07/22/07
Member Since: 07/21/07
1 lifetime posts
I live at 2400 ft above sea level outside of Beirut, Lebanon. During the winter months and on certain walls, mositure accumulates due to the cold air outside and the hot air inside.

I plan to create wall vents in the form of a 5 inch circular openings just below the ceiling.

Can you please advise as to the number of vents I should create?


07:57PM | 07/24/07
Member Since: 07/18/07
6 lifetime posts
Moisture condenses on cold surfaces which are below dew point. Choices:

A. Remove moisture to lower the dew point. Moist air is heavy. Hot air is light. Therefore locate moisture vent holes as low as practicle to allow moisture to escape and hold hot light air inside. Air circulation removes moisture only if the cold damp outside air is not allowed to replace the warm damp inside air. Eliminate the moisture but retain the heat.

B. Raise surface temp above dew point. Use Sunlight and Insulation combined to maintain temperature above dew point at coldest surface where condensation occurs.

C. Apply heat tape or heat at cold surface to warm the cold surface to a temp above dew point.

D. Caution: Too many vents allow cold damp air to enter which increases the condensation. Start with a limited number of vents on the opposite side of prevailing winds to allow wind suction to extract moisture from air. Do not locate vents directly facing cold wind side. If prevailing winds are from West, locate vents on east with none on North, South, West.


09:03PM | 07/24/07
Member Since: 04/25/05
1915 lifetime posts
"Moist air is heavy. "

Moist air is less dense (lighter).

"3) Humidity. Moist air is less dense than dry air (with all other factors equal); therefore, the aircraft will not perform as well on takeoffs and climbs if the air is moist. However, humidity is not generally considered a major factor in density altitude computations. Under high humidity conditions, the air density may be reduced by as much as three per cent. For example, an airport elevation of 6,000 feet would be converted to a 6,180-foot density altitude, all other factors being equal. That is not a significant change. However, changes in air density created by water vapor will affect engine performance, which again adversely influences the performance of the airplane."




The units of density are mass divided by volume (m/V). Density will increase if either mass increases while the volume remains constant or if volume decreases while mass remains constant.

Density of air will vary as the temperature and moisture content in the air varies. When the temperature increases, the higher molecular motion results in an expansion of volume and thus a decrease in density.

The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air."

"Air circulation removes moisture only if the cold damp outside air is not allowed to replace the warm damp inside air. Eliminate the moisture but retain the heat."

How can you remove moist air from the house and have it replaced with outside air?

"Caution: Too many vents allow cold damp air to enter which increases the condensation."

EXCESS "COLD MOIST" AIR ACTUALLY DRIES OUT A HOUSE. That is why old drafting house often need HUMDIFIERS while new tighter houses heat ventalation system to remove excessive humidity.

Use a psychrometric chart or calculator such as this one.

For example start with 30 F, 70% RH air and bring it in and warm it up 70F and 16% RH.


02:39PM | 07/25/07
Member Since: 01/30/05
360 lifetime posts
heat and air exchange? heat conductivity, convection? diffusion gas exchange?

the difference in density of the air inside and outside, add the wind factor? radiation effects?


05:35PM | 07/25/07
Member Since: 04/25/05
1915 lifetime posts
Things I don't know.

What the humdity level in the house is.

How big the house is.

What the construction of the house is.

How tight the houseis.

What the winter temperature are.

What the winter humidity levels are.

How the house is heated.

How many stories that the house has.

What kind and how much insulation that the house has.

Things that I do know.

Moist air is less dense than dry.

That the affect of difference in desnsity from moisture is slight enough not to have much driving force.

That you can exchange moisture from the inside to the outside just by venting and not have the warm air go out and the cold air come in.

That sticking vent hole in wall until the moisture level is reduced is a fools game.

That controlled ventalation is much better. Even using a simple exhaust fan helps. Specially if it is at the source of moisture addtions to the air (bathroom and kitchen). But the house is tight enough the an ERV.

That bring in COLD MOIST AIR and heating it up does not increase the RH in the house.


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