High Performance HVAC Humidity & the Properties of Air

The Properties of Air and Humidity

Humidity is important to our home and health and it is important to maintain the proper humidity levels in our homes and workplaces for our health. What is humidity? Humidity is simply little water droplets in the air. Cold air is nearly saturated with these little water droplets because the properties of cold air allow the air to hold more water droplets or humidity. As air is heated it expands and holds less humidity. When you heat the air in your home using a furnace, boiler, or heat pump this air expands and becomes drier. It is air with less humidity relatively speaking before it was heated and compared to the outside air. Air that is 30° F with 80% humidity will only be 15% humidity at 75° F. That is a big difference in humidity levels in the home after the air is heated. Granted, you are not heating air from 30° F to 75° F with your furnace. Depending on the set up of your thermostat and heating system you are most likely heating air that is 68° F to around 70° F to 75° F depending on the thermostat setting. The point is unless you have a source providing humidity to the airstream you are simply reheating dry air from inside the structure. There will be minimal infiltration humidity but the infiltration humidity will never be enough to bring the humidity levels up to healthy levels inside the structure for the inhabitants, plants, and furniture.

The Study of Psycrometrics

Yes, that is psycrometrics and it has nothing to do with psychology. It is the study of air and its properties. It is used in HVAC by HVAC technicians and HVAC engineers to determine the properties of air. The HVAC industry uses Psychrometric Charts to plot the properties of air. Using the Psychrometric Chart a technician or engineer can determine the relative humidity, enthalpy value, dry-bulb temperature, wet-bulb temperature, dew point and vapor pressure if they have a few of the listed values. With this data engineers and technicians can solve problems or assign values and set points. For example, an economizer system that is controlled by the building automation system can be controlled by an enthalpy value. This value associated with an And Gate will only allow the economizer to open under certain conditions that will allow the mechanical cooling systems to shut down and take in free cooling from outside the structure. The temperature, humidity, and all the properties of the air will be optimum for free cooling using the economizer. By knowing the properties of air one can set up HVAC systems to work better and one can also use the data provided by knowing the properties of air to troubleshoot problems with HVAC and building systems.

Indoor Air Quality and Proper Levels of Humidity

It is important to maintain proper levels of humidity in the home for many reasons. Comfort is on factor, indoor air quality is another factor, and saving energy is another factor. All three are related in one way or another but each are separate factors. If you reside in the northern climate regions where the average temperatures are less than 20° F you want the indoor humidity to be relative to outside air humidity. Temperatures about 20° F the recommended relative humidity inside the home or business is 35% to 45%. The construction of the home or business will also determine the amount of humidity. If you have proper vapor barriers and good tight windows and doors you need a good mechanical ventilation system. Humidity levels can rise above 60% causing excessive moisture inside the structure which will lead to mold, mildew, and possible water damage. Make sure you have a good mechanical ventilation system inside your structure to avoid moisture issues. Bathroom ventilation fans, hood fans over stoves, and other type of exhaust fans inside the structure pull out excessive moisture when cooking or showering. If you have this problem it most likely shows up in the form of mold growth inside closets, kitchens, bathrooms, and ceilings. Additionally, if you have excessive moisture problems your windows will frequently fog or have a lot of moisture on the windows.

Low Humidity Levels

Low humidity levels cause problems also. Static electricity, dry skin, dry sinuses, and other health problems including increased flu illness can occur. Additionally, low humidity levels cause problems with wood inside the home. To test for low humidity levels in the home make a glass of ice water and set it somewhere away from the kitchen or bathroom. If after five minutes water droplets do not form on the outside of the glass then you have a problem with low humidity levels in your house and you need to get a humidifier. If your structure is suffering from low humidity levels you will also feel cooler resulting in a desire to turn the thermostat up. Therefore adding a humidifier to the HVAC system will cause you to turn the thermostat down. The proper levels of humidity in your home can increase comfort, save you money on your utility bills, and make you healthier.

Additional HVAC related links to help you with Air Conditioning, Heating Systems, and staying comfortable

Air Handlers | Condensing Units | Filters | Thermostats

Your Resource for HVAC Humidity Information

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Comments (3)

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Proper Humidity Levels
written by Bryan, June 19, 2010

I came across this page (http://highperformancehvac.com...crometrics)and questioned some of the content.

In reading this page one draws the infers that you're saying that cold air hold more moisture than warm air, which is obviously not true. I assume you know the difference, and that warm air can "absorb" vapor and hold more moisture than cold air, but this page is misleading.

In any case, I still have questions about basement humidity and AC performance in regard to ducting if I can get a moment of your time and/or advice.

thanks,
Bryan D Kiko

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Warm air holds more moisture
written by Richard, June 28, 2010

50 degree air holds approximately 4.08 grains of moisture while 70 degree air holds approximately 8 grains of moisture according to the psychrometric chart. Put your humidistat in the air stream of air coming from a cold evaporator coil........this is saturated air or air that can not hold anymore moisture. This same air once it gets mixed into the space where it is destined to condition rapidly increases in temperature so therefore the humidity level goes down. This is relative humidity which depends on the temperature of the air - that is the RELATIVE in relative humidity. Colder air and the humidity goes up while the same air at a higher temperature would have a lower relative humidity. I believe I was referring to relative humidity and not how much moisture is in the air. If you have a question please send an email through the contact form. Thanks, Richard

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JW Consulting and Design / Knotty by Nature - Owner
written by Joshua Rather, August 25, 2010

My inlaws had a new HVAC system installed in their home. We do not know what the humidity level was previous to the installation but since the system has been installed the humidity in the home has never gone below 67%. The humidity outside will be 40% and inside still 67 to 71%. They state that the system is the correct size for the home and an addition of a sunroom would require an addition unit. Now they are stating that the system will be able to handle the extra space and if we want the humidity level dropped we need to purchase a dehumidifier from them at a cost of a couple thousand dollars. I have been told that if the correct system has been installed the humidity level should never be that high and something is wrong. Can you please advise? Thank you in advance.

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