Relative Humidity & A Few Ways to Reduce It Tech to Tech August 07 “Make daily deposits to your box of knowledge, soon it will have many reference cards.”--Randal S. Ripley Does a hot air system remove the Now say the sponge just suddenly moisture from the air circulating through doubled in size. The size of the sponge, its it? ability to hold water and the relative If this question means that it reduces humidity have changed but the amount of the actual moisture content in the air as it water in the sponge is the same. passes over the heat exchanger, the answer When you doubled the size of the is “no”. sponge, you are also doubling the amount of When an air sample is heated, the water it can hold. Since you didn’t add amount of moisture in the air sample anymore water, the relative humidity is now remains the same. This is referred to as the only 50%. absolute humidity and is measured in When air is heat it expands and grains per pound of air or as grains per some increases its ability to hold moisture. This unit volume. It takes 7,000 grains of causes the relative humidity to drop. moisture to make 1 pound of water. The reason our skin, nose and eyes, When water vapor is heated, what furniture, etc, dry out under low humidity do we get? You guessed it, water vapor. conditions is because the human body or There is no change of water quantity; anything else that contains moisture will therefore the amount of water vapor in the release this moisture at a higher rate as the air sample entering the heat exchanger will relative humidity goes lower. be the same amount in the supply air to the The reason we use humidifiers is not conditioned space. to replace moisture that is supposedly You may wonder if what I am removed from the air by the furnace, but to saying is true, why is there an increase in add moisture to raise the relative humidity static electricity shocks, cracking furniture, of the conditioned space. dry skin and nosebleeds, all sign of low So now you know that when air is humidity levels during the heating season? heated its ability to hold moisture increases, The answer for this is that while the well the opposite is also true, as air is cooled absolute humidity doesn’t change, the its ability to hold moisture decreases. relative humidity does. When you set a glass of ice water on Relative Humidity is defined as the the counter on a hot day it will get wet on ratio of water-vapor pressure—amount the outside of the glass, right? Where is that currently in the air to the saturation-vapor water coming from? It certainly isn’t coming pressure—the amount the air can hold—at a from inside the glass. given air temperature. What? The water vapor (humidity) in the If you were holding a sponge that air is coming in contact with the glass and was completely saturated with water, being cooled to its dew point, causing the meaning if you added one more drop of water to condense from a vapor to a liquid. water it would drip from the bottom of the sponge, it can be said the sponge is now Can a system with a variable speed motor holding the maximum amount of water that dehumidify better than a multi-speed it can and its relative humidity is 100%. motor? An air conditioning system does remove sensible heat (heat added or some dehumidifying as a byproduct of the removed to raise or lower temperature) but cooling process but is not designed to does nothing about removing the latent heat control humidity below 50% on a continual (heat added or removed to change the state basis. such as ice to water, water to vapor, or vise Today’s high efficiency systems versa) in the water vapor. In a nutshell, less struggle to control humidity and provide run time equals less humidity removal. energy efficiency at the same time. Changing the metering device to a The larger evaporator coils in these thermal expansion valve will give maximum higher efficiency systems have more evaporator capacity at part load conditions evaporative space and the industry still uses because it utilizes more coil surface for the 400 CFM per ton rule, which can cause moisture control. higher coil temperatures and less ability to remove moisture. Combination Temperature and The upside of these larger coils is humidistat’s controls can be used to help they also can remove more humidity from control humidity in cases of high humidity the air than the smaller 10 SEER coils when low load days. airflow is reduced making the coil colder. These controls when they sense The Goodman variable speed unit humidity outside the set parameters will has the ability for the motor to ramp up to change the system airflow speed from high 50% of the required airflow for 30 seconds; (the normal A/C speed) to a lower speed it will then ramp up to 82% of the required (multi-speed motor required) to remove airflow for 7 1/2 minutes and finally go to more humidity. They will cool to 3 degrees 100% of the airflow for the remainder of the below set point if needed and then shut off. cycle. If the relative humidity is still high the unit By reducing the airflow over the will cycle again. coil, the coil temperature will go further Proper sizing of the unit is the most below the dew point (the temperature that important thing a contractor can do. In hot, water vapor will condense to liquid if cooled humid weather a properly sized unit will any further) removing more humidity from have longer run cycles, removing more the air than the normal operating cycle. humidity from the air. Most of the variable speed units on the market today have some form of super In case you’re interested: dehumidification. Most experts agree that a range of 30-60 percent relative humidity is the safest Why is a TEV (thermal expansion valve) and healthiest for keeping occupants from better for humidity removal than a fixed feeling the air is too dry or humid. bore (piston) metering device? Humidity in basements should be When a unit doesn’t have capacity regulated to 60 percent or less to prevent control such as a fixed bore metering device, microbial growth. most humidity control issues occur at part Next month I will tell you the story load (lower dry bulb temperatures but still of the human furnace. An understanding of having high humidity) conditions. how the body controls its temperature is Because we use dry bulb necessary if you want to properly design, thermostats to control the equipment, run install, adjust and service equipment that times will be shorter on part load cycles aids the human body in its quest for personal which will also reduce humidity removal. comfort. Oversized equipment will make part Join the email club is easy, just send load conditions even worse for the occupant me an email [email protected] and because it will bring the space to its set dry you will be added to the list. bulb temperature. Short run times only Randal.