Our central air conditioning system has no trouble keeping the indoor temperature just where the thermostat is set. But the humidity indoors still seems way too high. It feels clammy and uncomfortable. Why?
An air conditioner both cools the air and makes it drier. Indoor air feels comfortable to most people when both the temperature and the relative humidity are in a certain range. Comfort is a relative term but for argument’s sake, suppose that in summer the goal is 74°F with a relative humidity of about 50%. When an air conditioner can keep up with temperature demands (what engineers call the “sensible load”) but not the humidity (the “latent load”), it’s not going to be very comfortable.
Heating contractors should be able to figure out both of these loads accurately by using Manual J, a set of calculations developed by the Air Conditioning Contractors of America. When sensible and latent loads are added together, the contractor gets the total cooling load. That’s expressed as a certain number of Btus of energy needed per hour. The contractor can choose equipment accordingly.
Sensible and latent loads vary quite a bit depending on where you live. The desert Southwest, for example, has vastly different conditions than the U.S. Southeast. One way of quantifying the difference is by looking at dew points—what the air temperature would have to be for the relative humidity to reach 100% and for moisture to begin to accumulate.
When the dew point is 70°, weather forecasters warn of “oppressive” conditions. When dew points fall into the 40s, you hear that conditions are “brisk” or “refreshing.” High dew points mean higher latent loads. Low dew points mean that an air conditioner will not be called upon to remove as much moisture from the air.
Why the problem occurs
To understand why indoor air can be cool but unpleasantly damp, think of how an air conditioner works. It uses what’s called the vapor compression cycle to cool down an evaporator coil. Air drawn over the coil by a fan will shed heat, and when the temperature of the coil is below the dew point, the air will unload some of the moisture it holds. The moisture condenses, drips off the coil, and eventually goes down the drain. The relative humidity falls, and when it gets down to 55% or so, most people will feel pretty good.
The problem is that air temperature may go down faster than relative humidity, and once the air temperature has reached the set point, the air conditioner turns itself off. Without it, there is no moisture removal. Life with a system like this is an unending cycle: quick bursts of cool air that lower temperature but don’t do much to lower relative humidity. It’s plenty cool, but it’s clammy.
The phenomenon is called short cycling, and it’s made worse by equipment that is too big for the space. There are many experts who believe AC contractors routinely install equipment that is oversized. They may or may not do the required Manual J calculations but even if they do, they keep their thumb on the scale to make sure they never get a call from an irate homeowner complaining the house is too hot.
Tight, well-insulated houses may exacerbate this imbalance because sensible loads tend to be lower while latent loads are still high. A whole-house ventilation system—a near necessity in very tight houses—can make indoor air more humid in the summer.
What you can do about it
An air conditioner is using some energy to lower temperature and some energy to remove moisture. Engineers call this proportion the sensible heat ratio (SHR), or the amount of energy used for the sensible load as a fraction of total energy consumption. A sensible heat ratio of 0.75 would be typical, meaning that 75% of total energy goes to cooling the air and the rest to removing moisture.
In a humid climate, it may be beneficial to choose equipment with a lower sensible heat ratio. Peter Yost, GBA’s technical director and a Vermont-based building consultant, says there are high-performance minisplit heat pumps with SHRs of less than 70%. On central systems, slowing down the fan speed makes the evaporator coil colder, and that will will help remove more moisture from the air. But as one expert explained, there are limits to this approach. Ken Gehring, the former head of Therma-Stor, said that over-cooling the AC’s evaporator coil can create problems of its own, such as condensation inside ductwork or a frozen coil.
David Butler, an Arizona-based consultant, suggests keeping as much moisture as possible out of the house in the first place, a strategy he calls source control. Make sure rain water is directed away from the house, use spot exhaust fans in the kitchen and bathroom, make sure dryer vents are working, and use a range hood that vents to the outside. All of those steps will help reduce indoor humidity and lower latent cooling loads.
Additionally, don’t buy equipment that’s too big for the space you’re trying to cool. As mentioned earlier, heating and cooling contractors have a reputation, deserved or not, for recommending AC systems that are way oversized. Hiring an independent consultant to specify the equipment is one way around this problem. Equipment that’s too big costs more to buy, uses more energy, and encourages short cycling.
Lastly, consider installing a dehumidifier. A whole-house unit will bring down interior relative humidity even when the AC equipment isn’t needed to lower indoor temperatures. This can be really helpful in the shoulder seasons of spring and fall, when there’s essentially no call for sensible heat removal but there are still substantial amounts of moisture in the air. Dehumidifiers are available in a wide variety of sizes—from units designed for houses of 5000 sq. ft. to small appliances that are mounted in a stud cavity in the wall—and they much less expensive to operate than central AC.
–Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.