A/C in the Southeast, Part 2: The Best Way to Cool Homes in Humid Climates?
Fix the Home First
This is the second article of a two-part series on cooling in the Southeast. Part 1 focused on what makes cooling in the region unique. Part 2 focuses on humidity control.
Determining the best way to cool homes in a high-humidity climate is a complex problem with many answers. According to Dane Christensen, senior engineer in Residential Building Efficiency at DOE’s National Renewable Energy Laboratory (NREL), best could mean best opportunity for maximum cost efficiency, best performance, or a single option that provides both. Knowing what’s going on in the home, planning ahead, and researching the options helps homeowners to make the best decision when their existing A/C system fails.
Christensen says that improving the home and installing a new A/C unit will give a better bang for the buck than just buying a new unit. He recommends that homeowners insulate and seal their ducts with mastic so they don’t leak cool air into unoccupied spaces, or pull hot, damp air from attics that can reach 120°F or from crawl spaces. While most ducts in newer homes are insulated to at least R-4, Christensen says there are many older homes in the Southeast with ducts that are not insulated at all. Improving the ducts in both cases reduces the amount of heat that is brought into the house when the air conditioner turns on, so a larger-sized air conditioner won’t be needed to remove that extra heat. He also recommends insulating the attic and air sealing the home, which can further help to reduce the cooling load.
Reducing the amount of heat that an air conditioner has to pull out of a home as much as possible means buying a smaller system—one that costs less to buy and operate. Those cost savings on a smaller system can be put toward purchasing a more efficient A/C unit.
Unfortunately, most A/C systems die on a day when it is really hot, and there is no time to decide whether the ducts need sealing, or the house needs more insulation. Christensen says that homeowners in the Southeast usually demand that the HVAC equipment be replaced quickly.
A 13 SEER system is often the most readily available option, but it is usually not the most cost-efficient option over the life of the equipment. “Energy savings from a 15 SEER air conditioner over the life of the system will more than compensate the initial cost,” says Christensen. “A homeowner strapped for cash may not be able to pay the added initial cost or may not live in the home long enough to make the money back, but overall for the country and whoever buys the house next, it’s really the best option. As a general rule, a 15 SEER unit is almost always the better option. Sometimes going even higher in SEER level may make sense, but some analysis would be necessary. In the Southeast, sometimes the 17 SEER would be justified in some areas, but it is a logical choice to push for the 15 SEER as the first option,” he explains.
Doug Garrett, a certified energy manager and president of Building Performance & Comfort, Incorporated, in Austin, Texas, stresses the importance of having correctly sized ducts and airflow. “Contractors should run a Manual J to specify a right-sized system that will do a decent job of removing moisture and size the ducts to Manual D before installing a unit or ductwork,” he says. “Remember, you can’t oversize a return grille or return duct. That [right-sizing the whole system] would be the right thing to do.”
Garrett says that it is rare to find both calculations completed at a jobsite—and that’s anywhere in the country, not only in the Southeast. He stresses the importance of proper airflow, quoting the opinion of the National Comfort Institute, a residential heating and cooling contractors association that trains in this field, that most systems are starved for air. “Fixing the airflow problem in a residential unit will increase capacity and efficiency. I find high total external pressure and airflow well below the manufacturer’s specification all the time, and add return grilles, return ducts, and additional supply ducts so the system works right and homeowners are happy,” he says.
On the hardware side, Garrett says that “combining thermidistats [combination thermostat and humidistats] and variable-speed or ECM blowers is the best option right now. They are both more expensive, but contractors have learned that they deliver better comfort and lower bills, so they are more often presented as an option to homeowners.” He estimates that 50% of builders in the Southeast include the combination in new construction, while the other half stick to a single-speed blower with a thermostat.
“In most retrofits, there is usually a humidity problem, so the combination is commonly proposed and many are going in. Homeowners are willing to pay the uptick in cost to achieve the comfort and lower energy bills. If it’s presented, homeowners love it, and it is usually a sale. My company encourages A/C contractors to pitch that combination in the South,” says Garrett.
—James Cummings, program director in Buildings Research at the Florida Solar Energy Center
Strategy for the Southeast: Combine A/C and Dehumidifier?
The question of whether to install a dehumidifier in the Southeast to battle the high humidity in homes with A/C has no clear answer. Removing moisture indoors is important for the comfort and health of the residents, and for the durability of the home itself. The controversy arises when one considers the cost in energy and money of solving this problem.
When the A/C runs long enough, as it does in the Southeastern summers, and in older, leaky homes, it can remove enough moisture from the air on its own. But in very small or very sustainable tight homes of say, 1,200 ft2, a 2-ton air conditioner set to 75–78°F may run for only 8–10 minutes to cool the home. That’s too short a cycle to dehumidify it as well. With the thermostat set to 68°F, the system could probably run long enough to dehumidify the space, but cooling below the dew point temperature increases the likelihood of moisture condensation and could wipe out the energy and cost savings of the energy-efficient A/C system.
During the rainy fall and spring seasons, the Southeast is still very humid but not hot enough to run the A/C. Christensen says that under these conditions a small dehumidifier may be the right way to go. Since most Southeastern homes are built on slabs rather than over basements, a small stand-alone dehumidification system is a reasonable option in his experience.
While there are many ways to deal with moisture in a building, running a dehumidifier can be an efficient way to remove moisture from the air. The moisture in the walls is harder to remove, so running a dehumidifier more often could be a good way to solve both problems, according to Christensen.
While dehumidifiers remove moisture from the air and from the walls, they also release the heat back into the house, making the A/C run more often and longer, and raising energy costs. Some brand-new split dehumidifiers release the heat outdoors, but most don’t.
Christensen says that when designing a system that includes a dehumidifier, HVAC contractors should complete a Manual S to deal with these loads so the heat from the dehumidifier is accounted for in the design. “In theory, all HVAC contractors would have both a Manual J and S before selling a system,” he explains. “In practice, it’s not usually true. They may have equipment on their truck likely to be a good match for a home and will often sell a like-for-like replacement. If there is a problem with moisture, the house, or the A/C, it may mean that a different-size A/C unit is needed.”
But James Cummings, a program director in Buildings Research at the Florida Solar Energy Center (FSEC), says to make sure you’re not simply throwing in a dehumidifier to slap a Band-Aid on the problem. He says to look at adding a dehumidifier as a measure of last resort, because dehumidifiers use so much energy. Although dehumidifiers are very effective at removing water vapor, they are not usually cost-effective.
Their cooling efficiency is rated using the energy efficiency ratio (EER) scale. Measured throughout the entire cooling season, dehumidifiers have an EER rating of –3.4, meaning that they do not cool, but rather heat, the building. Combining a dehumidifier with an air conditioner reduces the effective SEER. For this reason, Cummings doesn’t promote the use of dehumidifiers in the Southeast except as a backup. Before homeowners run the dehumidifier full time, he suggests that they explore other options.
Before Using a Dehumidifier
To reduce humidity before using a dehumidifier, homeowners can take, or have their contractor take, the following steps.
Reduce airflow. If central A/C can’t reach a target indoor humidity of 55% RH, most air conditioners have two to four fan speeds that an A/C contractor can set. Have the contractor measure and reduce the airflow down to 32°–36° CFM per ton for better humidity control and improved sensible heat ratio (SHR). The SHR is a measure of efficiency for the ability of A/C system to remove moisture or humidity; the higher the SHR number, the less capable the system. The energy and efficiency penalty of low airflow rates is rather small. An airflow measurement is important when changing the speed, because if it is already low, reducing it further will result in icing the coil.
Repair any duct leaks. There may be significant leakage that draws air into the house from outdoors.
Check for and eliminate unbalanced return airflow. In some homes, supply registers provide cool air in each room of the house, and return air is located centrally in the hallway or living room. If the bedroom door closes with a click when the A/C is on, this may be a sign that there is too great a pressure difference between the supply and return. If the room is at a positive pressure and the house is at a negative pressure, unbalanced return airflow can cause air infiltration, drawing humid air into the house from the attic and outdoors and raising indoor humidity. Florida Building Mechanical Code requires that the pressure differential not exceed 2.5 Pa, so have an inspector measure the pressure with closed doors. Installing extra transfer grilles in the wall or other ducted returns can move the return air back to the air handler and reduce most of the imbalance. Implementing this portion of the code can eliminate a source of humidity. Don’t try to use a dehumidifier to overcome this problem.
Check the refrigerant level in the air conditioner, using the subcooling/superheat method. Have a contractor verify that the refrigerant level is set properly. If it is not, the coil may not be cold enough to remove water vapor, and the A/C system will not operate at peak efficiency. Solving this problem has a double benefit; the system will operate more efficiently and will dehumidify the air more effectively.
Improve the airtightness of the house. Hot, humid air can flow into a leaky house. Improving airtightness can be expensive, but a house that is reasonably airtight saves a lot of energy.
Use the Auto fan setting instead of the On setting on the air conditioner. Sometimes homeowners themselves can cause humidity problems by running the air handler fan at the On setting. (The thermostat setting should be On.) Using the Auto setting, which cycles the fan on and off when the cooling system compressor cycles on and off, is the correct way to operate A/C in the Southern climate for four reasons.
- At the On setting, the fan runs all the time, whether the compressor is providing cooling or not. This moves room air through the ducts 24 hours a day. If there are duct leaks, more moisture enters the house through this air.
- If the ducts are in unconditioned spaces like attics or crawl spaces, they will pick up heat and move it to the rooms, reducing the efficiency of the A/C unit.
- Running fans all the time wastes energy, and fans use a lot of energy.
- Changing from the Auto setting to the On setting may quickly increase indoor humidity by as much as 20% because of the way the cooling coil removes moisture from the air. In hot, humid weather, moisture condenses on a cold coil. After water droplets form, they gradually get bigger until they are large enough to drain by gravity and drip into a drain pan. Cummings says that this takes up to 14 minutes, according to recent research. This is important when using the On setting, because the compressor often cycles on for less than 14 minutes, so the moisture hasn’t yet drained. The fan continues to pass warm room air over the wet coil, where the moisture evaporates into the air and is returned to the room.
If after trying all of the more energy-efficient solutions it isn’t possible to control RH at 45–55%, then Cummings says to consider a dehumidifier. There are times when it is appropriate to run a small unit for a short time—for example, during a humid period in winter when it is too cool to run the air conditioner. Cummings says that a small dehumidifier can be an efficient space-heating system, because lowering the humidity helps to heat the house.
Another time to use dehumidifiers is when the home is not occupied seasonally. It is common in Florida for occupants to leave for five months, and using a dehumidifier can be a more efficient way to keep humidity under control while they’re gone and don’t care about the indoor air temperature. Raising the air temperature by 1ºF lowers the indoor RH by 2%. Raising the thermostat temperature in the summer when the home is unoccupied to 850F, or 100F higher, drops the RH by 20%.
Stand-alone dehumidifiers are a relatively new thing in the Southeast; Garrett says they started appearing only around five years ago. “People didn’t think to use them. When homeowners told contractors they had humidity problems, they would install larger A/C units. That actually made the problem worse. Oversized systems would never get the coil cold enough to dehumidify the air. But now contractors are getting it. They either slow down the blower speed or install a dehumidifier to make their customers happy.”
Dehumidifying Ventilation Air with Mini-Splits
As ASHRAE Standard 62.2 becomes more widely adopted, and more ventilation is built into new homes, researchers at FSEC are finding that hot, humid weather tends to raise indoor humidity. More moisture removal capacity is needed with higher ventilation rates. While the standard recommends that a certain amount of outside air be brought inside, in the Southeast it takes a significant energy load to cool that air because there is so much moisture in it. FSEC researchers are trying to figure out better ways to condition the air without wasting electricity.
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One way to do this is to lower the airflow rate of the air conditioner, but if this is insufficient, Cummings says to use mini-split heat pumps instead of a dehumidifier. Mini-splits have a 21.5 SEER rating, compared to the dehumidifier’s –3.4 EER. Initial testing has shown that using a mini-split is an efficient way to cool a house, condition ventilation air before it is brought indoors, and control indoor humidity.
The pilot test was run on a small commercial test lab building at FSEC with a much higher ventilation rate than would be seen in a home. In the middle of the summer of 2013, hot ventilation air introduced directly into the intake of a very efficient mini-split produced 46% RH, and the unit operated with higher-than-usual efficiency as it responded to the high dew point air run across it.
Cummings says that these very optimistic results have generated a lot of interest, and testing will continue as part of a Building America program. By the end of 2014, the researchers expect to have data showing how efficient it is to use a mini-split to condition ventilated air. Look for the results of the testing in a future Home Energy.
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