A/C in the Southeast
Part 1: Challenges of Cooling in Hot-Humid Climates
It takes a lot of energy to keep a home cool in the hot-humid climate of the southeastern United States, and in some places cooling systems work year-round. The population in the Southeast has grown over the last decade, and the need for cooling in homes is greater than ever.
Homeowners in the Southeast are becoming more aware of their energy use. And now they have access to more tools they can use to manage it, as utility companies update to smart meters. In 2013, Florida Power & Light Company installed smart meters in the 4.6 million residences it serves, at no charge. Everyone in FPL’s service area now has access to energy dashboards that show more data than ever before, regardless of type of structure, household, or income. Data on local temperature, humidity, and energy use down to the hour allow users to make comparisons and wise decisions.
Many parts of the Southeast stay hot and humid for months at a time. These conditions combine to create a cooling problem that other regions of the United States do not face. There are two cooling loads of concern inside a dwelling: sensible cooling, which lowers the air temperature, and latent cooling, which controls the moisture in the air. Most homes have a thermostat that controls temperature to a set point, but the humidity is usually a byproduct of how well the A/C operates.
The cooling industry in the Southeast was built on immensely oversized high-profit outdoor systems that needed servicing often. Only recently has it begun to look at removing humidity as part of cooling. Historically this region has been slow to adopt new systems. There were no incentives for builders or contractors to install new system types with which they were unfamiliar. But the industry is slowly changing.
Typical Cooling in the Southeast
Most cooling systems installed in homes in the Southeast are fixed-capacity, central, ducted systems. A fixed 3-ton system, when turned on, for example, provides 3,600 Btu/hr until the temperature setting at the thermostat is met. The units cycle on and off throughout the day, typically three to five times per hour.
Cooling with a heat pump is also common in southeastern states. Heat pumps cool and can run in reverse mode to provide heating during winter months. In north and central Florida, 80–90% of all new cooling systems are heat pumps, according to Jim Cummings, program director of Buildings Research, who specializes in researching energy use, airflow, and duct leakage at the Florida Solar Energy Center (FSEC). He says heat pumps work well in meeting cooling loads south of 35° latitude.
Most A/C systems are controlled by a thermostat and typically turn on based on temperature. Some contain a humidistat. But Cummings explains that the newest systems have different operating modes, making them excellent for use in hot-humid climates, because they can run at very low airflows when more moisture is present. Manufacturers are helping dehumidify the air by producing systems with lower airflows, so the coil gets colder and moisture condenses faster.
Every system has a sensible heat ratio (SHR) that indicates how much moisture it will remove. To remove more moisture than the SHR of the system, Cummings says, something about the system runs has to change. The compressor speed must be increased, or the fan speed must be reduced, for example. Sometimes hot gas is injected to make the system run longer and remove more moisture, but this method is less efficient.
Standard Practice: Oversizing
A National Renewable Energy Laboratory (NREL) analysis on oversizing A/C units found that sizing doesn’t have a huge impact on total annual energy consumption. Dane Christensen, senior engineer in the Residential Building Efficiency group at NREL, says that oversizing doesn’t hurt too badly, but it does increase the homeowner’s initial cost and the installer’s profit on a larger unit. A properly sized or slightly larger unit at the higher efficiency level is a tradeoff that homeowners might be willing to make based on the installer’s calculations and recommendations.
“Dramatically oversizing is a fairly standard practice, almost like an insurance policy. They oversize in case they may have misinterpreted the R-value in a wall, and they don’t want to be called to fix the unit on the one hottest day of the year at their cost,” Christensen says.
Doug Garrett is a certified energy manager, and president of Building Performance & Comfort, Incorporated. Garrett sees many errors in the single-family homes he serves near Austin, Texas. He says that the two most common errors he sees are undersized ductwork and oversized A/C equipment. “These are still ubiquitous problems, especially in the South, where bigger units are installed to handle the higher cooling load,” he says.
If more air is moved, bigger ducts are needed. Even though there are rules of thumb for sizing registers and ducts, Garrett’s airflow measurements commonly show that units move only 60–65% of the air that they should be moving. If less than the right amount of air is moved, the cooling capacity of the equipment and real output are reduced. The airflow to each room is also reduced, so residents end up with the same energy bill but lower comfort levels and lower efficiency.
Garrett says that when performing home investigations, he almost always adds return air grilles and more return ducts. “Ninety percent of the time, that’s part of the fix. The same companies are making the same mistakes installing units on homes constructed at all price levels. Crews are consistent.”
Efficient Cooling: Variable-Capacity Systems
The SEER ratings of the newest traditional fixed-capacity systems are typically 13–16. Two-stage systems have a lower first-stage capacity and a second stage with larger cooling loads. They’re more efficient and can reach SEER efficiencies of 16–19. To cool with the highest SEER ratings of 20–28, true variable-capacity systems are needed.
Instead of turning on and off, variable capacity systems run all the time, adjusting the cooling rate by changing the compressor speed and the output of the full range. Systems can be central and ducted, or ductless. Ducted central variable-capacity heat pumps are fairly new; they were introduced in 2009.
According to Garrett, 80–90% of new construction in the Southeast is built with true variable-capacity heat pump systems that are central and ducted. Variable-capacity heat pumps are A/C units that can also operate in the reverse cycle during the heating season.
The highest-efficiency variable-capacity systems cost more initially, but over the life cycle of the unit, they can cost less when the cost of energy is included. Garrett says that “new-construction contractors are sensitive to spending the extra money to include a high-efficiency unit, since they would have to sell the customer on the long-term advantage versus the additional up-front cost, so [high-efficiency units] are not widely adopted yet. They are adopted in retrofits, but not widely, because the customers aren’t educated about their choices. It’s a tough decision, between an extra few thousand dollars now and the additional energy savings over ten or fifteen years. The rate of adoption for new central variable-capacity systems hasn’t been rapid.”
More on Efficient Cooling: Mini-splits—an Important Option
A mini-split heat pump is one type of variable-capacity system that is designed to heat or cool only one room or open zone of a house. They are smaller than traditional A/C units and come in ¼-ton to 2-ton capacity. They are widely used in most other parts of the world, especially in hot-humid Southeast Asia. Only in the last decade have they become widely available in the United States.
Mini-splits have many appealing features. One is that they have very high-efficiency SEER ratings, up to 28. Another is their ability to be used in buildings without ducts to replace window A/C units, since mini-splits are not central or ducted systems. Mini-splits are considerably more expensive than window units, since they have to be installed by a contractor. However, they are much more efficient and much quieter than window units.
The technology is not new; for years mini-splits have been used in hot-humid climates overseas, and manufacturers have worked to produce extremely high-quality systems. There are a few manufacturers in the United States. But Cummings says that since heat pumps aren’t commonly used north of 35° latitude (except in the coastal regions of the Pacific Northwest, where temperatures aren’t that cold), we don’t have much hard data about their use—and most of what we do have is anecdotal.
Cummings says the logical line of thought is to use mini-splits for renovations, but some builders and HVAC contractors also install them to supplement the central A/C. “A 1-ton mini-split with a SEER rating of 25 could be installed for $3,500 in a house with a 13 SEER unit. It is almost twice as efficient, and there are no losses associated with distribution, since there are no ducts. A hot attic and possible leaking ducts can bring that home’s 13 SEER heat pump down to an effective 9.8 SEER. By installing a high-efficiency mini-split, annual A/C bills could be reduced, in some cases cutting annual cooling costs in half. They are not often installed as a backup—more as a primary—but if the mini-split can’t satisfy the temperature requirements, a central system is used.”
Garrett agrees. In the South, central systems have been the dominant player. “Since mini-splits haven’t been used here historically, they’re not as common,” he says. “But because of their superhigh efficiency and acoustic advantages they are starting to make inroads in the South and are becoming more popular. More contractors are starting to install and service them in the Southeast.”
Don’t Forget Ceiling Fans
Ceiling fans are very common in the Southeast. NREL’s Christensen says these low-tech solutions are one of the most efficient ways for people to stay comfortable and save energy, since they operate on a different principle from air conditioners. A/C absorbs heat from inside a building and moves it to the outside, using a lot of energy along the way. The blowing air in ceiling fans cools in two ways: It helps move already-cooled air inside the home to the occupant’s body, and it helps cool the occupant by evaporating off his or her sweat.
Using both A/C and a ceiling fan is very effective and can save energy. Normally, the air immediately surrounding the body doesn’t move, and clothes hold the body’s warmth close. If the air weren’t moving (from the air duct to the person, and then through the clothes), a lower setting would be needed to stay cool. When moving air flows over the skin, the body feels cooler, so with a ceiling fan, the A/C can be set to a higher temperature. It will cool to the same level of comfort and use less energy.
Two challenges with ceiling fans that Christensen mentions are remembering to turn them off when you leave home, and the fact that they don’t follow you as you move from room to room. As a result, ceiling fans are most commonly installed in the places where people spend the longest time: over a bed and in the living room. Like other units, ceiling fans have different energy efficiencies. They are rated in the ceiling fan category at www.EnergyStar.gov.
New Cooling Technology for Dealing with Humidity: High-Velocity HVAC
In an older home that hasn’t had an energy retrofit, many leaks that raise energy bills allow hot, humid air to enter the house. Dehumidification is also a problem in very small homes, because A/C doesn’t run very long in tightly sealed structures.
A new option for homes without ducts or retrofits is Unico, Incorporated’s small-duct, high-velocity cooling system. This technology cools a house in zones, and distributes air in small, round tubes.
The Unico system works with smaller volumes of air across a coil that is colder than in traditional A/C coils. It provides about 250–300 CFM/ton of cooling, while conventional systems provide around 400–450 CFM/ton. With a higher temperature drop across the coil, the Unico works like a dehumidification device to pull more humidity out of the air. Randy Niederer, LEED AP Homes and vice president of marketing at Unico in St. Louis, Missouri, explains that the system is used in both retrofit and older homes that don’t have A/C but need it.
Instead of using a big block vent, the Unico system supplies air in five small portals where tubes centrally heat and cool homes with no preexisting ductwork. The smaller air handler and the 10-inch round duct-work, which is one-third the size of conventional ducts, is easy for an A/C contractor to integrate into an existing structure, requiring only minor carpentry work.
A conventional system diffuses cold air out of registers and has lots of cold air returns throughout the house that draw the air across the room. The Unico system operates at high velocity and streams the air out of more small 2-inch-round outlets without the need for a cold-air return. It operates on the principle of aspiration, which pulls the room air into the stream of air coming out of the outlet. Niederer says it’s like jets swirling water around everywhere.
“The difference is in the way the air is delivered,” Niederer says. “There are more outlets that mix the entire room with air. It’s quiet, and there are no dead spots in the corners or hot spots at the ceiling and cool at the floor, which can happen in a system with registers.”
Unico has installations all over the country and in the Caribbean. Units have been installed in a large project in New Orleans, including LEED Platinum homes. Most of Unico’s installations in homes in the Southeast are used for their ability to both cool and dehumidify the air.
Niederer says that NREL recently completed humidity testing on a Unico unit that was used without fresh air, to make sure that it met the 42–55% relative humidity guideline in ASHRAE 62.2. This study compared performance during a humid summer month to a performance during a less-humid month in fall. Niederer says initial results show that the Unico unit met the guideline, which suggests that this unit may be a solution to cooling homes and handling moisture in homes without duct systems.
Stay tuned for the next article in this special two-part series on cooling in the Southeast. Part 2 will focus on humidity control.
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