If you live in a humid climate (for example, in Florida), you need an air conditioner that does a good job of dehumidification. But if you live in a dry climate (for example, in Nevada), dehumidification is almost irrelevant, because the outdoor air is so dry. In Nevada, all you need is an air conditioner that lowers the temperature of the air in an energy-efficient way.
In theory, air conditioner manufacturers could tweak the design of their equipment to focus mainly on lowering the indoor air temperature (a process known as sensible heat removal), or they could produce equipment that devotes a lot of energy to dehumidification (a process known as latent heat removal). Unfortunately, though, air conditioner manufacturers don’t offer special equipment designed for dry climates. Instead, they sell the same equipment in Nevada that they do in Florida.
Most residential air conditioners sold in the U.S. are designed to operate at a sensible heat ratio between 0.75 or 0.80. (The sum of the sensible load and the latent load is the total load; the sensible heat ratio is the sensible load divided by the total load.) If you live in a very humid climate, this sensible heat ratio might be too high. If you live in a very dry climate, this sensible heat ratio is definitely too low.
John Proctor is the president of Proctor Engineering Group in San Rafael, California, and a nationally known expert on residential cooling. Proctor has always been irked by the failure of air conditioner manufacturers to address regional needs. “Air conditioners are designed and installed the same for Florida and Arizona,” says Proctor. “It makes no sense, since Florida and the Southwest have different climates and different cooling needs.”
A retrofit control to save energy
To improve the efficiency of air conditioners in dry Southwestern states, Proctor has developed a simple control device (the Western Cooling Controlâ„¢) that can reduce cooling energy use by 8% to 12%. The cost of the control $35 — less if purchased in quantity — and it has a payback period of two years or less.
Here’s how it works: most air conditioner controls turn the air handler fan off when the compressor shuts down, or shortly thereafter. Even though the compressor is off, the indoor coil is still cold at that point. The coil is usually damp and dripping, and moisture has accumulated on the drip pan.
In a humid climate, you want the fan to be off at that point, so that all of the water droplets on the coil can drip down to the pan and find their way down the drain. If the fan continued to run after the compressor turned off, the water would re-evaporate and stay in the house — which would be bad, at least in a humid climate.
However, in a dry climate, you really don’t need any dehumidification, so you don’t really care if the moisture on the coil re-evaporates. “Since air conditioners were not designed for dry climates, they wring more water out of the air than they need to, wasting energy and costing you extra on your cooling bill,” Proctor says.
In a dry climate, there is an energy benefit to evaporating the water clinging to the coil and sitting in the pan: after all, the process of evaporation cools the air. (This phenomenon is known as evaporative cooling.) The Western Cooling Control (WCC) overrides the controls supplied by the air conditioner manufacturer and directs the air handler fan to run for a longer period of time after the compressor turns off. The WCC keeps track of compressor run time; the length of time that the fan runs after the compressor turns off is proportional to compressor run time.
It doesn’t take much energy to run the fan for a few extra minutes after the compressor turns off. By evaporating the moisture on the coil and the pan, the air handler temporarily acts like an evaporative cooler. “The energy cost is low, since the fan draws little power and only operates for a short time,” says Proctor.
Contractors can buy the WCC for about $35. For electric utility energy conservation programs, the price of the control is even lower. According to Proctor, “If you are a klutz, it takes 30 minutes to install. But it only takes 15 minutes if you’re good.”
New AC equipment has a fan time delay, but it isn’t long enough
Although newer models of air conditioners are more likely to have some length of fan time delay than older air conditioners, the programmed time delay has not been optimized for dry climates. “Air conditioner manufacturers provide a time delay setting on most new equipment as a means of increasing the SEER rating,” Proctor has written. “The delay is timed to maximize performance on the SEER test, not to provide maximum benefit in dry climates. The 90-second delay commonly used by manufacturers … produced [only] a 3% improvement in sensible EER [in California’s central valley]. There is a major research gap with respect to how longer time delays affect energy consumption, particularly when the time delay interacts with duct system efficiencies. A field survey of 1,687 residential air conditioners of all vintages found that 96% are operating without any time delay.”
Air conditioner manufacturers aren’t particularly interested in developing climate-specific equipment. Matt Lattanzi is the director of product management for Nordyne, a major manufacturer of residential split-system air conditioners. (Nordyne sells its air conditioners under a variety of brand names, including Maytag, Frigidaire, Tappan, and Westinghouse.) Lattanzi told me that Nordyne has no plans to develop equipment that is optimized for different climate zones.
The SEER test is flawed
Unfortunately, air conditioner manufacturers are more interested in obtaining a high number on the SEER test than on maximizing actual energy efficiency in different climates.
Like many other cooling experts, Proctor believes that the SEER test is flawed. “There are three tests used to determine SEER,” Proctor told me. “One is a steady-state test at 50% relative humidity. The other two tests are performed with a 82 degree outdoor temperature and an 80 degree return-air temperature, with an extremely low indoor humidity level, so that there is absolutely no moisture build-up on the coil. These conditions are never found in the field — these conditions are dryer even than conditions in Saudi Arabia. So to improve a unit’s SEER rating, manufacturers have learned to run the fan for 90 seconds after the compressor goes off, because the coil is still cool. But because the test assumes there is no moisture on the coil, they can’t take advantage of any evaporative cooling. If you install this type of unit in Florida, with the fan running for 90 seconds after the compressor goes off, you end up putting the water back into the house, which is a bad idea. And in a dry climate, the fan isn’t running long enough. So the compromise design isn’t right for anyone.”
The test also has unrealistic assumptions concerning the watt draw of the fan and the external static pressure of the duct system. “For units that don’t have a specified air handler, the SEER test assumes that you have a fan with a watt draw of 365 watts per 1,000 cfm, but that is less than what we find in the field,” said Proctor. “Installed units have a median watt draw of over 500 watts. And when they do specify an air handler to go with the unit, the SEER test is run at an external static pressure as low as 1/10 of an inch of water column, and you never find a static pressure that low in the field. The SEER test makes ECM motors look good because the test is run against almost no static pressure. But in the field, the watt draw will be much higher than during the test.”
A retrofit motor to save energy
Proctor’s company, Proctor Engineering Group, has developed an energy-efficient replacement motor for air handlers and furnaces. The motor, dubbed the Concept 3â„¢, is manufactured by McMillan Electric, a motor manufacturer in Woodville, Wisconsin. It is intended to replace the type of motor found in most air handlers and furnaces — a permanent split capacitor (PSC) motor. The motor’s sophisticated controls help save energy in humid climates as well as dry climates.
The Concept 3 is a permanent magnet brushless DC motor that resembles an electronically commutated motor (EMC). Although Concept 3 does not provide one of the touted (but questionable) benefits of an ECM — the ability to maintain a constant airflow under changing static pressures — it is extremely energy-efficient and significantly cheaper than a replacement ECM.
The Concept 3 motor is relatively easy to install. “First you measure the watt draw of the old motor,” said Proctor. “Then you measure the static pressure in the supply plenum with the old motor running. PSC motors are usually set up with a cooling speed and a heating speed. In 95 percent of standard furnaces, there are four speed taps coming off the motor: high speed, medium high speed, medium speed, and low speed. The cooling speed is usually high speed, while the heating speed is usually medium high or medium.
“The speed of the Concept 3 motor is infinitely variable. Once the new motor is installed, you adjust the motor speed to get the same airflow as the original motor. The adjustment is based on the static pressure in the supply plenum — you adjust the motor to achieve the same static pressure. What the adjustment does is change the horsepower output of the motor. It’s a one-time adjustment. At that point you measure the new watt draw to verify the energy savings.”
Proctor warns installers to avoid the temptation to adjust the motor to increase airflow. “Sometimes it is good to just crank up the Concept 3 to its max,” said Proctor. “However, it is always better to increase airflow by addressing problems with the duct system.”
The Concept 3 is not a constant-airflow motor. “There is no feedback, so if restrictions happen in the duct system or filter, the static pressure goes up, airflow goes down, and the watt draw will remain the same,” said Proctor.
The Concept 3 uses line voltage connections for power and 24-volt signal wires to the furnace terminal block for speed selection. The controls are sophisticated, and provide the same intelligent features found in the Western Cooling Control — so if you install a Concept 3 motor, there is no need to install a WCC.
“For dry climates, the Concept 3 control directs the fan to continue running at the end of the cooling cycle, at a much lower speed and watt draw, to evaporate the water off the coil, providing increased sensible efficiency,” said Proctor. The Concept 3 control has efficiency-enhancing algorithms for different climates. “If you live in a wet climate, the 24-volt signal wires are hooked up differently so that you get a lower top speed, which provides more dehumidification in cooling mode,” said Proctor. “In a wet climate, the fan goes off at the end of the compressor cycle, allowing the coil to drain. If the thermostat is set to ‘constant fan’ — in homes with constant ventilation or filtration equipment — the fan still shuts down at the end of the compressor cycle so the coil can drain, and only comes back on after a 20-minute wait.
“If you live in a climate where it is sometimes dry and sometimes wet, you need to install an indoor Thermidistat. The motor controls will then switch from dry climate programming to wet climate programming, depending on the indoor conditions.”
The Concept 3 motor uses significantly less electricity than a comparable PSC motor. “The Concept 3 has a much lower watt draw for the same cfm,” claimed Proctor. “At the lowest speed, it draws between 60 and 100 watts. In a dry climate, the biggest energy savings come during cooling mode, because of the control which runs the motor at a very low speed at the end of the compressor cycle. The additional sensible capacity provided by that feature amounts to an energy saving of about 12 percent, in addition to the 8 percent saving attributable to the improved efficiency of the motor.”
According to Proctor, the Concept 3 “is compatible with all equipment except the Carrier Infinity series, which doesn’t use a standard 24-volt signal.” The motor is distributed by Fieldpiece Instruments of Anaheim, California. Contractors can purchase the Concept 3 for about $220 — significantly less than the cost of an ECM with a control board.
Improving air conditioner performance with the CheckMe! program
Proctor’s company has also developed a successful quality assurance system (CheckMe!¯) to verify the work performed by air conditioner technicians. HVAC contractors who are enrolled in the CheckMe! program collect information on the equipment they are servicing, and then dial a phone number to connect with a trained CheckMe! specialist. After the field technician reports a few required data points — including the model number of the installed equipment, the refrigerant level, the airflow rate over the indoor coil, and in some cases duct system air leakage — the CheckMe! specialist provides feedback to the technician in the field. The system ensures that the equipment’s operating parameters are in line with the manufacturer’s specifications.
According to Proctor, “The average time it takes to make a CheckMe! phone call is 2 minutes and 53 seconds,” says Proctor. “If there is something wrong with one of the reported numbers, the technician is told that something is wrong and needs to be corrected.”
Technicians enrolled in the CheckMe! program are far more likely to catch problems with two of the most crucial variables affecting air conditioner performance — incorrect refrigerant charge and low airflow over the indoor coil. The result: routine service calls can be used to improve the energy efficiency of residential air conditioners.
In many locations, the CheckMe! program is subsidized by a local electric utility. Over 6,200 service technicians have been trained in the CheckMe!
Dry climate recommendations
Proctor has developed a list of recommendations for optimizing the performance of air conditioners in dry climates:
- Air seal and test the building envelope and ducts.
- Verify that they system has a high airflow rate over the indoor coil; if necessary, upgrade the return air duct system to improve the airflow rate over the coil.
- Make sure that the fan runs for a period of time after the compressor shuts off.
- Better yet, install a Concept 3 motor to run the fan after the compressor shuts off.
- Make sure that the ducts are well insulated or are installed inside the building’s conditioned space.
Although the rule of thumb for residential air conditioners is to provide 400 cfm of airflow per ton of cooling capacity, most residential installations have much lower airflow rates. According to Proctor, “In California, a couple of studies have shown that the median airflow for installed systems is around 325 cfm per ton, with lots of systems worse than that.” Proctor advises a minimum airflow rate of 350 cfm per ton. More is better — at least in dry climates.
The main reason for low airflow over the indoor coil is an undersized return-air duct system. Other common problems include undersized supply ducts, convoluted ducts, or leaky ducts.
In a dry climate, the optimum air flow per ton of cooling is “probably around 500 cfm per ton,” Proctor told me. “There isn’t any downside to a high airflow, as long as the increased watt draw of the fan motor doesn’t exceed the savings from the energy-efficiency improvement.”
In a humid climate, on the other hand, a lower air flow rate (less than 350 cfm per ton) will do a better job of dehumidification than a higher rate.
More recommendations for humid climates
I asked Proctor to provide advice for homeowners in humid states like Florida. “Number one: do not run your fan all the time,” he answered. “Running your fan all the time is always a bad idea, but it is even worse if Florida than it is in a dry climate, because it totally eliminates your dehumidification.
“The second thing to do is to make sure you have no time delay,” he said. “With most air conditioners, you can turn off the 90-second time delay if there is one.”
Regional appliance standards
A coalition of energy-efficiency groups — including the Natural Resources Defense Council, the Alliance to Save Energy, and the American Council for an Energy-Efficient Economy — spent several years lobbying legislators in Washington and the U.S. Department of Energy to enact standards requiring air conditioner manufacturers to produce equipment optimized for different climates. Eventually, these lobbying efforts resulted in the passage of new regional standards for air conditioners. The new standards divide the country into three regions (see the map reproduced below). The standards are scheduled to take effect in January 2015. (For more information on the new regional efficiency standards, see the GBA news story on the topic, as well as Harvey Sachs’ article, Guide to Regional Energy Efficiency Standards.)
Unfortunately, the new regional standards are based on a political compromise rather than engineering principles. The new standards specify:
- In the North, split-system air conditioners must have a minimum SEER of 13.
- In the Southeast, split-system air conditioners must have a minimum SEER of 14.
- In the Southwest, most residential split-system air conditioners must have a minimum SEER of 14 and a minimum EER of 12.2.
While these standards have the effect of raising the efficiency bar, they don’t compel manufacturers to design equipment with an optimized fan time delay. “Rather than fixing the SEER test, they increased the SEER a bit and specified the EER for the Southwest,” Proctor told me. “The effort got subverted. It was a political decision. They didn’t think they could win if they addressed the technical problems.”
Martin Holladay’s previous blog: “Energy Upgrades for Beginners.”