Home Energy AC Pro Guide 2011 : Page 4

Average cycle (1,000 Btu) % of continuous running 70 60 50 40 30 20 10 0 0 1 2 3 4 Compressor on-time (minutes) 5 Cycling "city mileage" line PROCTOR ENGINEERING Moisture removal % of total cooling capacity > PRO GUIDE SERIES 4 100 90 80 Air Conditioner Effi ciency “Mileage” 16 Air Conditioner Moisture Removal Blower on and off with indoor unit Steady state (continuous running) highway mileage 14 12 10 8 PROCTOR ENGINEERING 6 4 Continuous blower 2 0 6 0 5 10 15 20 25 30 35 40 45 Compressor on-time (minutes) 50 55 60 Figure 1. The “electricity mileage” (kbtu/kWh) improves when the air conditioner stays on longer. The total energy consumption drops because there are also longer pauses between cycles when the air conditioner does not run. Figure 2. An air conditioner connected so that the blower turns on and off with the compressor provides the most moisture removal. In addition, that moisture removal improves dramatically when the compressor runs longer. Continuous blower operation, on the other hand, reduces moisture removal to zero, unless the compressor runs for more than 13 minutes. bulb temperature). The ability of an air conditioner to lower dry bulb temperature is known as sensible capacity. Homeowners in the rest of the United States, and in particular western microclimates, have to worry about the amount of moisture the outside air brings into the home. When it is hot and humid outside, the air entering the building is cooled, but unless moisture is removed from the incoming air, the indoor relative humidity rises, often making a home feel damp and uncomfortable. A standard thermostat does not measure the amount of moisture in the home. So setting the thermostat lower does not ensure com-fort. What we need in the hot-humid climates is to remove moisture. Can an air conditioner do that? Yes and no. Air conditioners almost always remove some moisture from the air, and that moisture stays on the inside coil. (The ability of an air conditioner to remove mois-ture is known as latent capacity.) But in order to remove a signifi cant amount of moisture, the air conditioner must run long enough for the condensed water to run off the coil and down the condensate drain. For a coil that is starting dry, this can be as long as 10 to 20 min-utes. A short run time—which is what larger air conditioners generally provide—fails to remove suffi cient moisture in a wet climate. An air conditioner connected so that the blower turns on and off with the compressor provides the most moisture removal. In addition, mois-ture removal improves dramatically when the compressor runs longer (see Figure 2). Smaller air conditioners will run longer and do a better job of removing moisture than larger units. The effect of running the blower all the time (an increasing practice) is dramatic as well—in a very different way. Under continuous blower operation, moisture re-moval is zero until the compressor is running over 13 minutes every time it comes on. Most compressor runs are less than 10 minutes. “Are you saying that running the blower continuously is a bad idea?” You bet! First of all, running the blower continuously will cost you big time. In Wisconsin, for example, running a standard blower year-round will cost you an additional 3,560 kWh ($365 at 10¢/kWh) just to pay for running the blower. On top of that, running the blower all the time increases duct system losses. The outside air leakage into the house increases by a factor of four in many houses when the blower is on. This brings hot, moist air into your home in the summer and cold air into your home in the winter. “A bigger air conditioner doesn’t cost that much more.” If your contractor is offering to upsize your air conditioner for very little more, he or she is cutting corners that will cost you comfort and money. A larger air conditioner requires a larger furnace (air handler) and larger ductwork. Without proper ducts and a proper furnace, the air condi-tioner will be noisy and ineffi cient. Furthermore, it is more diffi cult to install the larger ductwork to get proper fl ow to every room. The larger furnace has a larger blower motor that uses more energy. There is a signifi cant increase in blower energy consumption when a 4-ton air handler is used instead of a 3-ton air handler in otherwise identical homes (see Figure 3). “Any new air conditioner is as good as any other.” Most residential air conditioners consist of an outdoor unit and an indoor unit. The outdoor unit consists of the compressor, the condenser coil, and the outside fan. The indoor unit consists of an inside coil on a furnace (air handler). These components are shipped from the factory, and fi nal assembly takes place at your home. The outdoor unit is joined to the inside coil by a set of copper lines that carry the refrigerant to and from the inside coil. Builders or homeowners who look only to the lowest bid are pretty much guaranteed to get what we call a business-as-usual installation. These installations are fast and cheap, but they result in a number of problems. Those problems include ▪ too much or too little refrigerant; ▪ low airfl ow through the inside coil; Home Energy | Professional Guide 2011

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