Fig. 17-4. Evaporator (cooling) coil mounted in furnace plenum. 236 Air conditioning in .NET

Generation QR Code JIS X 0510 in .NET Fig. 17-4. Evaporator (cooling) coil mounted in furnace plenum. 236 Air conditioning

Fig. 17-4. Evaporator (cooling) coil mounted in furnace plenum. 236 Air conditioning
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method of removing the condensate is not very desirable in those areas where the water will not readily drain because of a high water table or a high clay content in the soil. Even though the amount of water discharging from the condensate drain is small, the introduction of additional water could aggravate a condition that makes the lower level of the structure vulnerable to water seepage. The condensate drain line should terminate at the plenum with a U-shaped trap. Since the condensate drain line is an open pipe leading directly into the cooling coil, the trap (which is partially filled with water) prevents any of the cool air from escaping through the pipe. Look for a trap. If it is missing, record that on your worksheet. Occasionally the condensate drain discharges into a small rectangular box located near the furnace. This box is the reservoir for a lift pump. The purpose of the pump is to lift the condensate to a level where it can then flow to any desired location. Without a pump, it is often necessary to position the drain line so that it blocks a portion of the room or interferes with foot traffic. Check the pump s operation. These pumps have a float control that is activated when the water reaches a preset level. If there is only a small amount of water in the reservoir, the pump can be checked by pouring water from a glass into the reservoir. A malfunctioning pump should be recorded on your worksheet. Now look at the overall furnace plenum around the evaporator coil. Rust and mineral deposits indicate a past or present problem in condensate removal. Water overflowing the condensate drain pan can damage the heat exchanger below. If you see this condition, you should have the furnace heat exchanger checked by a heating contractor for signs of deterioration. (See chapter 14.) When the evaporator coil is located inside the furnace plenum, the blower for the heating
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system is also used as the blower for circulating the cool air. Because cool air is heavier than warm air, when the blower is used for air-conditioning, it should operate at a higher speed. Most often, however, the furnace is equipped with a one-speed motor. Consequently, the air-conditioning system is often not as effective as it might be. A pair of doubleor triple-sheaved hubs can be installed to allow multispeed operation. When the blower is turned off by the master switch, check the tension in the fan belt. There should be no excessive slack. Press the belt midway between the pulleys. If the belt gives more than 3 4 inch to an inch, it is too loose, and adjustment is needed. When the blower is operating, listen for any unusual noises or vibrations. They should be recorded on your worksheet. You might also ask the owner when the unit was last serviced there is no substitute for periodic maintenance. Blower coil When the evaporator coil is housed in a separate casing that contains a blower for circulating the cool air, the coil is commonly referred to as a blower coil. Most often, the blower coil is located in the attic. However, it can be located in a closet or in the basement. The blower coil should be vibration-mounted to prevent the noise of the blower unit from being transmitted into the living area. Vibration mounting can be achieved by placing the unit on rubber, cork, or styrofoam pads. (See FIG. 17-5.) The vibrations might also be isolated in the attic by suspending the unit from the roof rafters. The base of the blower coil is basically a condensate collection pan. The accumulated condensate is removed by means of a drain line that will extend through the exterior wall, terminating on the outside, or extend through the lower portion of the roof, terminating in the gutter. Sometimes the condensate drain line terminates in the plumbing vent stack located in the attic. (See FIG. 17-6.) In many
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Fig. 17-5. Attic-mounted blower coil. Unit is resting on a styrofoam pad to minimize vibrations. Below the unit is an auxiliary condensate drain pan and associated drain line. Note that the main condensate drain line does not have a Ushaped trap. communities, this type of termination is not permitted because it is not in compliance with the plumbing code. If the drain terminates in the vent stack, record that on your worksheet. The legality of this type of connection should then be verified with the local building department. The purpose of the vent stack is to channel sewer gases in the plumbing system to the outside. If the condensate drain line is connected to the vent stack and there is no trap on the drain line, the sewer gases may back up into the condensate drain line, enter the blower coil, and be circulated throughout the house. The condensate drain line should have a U-shaped trap near its connection to the blower-coil housing. On many installations, this trap is omitted. Look for it. If it is missing, one should be installed. When the blower coil is located in the attic, certain steps must be taken to prevent cosmetic damage to the ceiling below in the event of a blockage in the main condensate Air-conditioning systems 237
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Fig. 17-6. Air-conditioning condensate drain line terminating in plumbing vent stack. In many communities, this type of termination does not comply with the plumbing code. drain line. Some blower-coil housings have a fitting for an auxiliary drain line that is located just above the main condensate drain fitting. If the main drain becomes clogged, the level of the condensate will rise and be drawn off by the auxiliary drain. For those blower coils that do not have a fitting in the housing for an auxiliary drain line, there should be an auxiliary drain pan below the unit. The auxiliary pan will collect any condensate that overflows from the main pan when there is a blockage in the main drain line. Look for an auxiliary drain pan. In some parts of the country auxiliary drain pans are installed when the blower coil is located over any furred space, even when the blower housing has an auxiliary drain fitting. Record the absence of one on your worksheet. Unfortunately, many air-conditioning contractors do not install the auxiliary drain or drain pan. Because rising costs make it difficult to remain competitive, they cut costs wherever they can. 238 Air conditioning
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The auxiliary drain pan must have a separate drain line that discharges to the outside. It should not be connected to the main drain line. (See FIG. 17-7.) If it is, it reflects poor-quality workmanship; if the main drain line becomes clogged near the discharge end, the auxiliary drain line will also not function. If the evaporator coil is accessible, it should be inspected for frost buildup. From an efficiency point of view, the attic is the least desirable area for locating the blower coil because of the high temperatures, easily reaching 140 F to 150 F that normally occur during the summer. Even though the blower coil is insulated, there will be a heat gain because of this high temperature. The overall attic temperature, however, can be lowered by increasing the number or size of the attic vent openings. A ridge vent is quite effective, as is a thermostatically controlled power ventilator. Ducts After the air-conditioning system has been operational for about fifteen minutes, the air discharging from the registers should be felt to determine whether it is relatively cool. The temperature of the air discharging from the supply registers should be about 15 degrees lower than the temperature in the room. If the air does not have a slight chill, it might be because there is a heat gain along the duct leading to that register as a result of inadequate insulation, or the system may be undersized or low in refrigerant. While checking the temperature of the air leaving the supply registers, also check the airflow. If the air discharging from the registers has a low flow and appears to be sluggish, it might indicate that there is an obstruction within the system caused by dirty filters or icing on the evaporator coils. Sometimes the condition is caused by an undersized fan or the need for balancing the airflow between the registers. In any case, the condition is abnormal and should be recorded on your worksheet.
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Fig. 17-7. Auxiliary condensate drain line connected to main condensate line. This negates the use of an auxiliary drain and reflects poor-quality workmanship. If the main drain becomes clogged near the discharge end, the auxiliary drain will not function. Note that the U-shaped trap is missing.
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As with a heating system, the location of the supply registers is important for effective air-conditioning. Since cool air is heavier than warm air, the cool air will tend to accumulate near the lower portion of the room and the warm air near the top. As a result, there is usually a temperature difference between the ceiling and floor. This stratification of heat layers can be minimized by adequate circulation in the room. Adequate circulation can be achieved by locating the supply registers on the opposite side of the room from the return grille. When the return grille is near the supply register, the air discharging from the supply is drawn in by the return grille and does not have a chance to circulate adequately around the room. In many houses, the rooms do not have individual return grilles. Instead there might be a large central return located in the hall. In these cases, the supply registers should be located on a wall that will allow the supply air to circulate completely prior to being drawn off and returned to the central grille. Also, the doors to the individual rooms must be undercut so that when they are closed, the supply air will be able to flow to the return grille. Ideally, air-conditioning supply registers should be located in or near the ceiling. To
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minimize air-conditioning installation costs, rather than install new ducts many new homes use the ducts and registers provided with a forced-air heating system. These registers are usually located near or at the floor level and are quite effective for heating purposes. However, when they are used for airconditioning, they are less effective and tend to increase the stratification effect. Some houses that have forced-warm-air heating have what are called high-low registers. The duct supplying the heat register is extended vertically to a point near the ceiling level where it terminates at another register. When the system is used for heating, the damper controlling the upper register is manually closed, and the lower register is opened. When the system is used for cooling, the damper controlling the lower register is closed, and the upper register is opened. This type of arrangement is very desirable and is often found in high-quality construction. The presence of a central air-conditioning system does not mean that the entire house is air-conditioned. Look specifically for registers as you walk through the house. In many raised ranches, I have found that the upper level is air-conditioned and the lower level is not. If Air-conditioning systems 239
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you have any questions, check each room while the air-conditioning system is operating. There are two basic types of ducts used in residential structures sheet metal and glass fiber. While checking the distribution portion of the air-conditioning system, look for exposed ducts. The glass-fiber type of duct is by its very nature insulated. However, the metal duct may or may not be insulated. The fact that there is exposed metal on the outside does not mean that the duct is not insulated. The insulation might be located inside the duct. Whether the metal duct is insulated can be determined by feeling the duct when the system is operating (if no insulation, the duct will be quite cool) or by striking the duct with your fingernail. If there is no insulation, you will hear a ringing sound, and if there is insulation, you will hear a dull thud. All ducts that lead through unfinished areas such as crawl spaces and attics must be insulated so that the cool air flowing through the ducts will not absorb heat from its surroundings. Pay particular attention to the joints for indications of air leakage. Any open joints should be sealed with inexpensive duct tape. Also, whether the evaporator coil is located inside the furnace plenum or in the attic, check the joints around the housing for air leakage. Very often, there are open joints that must be sealed. Heat pump A heat pump is a year-round air-conditioning system that provides warm air during the winter and cool air during the summer. It is basically a compressor-cycle air-conditioning system (similar to the one described previously) that can operate in reverse. During the reverse operation, the condenser functions as an evaporator, and the evaporator functions as the condenser. The overall refrigerant cycle, however, remains the same. (See page 230 and FIG. 17-1 for the operational flow details.) 240 Air conditioning
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When the system is operating, the condenser (which is located in the house) is cooled by the air that is circulated around the house. As this air passes over the condenser, it absorbs heat that is used for heating the house. For the cycle to operate properly, the evaporator (which is located on the outside) must absorb heat from the outside air. Even when the outside temperature drops to as low as 20 F. the evaporator can absorb heat because the refrigerant within the evaporator is at a lower temperature. However, as the temperature of the outside air drops, the ability of the evaporator to absorb heat decreases, decreasing the effectiveness of the heat pump. Even though a heat pump may be operational at temperatures as low as 20 F, the Btu output is sufficiently reduced so that auxiliary heaters are usually required. A heat pump should be sized for the airconditioning load and not the heating load. Otherwise, the air conditioner will be oversized. Except for a small section of the Deep South, the heating load on a house will always be greater than the cooling load. A heat pump will produce approximately 20 percent more Btus per hour for heating than it does for cooling. Consequently, in most parts of the country, when a house is heated by a heat pump, auxiliary heat will also be needed. For example, in the New York area, a typical eight-room house would require a furnace that could produce about 100,000 Btus per hour for winter heating and also a 31 2 ton (42,000 Btu/hour) air-conditioning unit for summer cooling. If a properly sized heat pump was used for heating, it would produce only 50,400 Btu/hour. The remaining 49,600 Btu/hour would have to be provided by auxiliary heaters, which are usually electrical resistance heaters. The auxiliary heaters in heat pumps are automatically activated when the pump cannot supply sufficient heat to keep up with the heat loss of the structure during the winter. In
northern communities where there is a considerable amount of moisture and low temperatures during the winter, there is a tendency toward an ice buildup on the metal fins of the outdoor evaporator coil. An excessive ice buildup could cut off air circulation across the coil and result in a loss of heating capacity. With some heat pumps, this icing condition is automatically controlled by a defrost cycle that reverses the flow of the refrigerant for a short time. The hot refrigerant heats the outdoor coil and melts the ice. During the defrost cycle, the auxiliary heaters are usually energized to offset the cycle s cooling effect on the indoor circulating air. A heat pump can operate in either the heating or air-conditioning mode. Most manufacturers suggest that the unit be operated in the air-conditioning mode when the outdoor air temperature is above 65 F (unlike a regular air conditioner, where the recommended temperature is 60 F) and in the heating mode when the temperature is below 65 F. Operating a heat pump in the wrong mode can result in damage to the compressor. The components and problems of heat pumps are basically similar to those of air-conditioning systems. Consequently, the overall inspection procedure outlined earlier in this chapter for air conditioners should be used when inspecting heat pumps. However, you should not check both modes of operation. As long as the unit is functioning properly in the mode tested, it is an indication that the major and most costly components (compressor, fans, and coils) are operational. The system should then also function in the opposite mode. If it doesn t, a faulty reversing valve is usually the cause. Evaporative cooler One of the benefits of air conditioning is the dehumidification of the circulating air. This
benefit is not without cost. Cooling and dehumidifying the air is more costly than cooling alone. In the southwestern part of the United States, the outdoor air is relatively dry, and dehumidification is not necessary. In this area, cooling can be achieved by means of an evaporative cooler. Because of the low humidity, water readily evaporates. In the evaporation process, the water absorbs heat from its surroundings and lowers the temperature. The typical evaporative cooler consists of a sheet-metal and plastic casing containing a fan, pads, filter, and a water source. The pads, which hold the water, can be wetted by a spray, by a trickling stream, or by passing through a reservoir on a rotating drum. In some units, the wetted pads also function as air filters. As the air passes over or through the pads, it is cooled by the evaporating water and then distributed throughout the house. When inspecting an evaporative cooler, turn the unit on and listen for any unusual sounds or vibrations in the blower compartment. Also look for signs of water leaks and check the pads for deposits and crusting. For efficient operation, the pads may require cleaning or replacement.
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