Rooftop Acs

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A S H RA E

JOURNAL

The following article was published in ASHRAE Journal, December 1997. © Copyright 1997 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

Operating and Maintaining Rooftop Air Conditioners
By David Houghton, P .E.
Member ASHRAE

C

limb up on the roof of a commercial building, and you’ll probably find a packaged air conditioner like the one shown in Figure 1. These units provide cooling for 49% of U.S. commercial building space.1 Although they are relatively durable, they often don’t get the attention they need to run efficiently and effectively. Rooftop air conditioners usually provide between 5–20 tons (17–70 kW) of cooling. Some commercial buildings also use larger prefabricated or custombuilt units (20–100 tons [70–351 kW]), smaller units that cross over into the residential category (1–5 tons [3–7 kW]), or “split systems” that separate the indoor and outdoor functions. Most units include a built-in gas furnace, electric resistance heater, or heat pump capability. This article focuses on the cooling and air handling equipment inside rooftop units. A typical 10-ton [35 kW] rooftop unit costs about $2,000 per year to operate and $10,000 to replace. The mainte-

About the Author
David Houghton, P.E., is a consulting engineer based in Boulder, Colorado specializing in the design of energy-efficient building systems. Mr. Houghton received a B.S. degree in Civil Engineering from the University of California at Berkeley and is a Certified Energy Manager. 50 ASHRAE Journal

nance steps rec- Fig. 1: Although rooftop units are relatively durable, they ommended in this often don’t receive the attention they need to run efarticle cost very feciently and effectively. little per year to implement, and pay dividends in lower improve efficiency and operation, roofoperating costs, better occupant com- top unit maintenance is a continual process, and “tune-ups” need to be done fort, and longer equipment life. annually. Potential for Improvement The operation and maintenance Rooftop air conditioners must endure opportunities for rooftop units can be radiative heat, high ambient tempera- categorized according to their two intertures, leaves, dust, insects, pollen, rain, nal fluid loops: air and refrigerant. Airhail, snow, and even lightning. In many side maintenance and repairs should be cases no one on-site is responsible for done before tackling the refrigeration operating them, so the units are ignored system, because most refrigeration probuntil they break down. Field measure- lems can’t be effectively fixed until the ments of rooftop unit efficiency show proper airflow is established. For examperformance substantially below catalog ple, refrigerant charge measurements ratings. A project that examined the per- will be inaccurate if airflow is restricted formance of rooftop units in Mississippi by dirty filters. found that two 10-ton (35 kW) units Filters rated at EER 9.0 operated at EER 6.6 and Filters play two important roles: they 7.1, respectively,2 while a test in Conhelp maintain indoor air quality, and necticut that measured two 10-ton (35 kW) units rated at EER 8.7 found aver- they protect downstream components of age efficiencies of EER 6.6 and 8.6 an air handling system (the evaporator coil and fan) from accumulating dirt. respectively over a three-week period.3 Pleated filters made with cotton or A program of regular rooftop unit synthetic fabrics cost more but perform maintenance can improve performance. better than flat filters made of fiberglass A project that studied efficiency or spun polyester mats (Figure 2). The improvements in 25 commercial rooftop fabric boosts filtration efficiency from units in New England, for example, below 20% to about 30%, and the pleats resulted in average energy savings of increase the filters’ effective area, 11%, demand reductions of 2%, and reduce pressure drop, and extend useful paybacks of just under three years,4 and life. For example, using 2 in. fiber or a similar project in Louisiana that per- polyester filters in a 10-ton rooftop unit formed “complete professional tune- costs about $100 per year in material and ups” of 23 air conditioners in motels, labor costs, while using 2 in. pleated fabrestaurants, and grocery stores resulted ric filters costs about $60 per year. The in efficiency improvements ranging longer lifetime of the pleated filters (six from 22 to 42%.5 Although these exam- months vs. two months) outweighs their ples are from one-time efforts to higher purchase cost.
December 1997

AIR CONDITIONER
Rooftop unit filter racks are either 1 or 2 in. (25 or 50 mm) thick. Two-inch (50 mm) pleated filters are better, since the greater media volume provides more surface to trap pollutants, and their lower face velocity causes less pressure drop. Some 1 in. (25 mm) racks can be retrofitted to 2 in. (50 mm) simply by rotating the rail that holds the filters in place—an easy way to upgrade a unit’s filtration system. Filter-changing intervals can be based on the pressure drop across the filter, calendar scheduling, or visual inspection. Although the latter two methods are the most common for rooftop units, measuring air-side pressure drop is the most reliable way to rate filter loading. A technician can install pressure-tap tubing and then use a hand-held pressure meter or manometer to check filter status; when the pressure drop is higher than a specified level—typically, about 0.5 to 0.75 in. of water gage (125 to 188 Pa) above the brand-new pressure drop—it’s time for new filters. Facilities with regular filter loading can use pressure measurements to establish the proper filter change interval and use calendar scheduling thereafter. Scheduled intervals should be between one and six months, depending on the pollutant loading from indoor and outdoor air and the filter type. Conventional wisdom holds that dirty filters reduce the efficiency of rooftop units, but in reality, system effects can result in very small energy savings. Analysis of a 10-ton (35 kW) unit shows that a static pressure increase of 1 in. of water gage (250 Pa) from dirty filters reduces compressor efficiency but actually boosts fan efficiency, with a net penalty of only about $21 per year (1%) in energy costs. However, dirty filters also reduce total airflow by 23% and cut cooling capacity by 7%. Although it’s worthwhile to change filters regularly, don’t expect big energy savings. Evaporator Coil Dirt on the evaporator coil causes two problems: it reduces system airflow, and it directly degrades the coil’s heat transfer efficiency, which significantly cuts cooling capacity. Field work in Louisiana showed that the evaporator coils in 87% of 23 units investigated for tuneups needed cleaning.6 It is worthwhile to inspect the coil at least annually to make sure that the filters have been doing their job. Check
December 1997

Fig. 2: Pleated filters offer better filtration efficiency and lower total operating costs compared to the mat filters shown here.

coil cleanliness by measuring supply fan amperage and filter/coil pressure drop (with fresh filters). If the amps are lower and pressure drop is higher than last year ’s measurement (also with fresh filters), then the flow through the coil is lower—which means the coil is dirty and needs cleaning. Evaporator coil cleaning should be done with a power washer. Supply Fan Fans in older rooftop units have sleeve bearings, which are oiled metalto-metal running surfaces. These should be lightly oiled two or three times per year with the recommended lubricant. It helps to place a label near the bearings with the lubrication interval, lubricant type, and a service log. Newer fans are equipped with selflubricating bearings—sealed-cassette ball bearing cartridges pre-loaded with grease. There is no way to re-grease these bearings, so when they finally fail—typically after several years of service—the bearing cassette must be replaced. Warning signs of impending bearing failure are excessive noise, vibration, or heat emanating from the bearing. Conventional greased ball bearings are occasionally found in rooftop units. The most common problem with these bearings is over-greasing, which can be as damaging as under-greasing.7 The

proper procedure is to open the drain plug and inject grease through the fill fitting until clean grease comes out of the drain. Take care not to get grease or oil on the pulley wheels or belt, as this causes a slip-stick action. Most HVAC technicians have at least one story of finding a fan motor running in the wrong direction. Centrifugal fans will supply some air even if running backwards (about 50% of rated flow), so this situation may not be apparent. The most common cause of reverse fan operation is switched wire leads on the motor. Clear labels on the fan housing, pulleys, motor, and wires can help prevent this problem. Fan Belt Loose belts slip on the pulley wheels (Figure 3), causing torque loss and rapid belt wear, while tight belts put an excessive lateral load on the motor, causing rapid bearing wear. Proper belt tension can be achieved with a deflection strain gauge, but most technicians adjust tension simply by pressing on the belt with a finger. Either method works well if performed consistently. Belts should also be aligned with a straightedge. Some technicians advocate annual or bi-annual belt changes, while others let them run until they break. Since a typical belt set for a 10-ton (35 kW) unit’s supply fan costs about $5 to $10, while a service call to replace a broken belt costs
ASHRAE Journal 51

$65 or more, it makes sense to preempt breakage with scheduled replacement. An easy upgrade that can improve drive train efficiency by 2 to 10% is to switch from standard to cogged V-belts.8 The cost premium for cogged V-belts is about 20%. Fan Motor Supply fan motors installed by the original equipment manufacturer are generally standard-efficiency induction motors. It pays to specify premium-efficiency motors for new rooftop units or as replacements for burnouts in existing equipment. For example, using the best available 2 hp (1.5 kW) induction motor (89.5% efficiency instead of 81.5%) in a 10-ton (35 kW) rooftop unit will save about $60 per year in operating costs for an extra cost of about $85—a payback of 17 months. Outside Air Dampers Improper damper operation is a very common rooftop unit problem. A study of 13 rooftop units on small commercial buildings found that none of them had properly operating outside-air dampers.9 This problem can have major energy consequences in regions that could take advantage of economizer operation, and potentially serious indoor air quality impacts in all climates. Damper servicing consists of cleaning, lubricating, and testing damper movement, which costs about $10 to $20 of a technician’s time. If this prevents one of the five-ton (17 kW) compressors in a 10-ton (35 kW) unit from running for 500 hours per year, it will save about $185 per year. After cleaning and lubrication, a damper should be run through its full range of motion. Then the economizer set point should be checked. Although many economizers are set at about 60°F (16°C), the set point can be as high as the return air temperature (about 74° F [23°C]) to provide beneficial ventilation. In highhumidity climates or where outside air is very polluted, however, it may not make sense to maximize outside airflow at low dry-bulb temperatures. Cabinet Integrity Many rooftop units spill expensive chilled air onto the roof through cabinet leaks. Most rooftop units are covered with access panels held in place by sheet metal screws, but often the panels have
52 ASHRAE Journal

M AINTENANCE T IPS
Finding a maintenance service contractor who exhibits quality and integrity is essential for high quality maintenance. Be sure to check references, ask to see sample reports, and find out if the contractor is committed to training their technicians and equipping them with the right tools. A thorough maintenance program includes checkups before each heating and cooling season and complete reports showing the findings of the service technicians. • Don’t let service contractors keep the only copy of performance documents. • Keep performance information (wiring diagrams, fan curves, etc.) close to the unit. • Include a service log sheet and a record of “alarm” conditions for each unit. • Records and record containers must be rugged enough to withstand abuse. Consider using reinforced, heavy-duty paper sheets and binders, and waterproof metal boxes. • Highly visible signage encourages the use of service logs and other tracking documents. • Contractual agreements with service providers can enforce use of service logs and other tracking documents.

Fig. 3: Loose fan belts cause rapid belt wear and loss of torque, while tight fan belts cause rapid bearing wear.

only one or two screws left after a few careless service calls. A cordless drill with the right nut driver makes panel access quick and easy, and is conducive to screw replacement. Technicians should also keep a bag of screws on hand to replace those that are missing. Losing 200 cfm (94 L/s) from a 10-ton (35 kW) rooftop unit reduces cooling and airflow capacity by about 5% and wastes more than $100 per year in energy costs.10 Refrigerant Charge Methods for verifying and correcting the refrigerant charge in a direct expansion (DX) cooling system range from measuring the length of sweating pipes to peering into sight glasses. A survey of 25 refrigerant circuits in 18 rooftop units found that 10 (40%) were overcharged and eight (32%) were undercharged.11 Undercharged systems usually result from a leak, while overcharged systems can occur when a technician charges the unit on a cool day or adds refrigerant to “correct” evaporator coil icing that is more likely the result of low airflow. Figure 4 shows the impact of incorrect refrigerant charge on unit efficiency. The most accurate way to check and correct refrigerant charge is by measuring superheat and/or subcooling (or, if the unit has been evacuated, by weighing in the correct amount of refrigerant). Superheat and subcooling measurements, however, are only meaningful when correlated with the loads on the condenser and evaporator. Technicians should have a blanket or other means of blocking condenser airflow to simulate design ambient conditions, or a lookup

table from the manufacturer that specifies the proper superheat for different ambient temperatures. The superheat for most DX systems should be between 10° F and 20° F (6° C and11° C). Because the idea of superheat is to protect the compressor, some manufacturers now specify superheat values measured at the suction line entrance to the compressor rather than the traditional location at the expansion valve temperature bulb. The superheat difference between these two locations can be several degrees—enough to make a significant error in refrigerant charge. Expansion Valve Some service technicians blame the thermostatic expansion valve (TXV) for system malfunctions, but adjustments to the internal spring’s screw setting should
December 1997

AIR CONDITIONER
be attempted only after other possible problems have been ruled out. The valve’s temperature sensing bulb should always be attached to the suction line with copper straps (rather than steel hose clamps or plastic “zip-ties”) to ensure complete heat transfer between the bulb and the pipe.12 Compressor are intended to be used with a brush and a hose will not do a good enough job of cleaning the coils, even though they may brighten the outer surface.)18 This is money well spent— cleaning the unit in this example has a payback of just over two months, with a net annual savings of $200. A serious condenser cleaning will include before-and-after measurements of the temperature difference across the coil to verify the effectiveness of the job.

Ninety percent of rooftop unit replacements are because of failures in the hermetic motor-compressor.13 Replacing a compressor is major surgery, costing $1,500 to $5,000 or more Condenser Fan and Motor Most condenser fan motors are equipped with sealed-casdepending on tonnage. Electrical and oil tests should be part of annual checkups for rooftop sette ball bearings that need units. no lubrication. Excessive Electrical testing checks noise, vibration, or heat at compressor motor health by the bearing indicate that the measuring the ground resiscassette needs to be tance of motor windings replaced. with a megohmmeter (“megRapid on-off cycling of a ger”), a process that requires condenser fan (three minutes a $300 instrument and takes or less) leads to poor control about 10 minutes. Low megof the refrigeration system ger readings (typically, and can wear out the fan below 100 MΩ) call for motor prematurely. This “drying out” the system by problem is often caused by installing a fresh filter-dryer a narrow deadband on the and/or dehydrating the sys- Fig. 4: Because most rooftop units are “critical-charge” syshead pressure controller for tem with deep evacuation.14 tems (there is no liquid receiver to buffer the flow of refrigthe condenser fan. A Oil testing is performed erant) they are very sensitive to the amount of refrigerant in healthy deadband (20 to 50 the system. on site by plugging a small psi [138 to 345 kPa]) indicator vial onto a between cut-in and cut-out schrader-valve service port. If the oil contains acid, the vial pressure set points prevents rapid condenser fan cycling.19 changes color. Test vials cost about $10 each and can be reused Like all air conditioners, rooftop units are complex until a compressor fails the test. The most important time to machines that quickly consume their purchase price in operatest oil for acidity is after a compressor failure, but it can also tion costs. Maintaining and servicing these units to reduce be used as part of the annual checkup. Systems that fail this test energy costs and avoid expensive repairs is a significant techshould be fitted with one or more filter-dryers designed to nical and organizational challenge that deserves the full attenclean acid and moisture out of the system. tion of facility managers and service providers. If the wiring to the unit is undersized or otherwise comproReferences mised, the compressor motor can be damaged by low voltage 1. U.S. Department of Energy, Energy Information Administraconditions (the motor responds to low voltage by drawing tion, “Commercial Buildings Characteristics 1992,” DOE/EIA-0246 more current, which dramatically increases resistive heating in (92), page 201 (April 1994). the motor’s windings). If this problem is suspected and the 2. Mukesh Khattar and Michael Brandemuehl, “Dehumidification cause of the voltage problem is too difficult or expensive to fix, Performance of Unitary Rooftop Air Conditioning Systems: Kmart a protective phase monitor can be installed in the rooftop unit Demonstration,” EPRI TR-106066, 3565-06, Final Report (May just before the unit’s main breaker, at a cost of $200 to $300.15 1996), Electric Power Research Institute, Palo Alto CA, 415-855These devices cut power to the unit when voltage is out of tol- 2514, p. 4-7. erance (typically, plus or minus 10%) and restart when condi3. Scott Silver, Philip Fine, and Fred Rose, “Performance Monitions are safe again. toring of DX Rooftop Cooling Equipment,” Energy Engineering, vol. Condenser Coil A dirty condenser coil that raises condensing temperature from 95°F (35°C) to 105°F (41°C) will cut cooling capacity by 7% and increase power consumption by 10%, with a net (compressor) efficiency reduction of 16%.16 Such performance degradation on a 10-ton (35 kW) unit operating at 9.0 EER for 2,000 hours per year wastes about $250 per year in operating costs.17 It costs about $50 in labor and materials to clean the condenser with a power washer that feeds cleaning solution into a high-pressure water flow. (Spray-on cleaning solutions that
December 1997

87, no. 5 (1990), pp. 32-41. 4. Martha Hewett, David Bohac, Russell Landry, Timothy Dunsworth, Scott Englander, and George Peterson, “Measured Energy and Demand Impacts of Efficiency Tune-Ups for Small Commercial Cooling Systems,” Proceedings, ACEEE 1992 Summer Study on Energy Efficiency in Buildings (1992), pp. 3.139-3.140. 5. Michael Carl and Joseph Smilie, “How Maintenance Impacts Air Conditioning Performance and Demand,” proceedings of the 1992 International Winter Meeting of The American Society of Agricultural Engineers, Nashville, TN, (December 1992), p. 7. 6. Carl and Smilie [5]. ASHRAE Journal 53

7. Bill Howe et. al., E SOURCE Drivepower Technology Atlas, (1996), p. 92. 8. Howe [7]. 9. Alan Vick, John Proctor, and Frank Jablonski, “Evaluation of a ‘Super Tune-Up’ Pilot Program for Forced-Air Furnaces in Small Commercial Buildings,” Proceedings, International Energy Program Evaluation Conference, Chicago Illinois, p. 503 (1991).

10. This flow rate would result from a total leakage area of 10 in.2 (6450 mm2) and exit velocity of 33 mph (15 m/s). Assuming 4000 cfm (1888 L/s) unit capacity, 5% cabinet leakage, EER of 9.0, runtime of 2000 hours per year, and electricity cost of 8¢/ kWh, the annual cost of air leakage is $106.40. 11. Hewett et. al. [4], pg. 3.131.

12. Dale Rossi, personal communication (September 20, 1996), Chief Technical Officer, Four Seasons Mechanical Inc., 1979 Stout Drive, Ivyland PA 18974, tel 215-6729600, fax 215-671-9658, e-mail dtrossi@ acrx.com, web www.acrx.com. 13. David E. Stouppe and Tom Y. S. Lau, “Refrigeration and Air Conditioning Equipment Failures,” The Locomotive (the quarterly magazine of the Hartford Steam Boiler Inspection and Insurance Co., Hartford CT), Spring 1988 vol 6 no. 1, pp. 3-9. 14. Leon Neal, personal communication (September 17, 1996), Senior Product Engineer, North Carolina Alternative Energy Corporation, 909 Capability Drive, Suite 2100, Raleigh NC 27606-3870, tel 919-857-9018, fax 919-832-2696. 15. Keith Clark, personal communication (August 28, 1996), Service Manager, Design Mechanical, 5637 Arapahoe Road, Boulder CO 80303, tel 303-449-2092, fax 303-4498739. 16. Robert W. Roose, Handbook of Energy Conservation for Mechanical Systems in Buildings (New York: Van Nostrand Reinhold Company, 1978) p. 281. Data is for a 15-ton (53 kW) reciprocating R-22 compressor. At 95° F (35° C) condenser temperature, head pressure is 181.8 psig ((1253 kPa), capacity is 18.3 tons (64 kW), and compressor input power is 14.3 boiler hp (140 kW). At 105° F (40° C) condenser temperature, head pressure is 210 psig (1448 kW), capacity is 17.0 tons (60 kW), and compressor input power is 15.9 boiler hp (156 kW). (All values at 45° F [7° C] suction temperature and 76.6 psig (528 kPa) suction pressure.) 17. Assuming 75% of energy goes to the compressor, and total energy costs of 8¢/kWh. 18. Keith Clark [15]. 19. Ira Richter, “Condenser ShortCycling,” Refrigeration Service & Contracting, vol.64, no. 8, p. 34 (August 1996).

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Acknowledgements
This article draws on research performed for E Source, an independent source of information on energy technologies based in Boulder, Colorado. Please circle the appropriate number on the Reader Service Card at the back of the publication. Extremely Helpful ........................ 462 Helpful ....................................... 463 Somewhat Helpful ....................... 464 Not Helpful................................. 465 54 ASHRAE Journal
December 1997

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