Air-Conditioning Best Practices

Recent studies of charge and airflow revealed that as many as eight out of ten A/C systems have incorrect airflow, and seven out of ten systems have incorrect charge. Potential savings from correcting these deficiencies are substantial.

May 01, 2011
May/June 2011
A version of this article appears in the May/June 2011 issue of Home Energy Magazine.
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We are at a crossroads in the HVAC industry. Simply put, the service practices we performed in the past to get the job done are not getting the job done today. Past practice and best practice no longer belong in the same sentence. The problems we are creating in this industry are not trivial. They are having a major impact on our wallets, our economy, our utilities, and our natural resources, as well as on our comfort.

A recent sampling of more than 1,300 A/C units from the Entergy Arkansas, Incorporated, CoolSaver Program (implemented by CLEAResult, Incorporated of Little Rock, Arkansas), revealed that corrections in charge and airflow netted an average increase in delivered Btuh of 38% and a peak kW demand reduction of 0.5 kW per tune-up. Potential savings from correcting these deficiencies would be substantial. A study of a sample of 1,500 A/C units from the CoolSaver program, conducted by CLEAResult, Incorporated, Consulting, found that corrections in charge and airflow netted an average increase in delivered Btu per hour of 38.1% and a peak kW demand reduction of 0.5 kW per tune-up.

Setting up A/C systems takes time. Evacuation, airflow measurement, refrigerant charging, performance testing using safe procedures—all take time. The old cliché “Time is money” rings as true today as when it was first said. Although we cannot make more time, we can save time, and a lot of it, if we use the right tools for the job.

How do you get it done faster? Back to the basics: Use the right tools and processes for the job.



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Practice Proper Brazing Techniques

Always, always purge the system with dry nitrogen during installation. To do this properly, first remove the valve cores, leave the core tools attached, and wrap the service valves with a wet rag. Work from one end and completely assemble the piping before you do any brazing inside or out. Purge the system with nitrogen from one side and allow the dry gas to vent out the opposite side of the system, at the condenser. In other words, push from the liquid line all the way through the evaporator coil into the suction line and out the suction service valve. There should only be enough pressure to move the flame on a lighter or match without blowing it out; 1–2 psi is more than adequate. Any more pressure than that is just wasting nitrogen. After brazing, isolate the system with the core tools until you are ready to perform the pressure test to prevent air from getting into the piping.

Pressure Testing

Perform a compensated-pressure test with dry gas. After brazing, sweep the system with dry nitrogen to move out any noncondensables and any water that could have gotten into the system during installation. (Believe it or not, just cooling a leaky fitting with a wet rag can cause this to happen by capillary action, if there is no positive pressure on the system.) Pressure-test the system with nitrogen and a trace of refrigerant at the manufacturer’s prescribed test pressure. Using a digital gauge with temperature-compensated pressure testing (unique feature of the Testo 550) will minimize the time required and significantly increase accuracy by taking into account both pressure and temperature. The inexpensive Testo 550 uses high-accuracy temperature-compensated pressure transducers and a separate air temperature sensor to perform a timed pressure drop test with ambient-temperature compensation. The gauge makes all the measurements and does the calculations. After the stabilization period, the technician starts the pressure test, and the initial pressure, current pressure, and pressure difference are displayed.

The pressure difference is where the rubber meets the road; it is the calculated difference based upon ambient temperature and Charles’s law. The initial and final pressure may differ with no change in differential pressure, as the gauge automatically compensates for the change in temperature. This, in conjunction with the higher accuracy, makes it possible to verify a leak-free system very quickly. In practice, I have done so, at a very high level of confidence, in half the time recommended by the industry standard. For systems up to 10 tons, this standard is currently 1 hour, and for systems over 10 tons, it is 24 hours. Whether or not you reduce your pressure-testing time may be a question for the high-pressure piping inspector, but a significant factor in pressure testing has been the resolution of the gauges we were accustomed to. Because the test is a timed test, you can easily determine the acceptable leak rate (if any) during testing. When venting the nitrogen again, do not drain the system completely. Leave 1–2 psi in the system, isolated with the core tools, and attach the vacuum lines to the system for evacuation.

Evacuation

Here are tips for doing the evacuation properly.

Use heavy-duty extension cords. Any vacuum pump operates best between 115 and 122 volts. Lower voltages result in lower speeds and lower performance. Smaller extension cords have higher resistance to electrical flow and result in voltage drops. Smaller cords get hotter over time, further increasing resistance. For evacuation, never use a cord smaller than 12 gauge; it is preferable to use a 10-gauge cord over 50 feet long.

Use vacuum-rated core tools and remove the valve cores. Valve core tools have several functions. First, removing the valve cores allows for full, unrestricted flow at the service valves. Second, the valve core tool allows you to isolate the hoses from the system when testing the ultimate level of vacuum. All hoses leak. Removing the hoses and the manifold from the system during testing minimizes the need to chase down leaks that are not system related.

Use clean synthetic vacuum pump oil. Clean oil saves time. Moisture, dirt, and other system contaminants decrease pump performance. Some new synthetic-based oils, like the Appion Micron-Dry, are designed to be hydrophobic by nature. This means that while the oil does help to draw the moisture from the system, it does not bond tightly with the moisture. Starting every job with fresh, clean synthetic vacuum pump oil can save you time on the evacuation and money on pump maintenance.

Use a large-bore manifold. Using a large-bore full-flow manifold like the Appion MegaFlow, with large hose connections, can drastically increase speed and reduce evacuation and recovery times. A high-quality vacuum-rated manifold with full ½-inch porting can provide up to 16 times the flow of a typical ¼-inch manifold.

Use large hoses. The deepest vacuum we can achieve is 29.92 inches of mercury (Hg) or about 14.7psi gauge. We are limited by the physics of pressure. No matter how big the pump or how long you let it operate, it simply cannot achieve any deeper vacuum. How do we increase flow volume if we cannot increase pressure? —Larger hoses. Even when you are restricted by ¼-inch system ports, using a 3/8-inch or ½-inch hose greatly reduces resistance and increases potential flow. This is especially true during system evacuation. Just using one ½-inch hose with ¼-inch fittings, instead of a ¼-inch hose, can reduce the evacuation time by 90%. Think of it this way: 60 minutes reduced to 6 minutes.

Use a micron gauge for evacuation. You cannot determine the level of vacuum without an accurate micron gauge. Proper evacuation ensures that adequate dehydration and degassing have taken place and the system is ready for refrigerant. If system contaminants are not removed, moisture will form acids in the system, which will significantly shorten equipment life; or noncondensables will increase head pressures, increasing operation costs; or both. It is imperative that the micron gauge be isolated from the vacuum pump during evacuation to verify that the level of vacuum is adequate and that moisture is not present in the system. Typically manufacturers look for evacuation levels to remain under 500 µ when the vacuum pump is isolated from the system. Core tools make it easy to do this and also to isolate hoses that may sometimes cause nonsystem leaks.

Use a vane anemometer to set airflow. There are many methods of setting airflow. They include the use of a vane anemometer, the temperature rise method, pressure matching, the use of a flow hood, and the use of the TrueFlow plate from The Energy Conservatory. If the duct system has a reasonably straight section, like a return air-drop, or is connected by a single or dual flex connector to a return air grille, using a vane anemometer is by far the fastest, and arguably the most accurate, method for any residential system.

A minivane anemometer like the Testo 416 is the ideal tool for measuring airflow in a duct, across a heat exchanger or evaporator coil, as required in the commissioning process. The minivane allows for a full duct traverse with an automatic calculation of the CFM in the duct (as the duct dimensions and the free area are input into the instrument before the measurement is taken). If the measurement is done carefully, the measurement error will be less than 3%, and often less than 1%. Changes in yaw and pitch of the probe head in the duct by as much as 10% will result in less than 1% error in the measurement, making the minivane an ideal probe for field in-duct air measurement. Because the probe is larger than the tip of a Pitot tube or hot-wire probe, stray eddy air currents will have little effect on the final measurement.

At the grilles or registers, a 4-inch-diameter vane anemometer like the Testo 417 may be used to determine air velocity at each terminal outlet. Again, no air density compensation is required; it is a simple one-hand operation; and most anemometers of this size are easy to carry and operate. Another advantage of using the Testo 417 is that it gives a more accurate average of true airflow over the sample area. The 4-inch vane (unlike a hot-wire probe or a Pitot tube) does not respond to stray eddy air currents created at grilles. Often, four to six service trucks can each be equipped with a large vane anemometer for what it would cost to equip one truck with one of the more-expensive options previously mentioned.

Use digital gauges to charge refrigerant. Digital instruments are faster, more accurate, and more reliable, and have a higher repeatability, than analog tools. Digital instruments stay in calibration, allow trending, allow more complex functions, and save time. Digital instruments allow data to be recorded and reported without human error, and provide reliable and accurate results for you and your customers. Data can be recorded much faster than any technician could ever do the calculations, and data can also be recorded whether or not the technician is there to see the recording done. In most cases, the data, once recorded, cannot be edited, so what you see is what was measured at the jobsite.

Charge directly by superheating or subcooling. Digital measurement leaves no room for interpretation; it is what it is. With digital, you will find yourself setting up the equipment exactly to the manufacturer’s specifications, because you can. If the manufacturer calls for 9°F of subcooling, you can charge the system to exactly 9°F. There is no learning curve beyond learning to navigate the menus of the analyzer. Simultaneous measurement of superheat and subcooling, with dual temperature and sensor porting standard, allows you to use instruments like the Testo 550 or the DigiCool 1250 with a second optional probe for charging and troubleshooting, without having to move the temperature sensor. Watching both sides of the system while charging lets you spot problems like a faulty expansion valve at system start-up before the system is overcharged. When performing any diagnostic testing, you must evaluate both sides of the system before you make mechanical repairs or adjustments to the charge.

Don’t use a sock to measure wet-bulb temperatures. Keep your socks in the drawer. Using a cotton sock and a thermocouple is a poor way to measure wet-bulb temperatures. While it may appear to be cost-effective, it can result in a lot of errors. These include incorrect air velocity readings across the sensor, and changes in water evaporation rate, caused by sock contamination, by the use of undistilled water, or simply by the sock drying out during testing. While the sock method can produce accurate readings when it is done properly, it's worth every penny to use a digital instrument. A digital psychrometer like the Testo 605 H2 not only is much more accurate than a sock but it is faster, more accurate, more repeatable, and more robust than a traditional psychrometer.

Performance test. The HVAC industry sells capacity, yet it very rarely measures capacity outside the lab. New, more accurate instruments allow us to make accurate airflow measurements and quickly determine the change in wet bulb with a digital psychrometer, thereby making it very easy to determine the capacity a system is producing—whether in Btu per hour, in kW, or in tons of cooling. This can be done quickly by hand, however, using an instrument like the Testo 435 equipped with a vane probe and two wireless sensors, to make quick work of a tedious process. While field calculations can provide a reasonable estimate of the capacity of a system, the Testo 435 can quickly and accurately measure the true airside performance of the system, including all the corrections required for nonstandard air. With an accurate electrical measurement, a field energy efficiency ratio can also be quickly and easily calculated.

Develop a process. You can put all of the right tools in the hands of a technician and still not obtain the results you desire if you don’t first consider the order in which the work is done. Training, training, and more training. Most technicians in the HVAC industry have never measured airflow! Many, if not most, verify what they deem to be correct airflow by simply measuring the temperature drop across the evaporator coil. You cannot put a test instrument in the hands of a technician and just expect results. Technicians need to be trained on how to make measurements. They need to be able to make repeatable and accurate measurements; to make them constantly; and not only to make these measurements for themselves, but also to make the same measurements as the other technicians in your program. It is equally important for them to understand what the measurement should be before they verify it. A measurement alone means nothing if you don’t know what it should be. While a digital instrument can eliminate calculation errors, as well as a host of others, an improperly used instrument still gives your technicians useless information.

A well thought-out, repeatable process keeps everyone on track, on time, and out of trouble. Remember that

  • dirt and duct leakage must be addressed before airflow is adjusted;
  • refrigerant charge may be adjusted only after airflow has been properly adjusted; and
  • the type of metering device determines how the system will be charged.

These are only a few of the key factors that a well-designed program will address. Finally, remember that paperwork is important. Not only is it important to keep accurate records, it is also important to be able to validate that the work that was done achieved the desired results.

Conclusion

While it may be impossible to do it more quickly the right way than the wrong way, you can save considerable time and add considerable profit to several processes your technicians should be performing in the field. The right way need not take a prohibitive amount of time. You can do it right in a lot less time with the right tools and processes, and see additional savings on the back end of every call.

Joe Kuonen is program manager for residential and commercial/industrial programs for CLEAResult Consulting and co-founder and chief training officer for Emerald Environments, Incorporated. Jim Bergmann is the chief technology officer at TruTech Tools and is an HVAC secondary and adult instructor at Cuyahoga Valley Career Center in Brecksville, Ohio.

>> learn more

Joe Kuonen, Program Manager
2010 Entergy Arkansas
CoolSaver Program
E-mail: jkuonen@clearesult.com
Tel: 501-772-1648

Contact Jim Bergmann at Jim2@trutechtools.com, or visit www.trutechtools.com.

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