True. Why. Truly. Today s current green-energy movement has stimulated

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1 Why True RMS Truly Matters Increasingly energy-efficient HVAC equipment is changing the way systems are maintained and making true-rms meters a must -have for every service professional. B y G r e g J o u r d a n, C M Today s current green-energy movement has stimulated the growth of new ultra-high-efficient HVAC equipment. This article describes some of these energy-efficiency trends, the equipment involved and the maintenance impact namely, that true-rms (root-mean square) meters are now more necessary than ever. In addition, this feature will clarify the differences between a true-rms meter and the standard averaging meter, as well as explain some common electrical measurements on energy-efficient equipment. Standards and terminology The federal government has spurred HVAC growth by mandating a minimum standard of 13 SEER or higher and a minimum HSPF of 7.7 or higher on A/C equipment. HSPF is specifically used to measure the efficiency of airsource heat pumps. The higher the HSPF rating of a unit, the more energy-efficient it is. HSPF is a ratio of Btu heat output over the heating season to watt-hours of electricity used. It has units of Btu/W-hr. Another measurement is AFUE, 20 RSES Journal JUNE 2010 w w w. r s e s j o u r n a l. c o m

2 In electrical terms, the ac RMS value is equivalent to the dc heating value of a particular waveform voltage or current. which calculates the amount of fuel converted to space heat in proportion to the amount of fuel entering a furnace. Fortunately, the aforementioned standards are only the minimum energy levels as required by the government on new equipment installations. Several HVAC equipment manufacturers have released products rated at up to 24 SEER in the cooling mode; 10 HSPF on heat pumps in the heating mode; and up to 97% AFUE on gas furnaces. The crux of complexity While these more efficient units offer many benefits, an unfortunate side effect of increased efficiency is increased cost and complexity. For example, several heat-pump manufacturers have fully computerized every function within the equipment with some units containing up to nine solid-state microprocessor controllers. These microprocessors electronically control everything within the unit, including: fan speeds on the indoor and outdoor blowers; compressor speed; both EXVs on the units indoor and outdoor coils; relative humidity; temperature; cfm; and more. As the inset image above illustrates, the typical controller board found on modern heat pumps puts the technician face-to-face with a myriad of electronics. These units are fully electronic and, for the average HVAC technician, not always easy to service. Even the thermostat is fully electronic, with only four wires on the sub-base; one set of wires on the base is for ac power, and the other two wires are for the communications cable between the indoor and the outdoor controllers. [Note: Older thermostat terminology, such as R = 24-V supply, W = Heat, G = Fan, and Y = Cooling or Compressor, is not used with modern modulating HVAC Circle Reader Service No. 73 w w w. r s e s j o u r n a l. c o m JUNE 2010 RSES Journal 21

3 Image 1 These side-by-side images illustrate the difference between a meter that provides an average versus one that supplies true RMS. Both meters are reading the same current, but the averaging meter (show in the photo on the left) has a reading that is low by some 32%. thermostats, since these thermostats are analog controllers not simply automatic on/off switches. Plus, new variable-signal thermostats can fully modulate system capacities from 40% 115%.] As this new equipment reaches the field, installation and service technicians need to fully understand the operation and service procedures of each piece of equipment. This often requires service professionals to be certified by factory representatives on each specific manufacturer s equipment. Plus, technicians need to use true-rms meters on equipment startup and servicing. True-RMS multimeters and other test tools respond accurately to alternating-current and voltage values, whether the waveform is linear or not. If a test tool is labeled and specified to respond to the true-rms value, it means the tool s internal circuit calculates the heating value according to the RMS formula. This method will give the correct heating value regardless of the current s wave shape. Average-responding tools do not have true-rms circuitry. Instead, they use a shortcut to find the RMS value by capturing the rectified average of an ac waveform and then multiplying the number by 1.1 to calculate the RMS value. The value these meters display is not a true value, but rather a calculated one based on an assumption about the wave shape. The average-responding method works for pure sine waves. However, when a waveform is distorted by nonlinear loads such as variable-speed drives or computerized controls the average-responding method can lead to reading errors as large as 40%. The table in Figure 1 gives some examples of the way the two different types of meters respond to different wave shapes. What does it mean to the tech? The term RMS comes from a mathematical formula that calculates the effective (heating) value of any ac wave shape. In electrical terms, the ac RMS value is equivalent to the dc heating value of a particular waveform voltage or current. For example, if a resistive heating element in an electric furnace is rated at 15 kw of heat at 240-Vac RMS, then we would get the same amount of heat if we applied 240 V of dc instead of ac. From a measurement perspective, the RMS value is equal to of the peak value of the sine waveform (V RMS = V peak x 0.707). Say an ac voltage source has a 22 RSES Journal JUNE 2010 w w w. r s e s j o u r n a l. c o m

4 Comparison of average-responding and true-rms meters Multimeter Type sine wave square wave single-phase diode rectifier 3Δ phase diode rectifier Average-responding Correct 10% High 40% Low 5% to 30% low True-RMS Correct Correct Correct Correct Figure 1 This chart details some of the differences between averaging and true-rms metering instruments. positive peak value of 165 V. Using the aforementioned formula: 165 x = V RMS. Remember that not all the problems found on new HVAC units are complex. A full range of basic mechanical issues can impact unit efficiency, such as air flow; duct leakage; dirty filters; and proper refrigerant charge, which typically requires accurate superheat and subcooling measurements. But, there also are sophisticated control-system problems and measurements that must be addressed during routine maintenance and service. Bear in mind that many of these electronic controls involve non-linear loads. Essentially, any control system con- Circle Reader Service No. 74 w w w. r s e s j o u r n a l. c o m JUNE 2010 RSES Journal 23

5 If a non-linear load such as a new high-efficiency heat pump is on that circuit, the tech will need a true-rms tester to accurately measure the true load current to determine where the problem is. taining semiconductors in the power supply or in the controllers would be considered a non-linear load. Normally, when troubleshooting HVAC equipment failure or nuisance trips due to an electrical problem, the technician s first instinct would be to check the panel for tripped circuit breakers or overloading. However, if a non-linear load such as a new high-efficiency heat pump is on that circuit, the tech will need a true-rms tester to accurately measure the true load current to determine where the problem is. Is the circuit faulty, is it overloaded, or is the problem with the load itself? The two meters shown side by side in Image 1 on pg. 22 demonstrate the difference between a standard averaging meter and a true-rms meter. Notice the readings differ by approximately 32% that is quite a difference. Measuring up energy-efficient equipment The following list offers a quick review of the HVAC electrical measurements that require a quality and accurate test tool on new energy-efficient HVAC equipment. Electronic temperature sensors for superheat/subcooling/air temperature g Most of these sensors are thermistor temperature-sensor types with an NTC-negative temperature-sensor response. This means that when the temperature goes down, the resistance goes up. g To test these sensors, technicians must refer to the manufacturer s data sheets to test for known resistance readings at a given temperature. For example, a temper ature sensor at 32 F in ice water, would read 50,664 ohms, but the same sensor at 50 F would read 30,343 ohms. Pressure transducers g Most of these sensors are pressure transducers with a dc voltage signal given out at a corresponding pressure. g To test these transducers, the tech must refer to the manufacturer s data sheets to test for a known voltage at a given pressure. For example, a pressure transducer at 45 psig using R-410A refrigerant ( 4 F) would read 1.1 Vdc, but the same transducer at 60 psig (8 F) would read 1.3 Vdc. Measuring current and voltage g Measuring the current when non-linear loads exist always requires the use of a true-rms meter. For most new indoor and outdoor high-efficiency units, these measurements largely occur on the main power supply. The compressor and the fans are controlled by a variablespeed control, so the only amps to be checked are on the supply side of each unit. For example, the total amps for the outdoor unit might be rated at 22 FLA. This means that on a hot day under a heavy load, the amps to the unit should not exceed 22 A. Checking the indi - vidual amps to the compressor is no longer possible, due to the unit s ECM-control system. g Measuring the voltage directly at the compressor also is no longer possible, due to the types of ECM controllers. However, the technician should learn to check the supply voltage to the outside unit and expect a supply-voltage reading of +/-10%. The best tool for the job With the proliferation of new solid-state VFDs, ECMs, and sophisticated electronic and computerized HVAC controls, successfully troubleshooting electrical problems relies on the technician performing the tests (and well-written manuals from the manufacturer), and on a high-quality true-rms tester as well. In today s high-tech HVAC environment, the best move is to use only true-rms test tools for the best results. Technicians who add these meters to their array of tools and test instruments will consistently get accurate readings and much more satisfied customers. Greg Jourdan, CM, is the Refrigeration Professor at Wenatchee Valley College, Wenatchee, WA. For more information, gjourdan@msn.com. 24 RSES Journal JUNE 2010 w w w. r s e s j o u r n a l. c o m