A Robust Feature Set for Residential Air Conditioners

Harvey M. Sachs, ACEEE
Hugh Henderson, CDH Energy
Don Shirey III, FSEC
Wayne DeForest, WECC

January 2008

Robust central air conditioners and heat pumps are units with special features and specifications. They are designed to remain very efficient and require minimum maintenance over their lifetimes, and to automatically signal problems that require service.  This report stresses the specifications required to identify the differentiated product, and the parameters that we varied in a simulation to study the benefits of the specification. We highlight several issues that define the robust air conditioner and will require discussion among stakeholders: 

• Base Efficiency Level.  The robust air conditioner meets the 2006 ENERGY STAR performance specifications, SEER 14 and EER 11.5.¹ Using the ENERGY STAR specification allows easy consideration of the additive energy-saving benefits beyond the ENERGY STAR specification, and suggests that robust and ENERGY STAR complement each other in achieving savings for customers.   
• Variable Cooling Capacity.  Modulating or two-speed compressors can improve air conditioner performance, particularly by mitigating the most severe aspects of oversizing.  Capacity modulation is not a prerequisite for the robust specifications.
• Adaptive SHR. SHR, or Sensible Heat Ratio, measures the ability of the air conditioner to remove moisture.  All air conditioners remove more moisture when the incoming air is more humid. The question is whether to require extended capabilities in this area.  We did not make adaptive SHR a prerequisite, feeling that this area would receive further consideration elsewhere.
• Alarms. The robust system thermostat will be required to display alarms corresponding to excessive pressure drop across the air filter, and when service is needed, as noted in the technical specification. 

In our simulations, we considered the effects of the following:

• Improper Refrigerant Charge. Because the 2006 SEER 13 minimum standard seems to have led to widespread adoption of refrigerant metering devices that incorporate feedback (such as thermostatic expansion valves, or TXVs), we simulated the effects but did not include these savings relative to orifice or other fixed metering devices.
• Adaptive Airflow. This is the potential savings with a BPM (brushless permanent magnet), ECM (electronically commutated motor), or other advanced air handler motor.  Our simulations suggest that it would save 6–11% of total electricity use, depending on climate.
• Slow Refrigerant Leak.  We simulated the effect of a leak slow enough that the unit would quit working when the refrigerant charge dropped to 80% of the proper level. Relative to an orifice-equipped unit, the robust air conditioner would save 6–8% of electricity use over its lifetime by sending an alarm to the thermostat (or service firm) when charge level dropped to 90%.  Savings relative to a TXV-equipped unit would be smaller.
• Reduced Air Handler Unit (AHU) Leakage. This would save 1–3% of total electricity use, primarily because the air handler has the largest pressure differential with the ambient, both negative (air handler end) and positive (supply plenum). Of course, for attic-mounted equipment the ambient temperature at the air handler/evaporator is generally much higher than stipulated in the test method, so drawing in “ambient” attic air gives a mix that is hotter than the 80ºF return expected from the rating method.

After initial discussions with manufacturers and others, we removed several possible features of the robust air conditioner.  These included (with reasons):

• Oversizing.  This is largely outside the control of the manufacturer. The OEM can influence this to some extent by supplying units with modulating compressors, but this is a feature we did not want to include, as noted below.
• Modulating Cooling Capacity.  This is an important premium feature and presumed to be a key profitability driver for these products. Our goal was to develop a specification for a unit that would be differentiated from “commodity” offerings or today’s ENERGY STAR specification, but not one that simply recognizes the most expensive models.
• Adaptive Sensible Heat Ratio.  To some extent, conventional units are already adaptive: the higher the wet bulb temperature, the more humidity will be removed at a given evaporator surface temperature.  To go beyond this requires advanced features, such as modulating compressors interacting with variable speed motors to tune the ratio of airflow to refrigerant flow and thus control evaporator temperature.  This is currently available in some premium products that are controlled by both temperature and humidity level. It can also be addressed by alternative designs, such as the Cromer cycle, that increase the potential for dehumidification.  These are not yet on the market, and have some intellectual property issues that may prevent multiple manufacturers from adopting these features.
• Condenser Fouling and Blockage.  A clogged condenser loses efficiency. Some coil and fin designs may be more resistant to this degradation than others, but there is little data on this aspect of long-term efficiency losses.  In any event, for a given manufacturer, the most likely design feature to ameliorate this decay would be to increase the condenser heat exchange area.  This “brute force” approach did not look like a smart inclusion for the robust criteria.

We conclude that the following features define a Robust Central Air Conditioner that has the potential to improve customer satisfaction and provide a platform for marketing premium, differentiated products.  It will save energy, but all of the savings are from characteristics that are not reflected in the federal test procedure.

• SEER 14; EER 11.5.  We aligned these performance parameters with the 2006 ENERGY STAR specification, to allow easy comparison of simulated energy use. We recommend that the robust air conditioner SEER and EER be set at ENERGY STAR levels, or CEE Tier 2 (currently SEER 15/EER 12) as a value-added feature of efficiency programs.
• Resistant to refrigerant charge errors. Maintains 90% capacity and efficiency with +/-20% mischarge.  In effect, this virtually mandates an adaptive expansion valve, such as a TXV.
• Resistant to refrigerant leaks and other degradation. Shuts down or calls for service if efficiency or capacity drops below 90% of nominal.
• Adaptive air handler. Air handler adjusts airflow in response to ductwork external static pressure, to maintain specified air flow. The specification requires maintaining airflow within 15%, with ESP from 0.1 to 0.9 inches of water. We would also require that the motor have current limiting or temperature rise control to prevent significant life reduction when operated in high external static pressure systems. This feature is required since so many houses have very high external static pressures. This can be caused by high-MERV air filters, or duct design and installation problems. 
• Air handler integrity.  We propose that the robust unit will have a tightly-sealing filter rack included as part of the package, and that the cabinet leakage be de minimus. We characterize this as “<1%,” pending a suitable test method.
• Alarms.  We specify that the unit should include an “alarm” (thermostat notification) if the air filter is loaded to the point that airflow is significantly reduced, or that the unit needs service (such as results from loss of refrigerant). The alarm should call for service or alert the owner when performance changes by 10% or more. This would include, for example, an airflow increase/ESP drop due to disconnect of a register boot.
• Quality. The robust air conditioner shall have a single source and be sold as an integrated package of the condensing unit, evaporator, and air handler (with or without furnace section) and thermostat.  This is necessary to assure proper sizing and component match, and to avoid finger-pointing among vendors in case of problems.

From simulations of the features selected for inclusion, we conservatively estimate that the robust unit would save on the order of 10%, which is more than the change from 14 SEER to 15 SEER.  That is, a robust air conditioner should give better performance than a conventional unit rated one SEER point higher.  However, there remain many uncertainties, particularly with respect to the fraction of units with slow refrigerant leaks, and the benefits of an alarm at 90% v. 80% for these units.

View the report for free in PDF or click to order hard copy.

60 pp., 2008, $30.00, Report Number A081