Reference Publication: Chasar, D., Chandra, S., Parker, D., Sherwin, J., Beal, D., Hoak, D., Moyer, N., McIlvaine, J., "Cooling Performance Assessment of Building America Homes", Fifteenth Symposium on Improving Building Systems in Hot and Humid Climates, July 24-26, 2006 Orlando, FL.

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Cooling Performance Assessment of Building America Homes

Dave Chasar, Subrato Chandra, Danny Parker, John Sherwin,
David Beal, David Hoak, Neil Moyer, and Janet McIlvaine
Florida Solar Energy Center

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ABSTRACT

Long-term monitoring of building energy use and environmental conditions has been a strong component of FSEC research since the 1980s. Fully-automated data collection, verification, archiving and management ensure accurate logging of large amounts of data simultaneously from numerous field sites prior to being made available for analysis and display via the internet. Homes are typically monitored using 15 to 50 channels of data to measure indoor and outdoor environmental conditions and energy use of heating, cooling, water heating, whole house, and other points (e.g. Solar PV or  Solar DHW) if needed.

Energy performance in many Building America homes has been documented with measured data collected over several years to verify savings projections. An evaluation of measured cooling performance is presented with data from nine homes in three climate regions. Data from potential zero energy homes and minimum code homes provide upper and lower performance bounds. Comparisons are based on regression analysis of daily cooling energy per 1,000 square foot of floor area versus average daily temperature difference (outdoor-indoor).

INTRODUCTION

Building America is a private/public partnership sponsored by the U.S. Department of Energy that conducts systems research to improve overall housing performance including durability, comfort and reduced energy use. The ultimate program goal is to achieve a 70% reduction in energy while making up the other 30% with on-site power to provide homes that can cost-effectively produce as much energy as they consume.

As of 2004, 46% of new single-family homes are currently built in the South where air conditioning makes up the largest portion of the annual electric bill (USDOE 2005). Through systems engineering, significant reductions in cooling energy have been successfully achieved in these climates by rigorous application of cooling load reduction strategies. Lower cooling loads lead to smaller air conditioners which, when coupled with high efficiency equipment, have led to reductions of over 70% in cooling energy use.

Long-term measurements of cooling energy use from eight research homes of varying performance levels, mostly in the Central Florida area, were compared with two “minimum-code” homes. The data is drawn from dwellings of various size and construction and with cooling equipment of varying efficiency. Plotting such data on a common graph required generalizations that limit the ability to make direct house to house comparisons, but instead provides a broad assessment of cooling performance.

DATA PLOTTING METHODOLOGY

Cooling equipment consisted of split systems with ducted central air handlers. Sub-metered energy from the condenser and air handler was stored at 15 minute intervals and subsequently combined and totaled on a daily basis during the summer months of various years from 1998 to 2005. Daily cooling energy totals were then divided by the total conditioned area of the home to arrive at daily cooling energy per 1,000 square feet. This provided a means of comparing all homes which range from 1,200 to 4,200 square feet.

The daily cooling energy totals were plotted against average daily temperature difference between outdoors and indoors. Weather stations installed at each site collected dry bulb temperature, relative humidity and solar radiation. Indoor temperatures were taken at or very near the thermostat. The x-axis for each data set consists of the difference between the daily average outdoor and indoor temperatures for the 24 hour period starting at midnight. The values generally fell between negative 10 and positive 15 degrees (outdoor minus indoor). Those residences with lower thermostat settings were characterized by large positive values during the hot summer months. The use of temperature difference is intended to account for both indoor and outdoor temperature variations due to occupant determined thermostat settings and outdoor weather variations.

One pair of homes in the data set can be compared without the generalizations discussed above (except for indoor set point) as they were constructed together with identical floor plans and orientation. These two dwellings located in Lakeland, Florida only differed in equipment efficiency and construction. One was built to minimum code requirements while the other was extensively engineered for reduced cooling load and high efficiency. The original measured results from this 1998 project have since formed the basis for the national Zero Energy Homes program (Parker 1998). The pair effectively sets the upper and lower bounds of the data plotted here.

Baseline For Comparison

A single baseline was needed to provide a common comparison point for cooling performance in the eight research houses. This was achieved with data from two minimum-code homes located in Central Florida. The Lakeland home provided the majority of this data collected over five summers from 1998 to 2002. The other home contributing to the baseline was a code-minimum frame structure located in Cocoa, Florida; built in 1991. Data from this home was collected over three summers from 2002 to 2004. Each of these residences is cooled by the originally installed, minimum efficiency equipment, SEER 10 in Lakeland and SEER 9 in Cocoa.

Figure 1 shows the data points used to develop the baseline as well as the associated trendlines established through linear regression and least-squares analysis. Regressions were performed on each control home to reveal their individual trends and again on the combined data from both homes. The combined data (black line) provided the baseline for determining cooling energy performance in all other homes. Data from the low-energy Lakeland house is also shown for comparison.

The cooling performance level of each research home was quantified by comparison of the areas under the least-squares line. This assumes the areas are directly proportional to energy use and are affected by the length chosen to makeup the bottom edge of the area along the x-axis. The length chosen for this analysis was from -5 to 10 along the x-axis, since the majority of data points fell between these values. This section of the x-axis is where moderate thermostat settings and outdoor temperatures are more likely to fall and tends to exclude high and low extremes.

Also shown in Figure 1 is the coefficient of determination (R2) for each regression line. This measure of “goodness of fit” of the line to its associated data points ranged from 0.50 to 0.89. Removing outliers will improve these numbers however no attempt was made to do so, except where obvious errors or extremes caused by unusual weather or occupant activity were found. For the most part, the data presented here includes fluctuations caused by occupant activity.

PERFORMANCE COMPARISON

The Lakeland high efficiency home was the oldest of those studied (8 years), yet it continues to set the bar for cooling efficiency. The data shown in Figure 1 is typical of the last two years of data collection (2002 & 2003) and represents 72% less cooling energy use than the baseline. While newer the research houses have higher efficiency and sometimes dual-speed cooling equipment, this particular home took advantage of well-designed cooling reduction strategies coupled with a smaller 2-ton cooling system.

CONCLUSIONS

Field-collected home performance measurements are needed to gauge progress toward the Building America goal of 70% whole house efficiency. The method developed here made use of measured cooling energy and temperature data analyzed through least-squares linear regression on both code-minimum and research homes. Figure 2 directly compares the linear regression of each data set.

The cooling energy savings of each research home was determined in reference to a combined baseline established with data from two homes built to minimum code. While the baseline houses do not necessarily represent “typical” code-minimum homes, they nonetheless provide a useful baseline for comparison of the eight research houses. Additional data from homes built to standard construction practices are needed to further refine the baseline.

Additional work is required to determine the influence of home size on cooling performance level. A greater number of people and equipment per square foot tends to concentrate internal loads in smaller homes more so than in larger ones. This may partially explain the MHLab performance, which was below the baseline despite its efficient design. The MHLab was 34% to 62% smaller than the other research homes in the same climate (Florida).

Further research on the influence of ground-coupling on cooling performance will improve the accuracy of comparisons between homes in different climate regions and with different levels of ground contact. All but three homes in this study were of slab-on-grade construction. The basement design of the smallest research home (Tennessee Habitat) was likely a strong contributor to its excellent performance, just as the crawlspace design of the MHLab negatively impacted its cooling efficiency.

ACKNOWLEDGEMENT

This work is sponsored, in large part, by the US Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Building Technologies Program under cooperative agreement number DE-FC26-99GO10478. This support does not constitute an endorsement by DOE of the views expressed in this paper.

REFERENCES

Christian, J.E., Beal, D., Kerrigan, P. “Toward Simple, Affordable Zero Energy Homes”, Proceedings: Performance of Exterior Envelopes of Whole Buildings IX, Clearwater, Florida, December 2004.
Lubliner, M. Hadley, A., Gordon, A. “Manufacture Home Performance Case Study: A Preliminary Comparison of Zero Energy and Energy Star”, Proceedings: Performance of Exterior Envelopes of Whole Buildings IX, Clearwater, Florida, December 2004.

Parker, D.S., J.P. Dunlop, J.R. Sherwin, S.F. Barkaszi, Jr., M.P. Anello, S. Durand, D. Metzger, J.K. Sonne, "Field Evaluation of Efficient Building Technology with Photovoltaic Power Production in New Florida Residential Housing." Report No. FSEC-CR-1044-98, Florida Solar Energy Center, Cocoa, FL, 1998.
U.S. DOE 2005 Buildings Energy Data Book August 2005. http://buildingsdatabook.eere.energy.gov/