August 19, Building Science. Corporation August 19,

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Lynette Dalton
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August 19, 28 Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot Dry Climate ACEEE Summer Study on

28 2 Background Oversizing of cooling systems common Greater first cost Short cycling less latent removal Greater utility

of development High performance enclosure & mechanicals Reduction of cooling equipment size AC monitoring to reassure builder and

1 August 19, 28 Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot Dry Climate ACEEE Summer Study on Energy Efficiency in Buildings Aaron Townsend and Kohta Ueno I Am Not the Primary Author Aaron Townsend August 19, 28 August 19, 28 2 Background Oversizing of cooling systems common Greater first cost Short cycling less latent removal Greater utility coincident demand (FSEC) Reduced energy efficiency (startup losses) 15% 12% of Manual J 6% energy savings (James 1997) 2-3% savings w. ducts in conditioned space; negligible if in attic (Sonne 26) Experimental Background Involvement with homebuilder during buildout of development High performance enclosure & mechanicals Reduction of cooling equipment size AC monitoring to reassure builder and HVAC contractor Building enclosure identical before/after recommended changes SM = Standard sizing/model (4) SO = Standard sizing/occupied (3) RO = Resized/occupied (6) Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot-Dry Climate 1 of 5

August 19, 28 Duct leakage ~4% (same Standard/Resized) Airtightness ~3 ACH 5 (same Standard/Resized) Cooling Equipment Sizing

1% 8% Monthly AC condenser consumption (T.E.D.

unused latent Standard ~ 15% Right-sized ~ 13% Monitoring Equipment Results: AHU Efficiency Monthly AC condenser consumption (T.E.D.

2 August 19, 28 Duct leakage ~4% (same Standard/Resized) Airtightness ~3 ACH 5 (same Standard/Resized) Cooling Equipment Sizing Monitoring Equipment kbtu/hr Capacity 6 181% 184% 16% % % 176% 171% 2% 18% 149% 152% 16% 14% 142% 142% 135% 14% 12% 1% 8% Monthly AC condenser consumption (T.E.D.) AC on/off runtime Coarse measurements 6% 12 4% 2% % Calculated Design Load Installed 15 F Calculated sensible + ½ unused latent Standard ~ 15% Right-sized ~ 13% Monitoring Equipment Results: AHU Efficiency Monthly AC condenser consumption (T.E.D.) AC on/off runtime Coarse measurements Interior temperature/rh (3 locations/house) Exterior temperature/rh Airport weather T/RH Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot-Dry Climate 2 of 5

3 August 19, 28 Results: AHU Efficiency Monitored Period (Daily Hi/Low) % Design T 2. 8 CFM/Watt WIC (125 Pa) external static pressure Temperature (F) 6 4 monitored period. Pa 5 Pa 1 Pa 15 Pa 2 Pa 25 Pa 3 Pa Static Pressure (Pa) Std Models Std Occupied Right Sized CFM/watt (Proctor & Parker) 2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Thermostat operation categories Constant off Constant setpoint Daily set up/set back Manual on/off Model homes unoccupied (locked T-stat) Substantial occupant variation in data/usage Programmable features not used Pay homeowners to maintain constant thermostat setpoint? Wide swings in temperature Long runtimes Weak correlation between runtime & ΔT Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot-Dry Climate 3 of 5

4 August 19, 28 AC Sizing vs. Runtime AC Runtime (Hottest Day) 16 F 75 F = 31 F ΔT Minutes of longest runtime AC Runtime (Longest Runtime) Minutes of longest runtime Long runs from changes in setpoint Also open door/windows? Variations between individuals >> pattern between groups E.g. long runtime at night Condenser Daily Avg. Use kwh/day average Condenser Daily Avg. Use kwh/day average No significant difference between groups Much larger sample would be needed to see differences of the magnitude expected Alternately, more detailed monitoring package & tighter control of operation Ducts in attic Runtime Fraction 4: PM to 7: PM Peak Demand Effects Run time fraction at peak time (4-7 PM) vs. daily maximum outside T y =.1x R 2 = Runtime Fraction 4: PM to 7: PM y = 74x R 2 = Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot-Dry Climate 4 of 5

5 August 19, 28 Peak Demand Effects Run time fraction at peak time (4-7 PM) vs. daily maximum outside T Runtime Fraction 4: PM to 7: PM y =.171x R 2 = Conclusions Resized equipment meets cooling demand (homeowners satisfied; no complaints) Resized has longer response to thermostat temperature change Difference in energy use too small to see given data variability, sample size, measurement resolution, duct location Cooling resizing limits likely due to recovery from setback rather than static/manual J limits Acknowledgements U.S. Department of Energy Building America Program D.R. Horton Inc. Questions & Comments 27 Part Load Efficiency Startup losses: C D (coefficient of degradation/cycling loss) SEER = EER 82 ( 1 C D / 2 ) C D.5 to.1 (Sonne, Sachs) Manufacturers motivated to reduce C D in order to increase SEER Effects of Air Conditioner Sizing on Energy Consumption and Peak Demand in a Hot-Dry Climate 5 of 5

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