Multifamily Central Heat Pump Water Heating

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Russell Hampton
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1 Multifamily Central Heat Pump Water Heating Elizabeth Weitzel Davis Energy Group Alliance for Residential Building Innovation (ARBI) January, 2015 The work presented in this presentation does not represent performance of any product relative to regulated minimum efficiency requirements. The field site used for this work was not a certified rating test facilities. The conditions and methods under which products were characterized for this work differ from standard rating conditions, as described. Because the methods and conditions differ, the reported results are not comparable to rated product performance and should only be used to estimate performance under the measured conditions. 1 Davis Energy Group DATE

The field site used for this work was not a certified rating test facilities.

2 Overview Background Objective Measure in-situ field performance and document reliability of central HPWH When does central HPWH fail to meet load, and frequency Compare economics of system to conventional options Monitoring performance Develop and validate TRNSYS model and develop results for other climates 2 Davis Energy Group DATE

Compare economics of system to conventional options Monitoring performance Develop

3 Why Heat Pump Water Heaters? More than 40% of US households use electric resistance In single family homes HPWHs have been shown to provide 50%+ energy savings Little data available on MF Central HPWH systems 3 Davis Energy Group DATE

4 Background University of California Davis, West Village Student Housing Hot-dry climate 45 Student apartment buildings 14,200 ft 2 12 unit apt building with 10.5 ton HPWH Two 120-gal storage tanks; each with 54 kw back-up electric heat Time/temperature recirc loop 4 Davis Energy Group DATE

5 HPWH System Monitored Oct 11 Jan 13 HPWH Energy Electric Resistance Energy DHW Delivery Recirculation losses 5 Davis Energy Group DATE

6 The case for monitoring Document in-situ field performance Monitoring as a commissioning aide Compressor not running, circulating pump and fan gave impression of operation Incorrect sensor placement Non-optimal setpoints 140 F heat pump upper limit set point (5 F dead band) 120 F resistance heat set point in each tank 120 F setting on the tempering valve that feeds the recirculation loop. Monitoring for early equipment failure Evaporator fan failure 6 Davis Energy Group DATE

upper limit set point (5 F dead band) 120 F resistance heat set point in each tank 120 F setting on the tempering valve

7 Monitored Data Student occupancy patterns atypical with multifamily kbtu/day Daily Electrical Consumption (kwh/day) HPWH Mode Electric RH Only y = x R² = y = x R² = Hot Water Use (gpd) and HPWH Energy Delivered (kbtu/day) Hot Water Usage Total Water Heating kwh Heat Pump Heat Delivered Resistance Heat kwh Evaporator fan bearings fail Daily Energy Delivered to Storage (kbtu/day) Energy Use (kwh/day) Resistance heat typically 2.5 times higher energy use than HPWH Oct Jan Apr Jul Oct Jan-13 7 Davis Energy Group DATE

8693 Hot Water Use (gpd) and HPWH Energy Delivered (kbtu/day) 800 600 400 200 0-200 -400 Hot Water Usage Total Water Heating kwh Heat Pump Heat Delivered Resistance Heat kwh

8 System Performance Mapping 140,000 Monitored data: Heating Capacity (Btu/hr) 120, ,000 80,000 60,000 40,000 Nominal Ratings at 25gpm flow rate and 100 F inlet water temperature Full Load Monitoring Data Avg. inlet F, Flow rate 19.6 GPM Manufacturer Data: Inlet 100 F, Flow Rate 25 GPM ,000 Bi-Quadratic Fit at 100F inlet Bi-Quadratic Fit R2=0.86 Manufacturer Data Outdoor Air Dry Bulb Temperature ( F) HPWH Performance Map developed using linear regression. Result is biquadratic relationship of power and capacity to outdoor dry bulb and entering water temperature. HPWH Steady State Demand (kw) Full Load Monitored Data BiQuadratic Fit at 65F Outdoor Bi-Quadratic Fit, R2= HPWH Inlet Water Temperature ( F) 8 Davis Energy Group DATE

86 Manufacturer Data 0 20 30 40 50 60 70 80 90 Outdoor Air Dry Bulb Temperature ( F) HPWH Performance Map developed using linear regression.

9 TRNSYS Model Validated with monitored data Hourly draw profile from ASHRAE service water heating guidelines Percent of Daily Total Gallons by Hour 7% 6% 5% 4% 3% 2% 1% 0% Hour of Day Hot water usage varied by climate to account for variation in distribution losses 9 Davis Energy Group DATE

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hour of Day Hot water usage

10 Cross-climate Comparison Compared with TRNSYS all-electric and natural gas storage unit models Natural gas storage nominally rated 309 kbtu/h, 80% efficiency EIA state average utility rates Annual Cost $7,000 $6,000 $5,000 $4,000 $3,000 $2,000 $1,000 $0 Phoenix Houston Sacramento Seattle Denver Chicago Electric Resistance Storage Heat Pump Natural Gas 10 Davis Energy Group DATE

Annual Cost $7,000 $6,000 $5,000 $4,000 $3,000 $2,000 $1,000 $0 Phoenix Houston Sacramento

11 Utility Rate Comparison 11 Davis Energy Group DATE

12 Summary System performance impacted by lower flow rate and higher inlet temperatures from recirculation mixing Control dead-band should be widened to improve steady-state operation, or multi-stage HPWH Installation and commissioning training is critical Sizing of 2,500 Btu/h compressor capacity per person was sufficient to meet the load with little need for resistance backup (for this application) 3-person single family typically sized for 6,000-8,000 Btu/h Savings relative to resistance heaters projected to be 49%-59% based on climate Local utility rates and natural gas availability can strongly impact economics 12 Davis Energy Group DATE

sufficient to meet the load with little need for resistance backup (for this application) 3-person single family typically sized for 6,000-8,000 Btu/h Savings

RESEARCH HIGHLIGHT. Performance Assessment of a Cold-Climate Air Source Heat Pump

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