Central Valley Research Homes: Evaluation of Centrally Ducted and Through-the-Wall Variable Capacity Heat Pump (VCHP) Systems, and Alternative Near-Zero Global Warming Potential Refrigerant Heating and Cooling Technologies 2020-2021

The purpose of this project was to evaluate the performance of:

1. Centrally ducted variable capacity heat pump (VCHP) systems.
2. Packaged through the wall VCHP systems.
3. Near zero global warming potential (GWP) heating and cooling technologies, including an advanced indirect/direct evaporative cooling system and a hydronic space heating system using a carbon dioxide heat pump water heater (HPWH).

These evaluations were conducted at the Central Valley Research Homes (CVRH) test houses, and work was sponsored by Pacific Gas and Electric and Southern California Edison. CVRH has been used to evaluate the energy saving potential of building shell improvements and advanced HVAC technologies through a series of projects supported by the California Energy Commission, PG&E, and SCE.  Occupancy is simulated, and the buildings are intensively monitored and controlled to provide a stable environment for energy evaluation.

Key results from this research include:

• Consistent with prior CVRH findings, there is no correlation between the AHRI seasonal efficiency rating metrics (SEER and HSPF) and installed performance of VCHP systems.
• One of the centrally ducted VCHP systems tested provided a test mode and subcooling target by which to evaluate refrigerant levels, as well as onboard monitoring of the relevant temperature and pressure measurements and a diagnostic display of the monitored superheat and subcooling relative to targets. This demonstrates that:
  • Contrary to the claims of some VCHP manufacturers, refrigerant charge test methods are possible for variable speed systems.
  • The technology exists for meaningful refrigerant charge verification to be completed without connecting external refrigerant gauges.
• The evaporative cooling system demonstrated 66% lower cooling energy use than a reference single speed heat pump system, with 76% lower peak demand on hot afternoons.  The evaporative cooling system was able to maintain indoor temperature at 78 °F on even the hottest days, while also maintaining indoor relative humidity below 60%. The hottest day that occurred during monitoring of the evaporative cooling system reached a high temperature of 107 °F dry bulb, 70 °F wet bulb. 
• The evaporative cooling system experienced a failure related to the water drain system during the summer of 2020.  Water use was significantly increased following repairs by the manufacturer.  The annual water use range shown in Table 2 represents water use characteristics before and after the repair. 
• The CO2 refrigerant HPWH hydronic heating system demonstrated a seasonal heating COP of 1.8.  The HPWH tended to operate to reheat the water tank during the early morning hours when heating loads were high.  There may be opportunity to improve efficiency and reduce morning demand by preheating the tank when conditions are more favorable. 

Key recommendations include:

• As there is no correlation between the SEER and HSPF seasonal efficiency ratings and installed energy performance in California, it is recommended that all types of VCHP systems be viewed similarly to the mini and multi split type systems that were studied in the 2014-2019 CVRH projects.  Title 24 compliance credit should reference performance information that is more reliable and more applicable to the California climate than the AHRI seasonal efficiency ratings.
• Heat pump systems of all equipment types should be required to provide discrete heating and cooling modes, and prevent unnecessary electric resistance heat operation.  The SPV VCHP was observed to operate in heating mode on some summer days, and was also observed to disable the compressor and operate electric resistance heat even at mild outdoor conditions when the compressor was capable of producing more heat than the resistance heater.  These control strategies should be prohibited in Title 24.
• VCHP systems that provide practical methods of test to verify refrigerant levels exist, and their adoption should be supported through utility or third-party incentive programs and/or Title 24 credit. 
• VCHP systems that provide onboard monitoring of superheat and subcooling relative to target, and communicate results to installation or maintenance technicians exist and their adoption should be supported through utility or third party incentive programs and/or Title 24 credit.
• Very low GWP cooling and heating systems are emerging technologies that should continue to be supported through research and demonstration projects. 
• The evaporative cooling system demonstrated very low seasonal cooling energy use compared to the SCHP system, very low peak demand, and ability to provide acceptable indoor temperature and humidity. Water use is an issue that should be explored through further research. 
• The CO2 heat pump water heater hydronic space heating system demonstrated a seasonal heating COP approaching that of conventional air to air heat pumps.  The technology is hampered by a lack of complete system solutions, resulting in increased probability of performance degrading problems when this type of system is installed by a typical HVAC contractor.  There is opportunity to improve the technology through more coherent solutions, including advanced control systems.