Central Valley Research Homes: Variable Capacity Heat Pumps Evaluation of Refrigerant Charge Effects 2019-2020

The purpose of this project was to evaluate the impact of low refrigerant levels on the cooling and heating seasonal energy performance of variable capacity heat pump (VCHP) systems operating in California homes.

Prior studies have found that incorrect refrigerant levels are common in field installed HVAC systems (Proctor 2002, Pigg 2008) and have documented efficiency impacts associated with incorrect refrigerant levels (Davis 2001, Kruse 2006, Domanski 2014).

The prior research has focused on single speed equipment operating at steady state conditions in a laboratory. This project evaluated the impact of low refrigerant levels on variable speed equipment operating at the range of conditions that occur in single family homes throughout the course of a heating and cooling season.

The summary conclusions were:

• The study documented substantial cooling mode efficiency losses and the potential for heating mode efficiency losses when the VCHP systems were operated at reduced refrigerant levels. It is recommended that:
• California Title 24 should continue to apply refrigerant charge verification requirements for VCHP systems.
• California Title 24 should continue to apply efficiency loss assumptions to VCHP systems with unverified refrigerant charge. The existing assumption of 10% efficiency loss for central air conditioners and heat pumps is appropriate for VCHP systems because the efficiency losses documented in this study are similar to the losses that would be expected to occur on a standard heat pump system with a thermostatic expansion valve.
• VCHP manufacturers should be encouraged to develop refrigerant charge methods of test that enable equipment installers and maintenance technicians to ensure field installed VCHP systems contain the correct amount of refrigerant. Providing a credit in Title 24 would be one way to encourage this effort. It would be reasonable to assume degraded lifetime efficiency for equipment that lacks methods of test to facilitate ongoing maintenance at correct refrigerant levels.  Methods of test must be feasible and practical for residential HVAC contractors.  Examples of practical tests include the superheat and subcooling tests that have long been standard for conventional heat pumps.  Removing all of the refrigerant and weighing it is not a practical maintenance procedure.
• VCHP manufacturers should be encouraged to develop onboard systems to detect incorrect refrigerant levels or leaks and alert occupants and the HVAC contractor that there is a problem. Providing a credit in Title 24 would be one way to encourage this effort.
• VCHP manufacturers should be encouraged to adopt refrigerant line connections that are less prone to leakage than flare connections.  At present, the installation instructions for nearly all residential mini and multi split type systems specify flare connections.
• Energy efficiency programs that install VCHP systems should promote the installation of systems that provide effective refrigerant charge methods of test and onboard refrigerant fault detection by incentivizing the systems that include these capabilities.
• Energy efficiency programs that install VCHP systems should include procedures to ensure refrigerant levels are properly adjusted and that all refrigerant line connections are tested and verified to not leak.
• Energy efficiency programs that install VCHP systems should include rigorous technician training and certification requirements. At minimum, training should include:
  • Evacuation and charging procedures
  • Refrigerant adjustment to compensate for lineset length
  • Correct installation of refrigerant line connections, including flare connections and options such as press fittings that are demonstrated to be less prone to leakage
  • Refrigerant charge verification methods.

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