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Residential Variable Capacity Heat Pumps

Laboratory Test of Residential, Variable Capacity Heat Pumps (VCHP)


Manufacturers are expanding their residential air source heat pump product lines to include high efficiency variable capacity heat pumps (VCHP) that can improve upon an electric forced air furnace or a lower efficiency Heating Seasonal Performance Factor (HSPF) fixed speed air source heat pump. To fully quantify the annual energy savings of this new technology BPA sponsored laboratory testing to develop VCHP performance curves. Our intent is to use these performance curves in combination with field testing and energy modeling to evaluate annual energy savings over a range of installations and outdoor air temperatures in the Pacific Northwest region.

Laboratory testing was led by Electric Power Research Institute (EPRI) in Knoxville, TN to develop performance curves over a range of outdoor air temperature and three levels of heating capacity of a nominal two-ton VCHP with a HSPF of 10.5. Testing for frost accumulation and cyclic degradation were also performed to help characterize the system operation. Using this data and energy modeling, a comparison was made between the VCHP and a federal minimum efficiency, HSPF 7.7, fixed speed system.

Key factors identified from laboratory testing:

  • The variable capacity system had the ability to operate at full nominal heating capacity (two tons) between 17º F - 62º F.
  • Increased efficiency is partially due to a larger heat exchanger surface area.
  • Performance of the system showed no degradation from cyclic testing.
  • Minimal capacity degradation was seen over the frost accumulation test period.
  • This VCHP may significantly reduce auxiliary heat in field operation for lower outdoor temperatures because it provides higher heating capacities at low outdoor temperatures.
  • Regression analysis based on laboratory data show the ability to predict air flow with either fan speed or fan power; and to predict system capacity as a function of compressor speed, blower speed, and outdoor temperature.

Key factors identified from energy modeling comparison:

  • Seattle climate model determined a 17% and Boise climate a 19% annual energy savings for the VCHP.
  • Reduction in backup heat is the primary reason for the energy savings.

Next Steps:

By itself, the steady-state laboratory testing and energy model within this report does not fully characterize the variable capacity system of interest. In follow-up, field data will be combined with the laboratory results and additional energy modeling to quantify the energy savings potential and fully characterize the system.

In evaluating new VCHP products, we will use the results of this project to improve lab testing protocols and simplify related field testing.  Full project details are available in the final report below.

Project Team
BPA: Kacie Bedney, Jack Callahan, Stephanie Vasquez
EPRI: Walt Hunt, Ron Domitrovic

2012 - 2013 (Completed July 2013)