Heat Pump Water Heater Ducting Strategies with Encapsulated Attics in Climate Zones 2 and 4

Stacey Rothgeb, M.L. Sweet, A. Francisco, S.G. Roberts, Stacey Rothgeb (NREL Technical Monitor)

Research output: NRELSubcontract Report


The focus of this study is on the performance of HPWHs with several different duct configurations and their effects on whole building heating, cooling, and moisture loads. A.O. Smith 60 gallon Voltex (PHPT-60) heat pump water heaters (HPWHs) were included at two project sites and ducted to or located within spray foamed encapsulated attics. The effect of ducting a HPWH's air stream does not diminish its efficiency if the ducting does not reduce intake air temperature, which expands HPWH application to confined areas. Exhaust ducts should be insulated to avoid condensation on the exterior, however this imposes a risk of condensation occurring in the duct's interior near the HPWH due to large variation of temperatures between the compressor and the duct and the presence of bulk moisture around the condenser. The HPWH's air conditioning impact on HVAC equipment loads is minimal when the intake and exhaust air streams are connected to a sealed attic and not the living space. A HPWH is not suitable as a replacement dehumidifier in sealed attics as peak moisture loads were observed to only be reduced if the heat pump operated during the morning. It appears that the intake air temperature and humidity was the most dominant variable affecting HPWH performance. Different ducting strategies such as exhaust duct only, intake duct only, and exhaust and intake ducting did not have any effect on HPWH performance.
Original languageAmerican English
Number of pages53
StatePublished - 2016

Bibliographical note

Work performed by Partnership for Home Innovation, Upper Marlboro, Maryland

NREL Publication Number

  • NREL/SR-5500-64860

Other Report Number

  • DOE/GO-102016-4753


  • Building America
  • conditioned attic
  • ducted
  • enclosed attic
  • energy efficiency
  • heat pump water heater
  • high performance
  • HPWH
  • PHI
  • residential
  • Residential Buildings
  • sealed attic
  • water heating


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