Assessment of a Hybrid Retrofit Gas Water Heater

Stacey Rothgeb, Marc Hoeschele, Elizabeth Weitzel, Christine Backman

Research output: NRELSubcontract Report


This project completed a modeling evaluation of a hybrid gas water heater that combines a reduced capacity tankless unit with a downsized storage tank. This product would meet a significant market need by providing a higher efficiency gas water heater solution for retrofit applications while maintaining compatibility with the 1/2 inch gas lines and standard B vents found in most homes. The TRNSYS simulation tool was used to model a base case 0.60 EF atmospheric gas storage water, a 0.82 EF non-condensing gas tankless water heater, an existing (high capacity) hybrid unit on the market, and an alternative hybrid unit with lower storage volume and reduced gas input requirements. Simulations were completed under a 'peak day' sizing scenario with 183 gpd hot water loads in a Minnesota winter climate case. Full-year simulations were then completed in three climates (ranging from Phoenix to Minneapolis) for three hot water load scenarios (36, 57, and 96 gpd). Model projections indicate that the alternative hybrid offers an average 4.5% efficiency improvement relative to the 0.60 EF gas storage unit across all scenarios modeled. The alternative hybrid water heater evaluated does show promise, but the current low cost of natural gas across much of the country and the relatively small incremental efficiency improvement poses challenges in initially building a market demand for the product.
Original languageAmerican English
Number of pages77
StatePublished - 2017

Bibliographical note

Work performed by Alliance for Residential Building Innovation (ARBI), Davis, California

NREL Publication Number

  • NREL/SR-5500-64550

Other Report Number

  • DOE/GO-102017-4707


  • ARBI
  • Building America
  • DHW
  • domestic hot water
  • hybrid water heater
  • residential
  • residential buildings
  • retrofits
  • tankless
  • water heating


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