Retrofit Integrated Space & Water Heating: Field Assessment, Minneapolis, Minnesota (Fact Sheet)

Stacey Rothgeb, T. Schirber, B. Schoenbauer, D. Bohac, J. McAlpine, M. Hewett

Research output: NRELFact Sheet


This project analyzed combined condensing water heaters or boilers and hydronic air coils to provide high efficiency domestic hot water and forced air space heating. Called 'Combi' systems, they provided similar space and water heating performance less expensively than installing two condensing appliances. The system's installed costs were cheaper than installing a condensing furnace and either acondensing tankless or condensing storage water heater. However, combi costs must mature and be reduced before they are competitive with a condensing furnace and power vented water heater (EF of 0.60). Better insulation and tighter envelopes are reducing space heating loads for new and existing homes. For many homes, decreased space heating loads make it possible for both space and domesticwater heating loads to be provided with a single heating plant. These systems can also eliminate safety issues associated with natural draft appliances through the use of one common sealed combustion vent.
Original languageAmerican English
Number of pages2
StatePublished - 2014

NREL Publication Number

  • NREL/FS-5500-61314

Other Report Number

  • DOE/GO-102014-4379


  • boiler based combi systems
  • Building America
  • combi heating systems
  • combi system summer efficiency
  • DHW performance
  • direct vent burner
  • dual integrated appliances
  • forced air combi systems
  • hydronic air handlers
  • NorthernSTAR
  • residential
  • residential buildings
  • storage water heater based combi systems
  • tankless water heater based combi systems


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