Combined Space and Water Heating: Next Steps to Improved Performance

Stacey Rothgeb, B. Schoenbauer, D. Bohac, P. Huelman, Stacey Rothgeb (NREL Technical Monitor)

Research output: NRELSubcontract Report


A combined space- and water-heating (combi) system uses a high-efficiency direct-vent burner that eliminates safety issues associated with natural draft appliances. Past research with these systems shows that using condensing water heaters or boilers with hydronic air handling unitscan provide both space and water heating with efficiencies of 90% or higher. Improved controls have the potential to reduce complexity and improve upon the measured performance. This project demonstrates that controls can significantly benefit these first-generation systems. Laboratory tests and daily load/performance models showed that the set point temperature reset control produced a 2.1%-4.3% (20-40 therms/year) savings for storage and hybrid water heater combi systems operated in moderate-load homes.
Original languageAmerican English
Number of pages49
StatePublished - 2016

Bibliographical note

Work performed by NorthernSTAR Building America Partnership, Minneapolis, Minnesota

NREL Publication Number

  • NREL/SR-5500-63459

Other Report Number

  • DOE/GO-102016-4600


  • Building America
  • condensing
  • HVAC
  • low load homes
  • NorthernSTAR
  • residential
  • Residential Buildings
  • space heating
  • water heating


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