Retrofitting Forced Air Combi Systems: A Cold Climate Field Assessment

Stacey Rothgeb, Ben Schoenbauer, Dave Bohac, Jake McAlpine, Martha Hewett, Stacey Rothgeb (NREL Technical Monitor)

Research output: NRELSubcontract Report


This project analyzed combined condensing water heaters or boilers and hydronic air coils to provide high efficiency domestic hot water (DHW) 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 a condensing 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 (energy factor 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 domestic water 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. The combined space and water heating approach was not a new idea. Past systems have used non-condensing heating plants, which limited their usefulness in climates with high heating loads. Previous laboratory work (Schoenbauer et al. 2012a) showed that proper installation was necessary to achieve condensing with high efficiency appliances. Careful consideration was paid to proper system sizing and minimizing the water temperature returning from the air handling unit to facilitate condensing operation.
Original languageAmerican English
Number of pages97
StatePublished - 2017

Bibliographical note

Work performed by NorthernSTAR Building America Partnership, University of Minnesota, St. Paul, Minnesota

NREL Publication Number

  • NREL/SR-5500-61313

Other Report Number

  • DOE/GO-102017-4378


  • 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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