Measure Guideline: Combined Space and Water Heating Installation and Optimization

Stacey Rothgeb, B. Schoenbauer, D. Bohac, P. Huelman

Research output: NRELSubcontract Report


Combined space and water heater (combi or combo) systems are defined by their dual functionality. Combi systems provide both space heating and water heating capabilities with a single heat source. This guideline will focus on the installation and operation of residential systems with forced air heating and domestic hot water (DHW) functionality. Past NorthernSTAR research has used a combi system to replace a natural gas forced air distribution system furnace and tank type water heater (Schoenbauer et al. 2012; Schoenbauer, Bohac, and McAlpine 2014). The combi systems consisted of a water heater or boiler heating plant teamed with a hydronic air handler that included an air handler, water coil, and water pump to circulate water between the heating plant and coil. The combi water heater or boiler had a separate circuit for DHW. Past projects focused on laboratory testing, field characterization, and control optimization of combi systems. Laboratory testing was done to fully characterize and test combi system components; field testing was completed to characterize the installed performance of combi systems; and control methodologies were analyzed to understand the potential of controls to simplify installation and design and to improve system efficiency and occupant comfort. This past work was relied upon on to create this measure guideline.
Original languageAmerican English
Number of pages45
StatePublished - 2017

Bibliographical note

Work performed by NorthernSTAR, St. Paul, Minnesota

NREL Publication Number

  • NREL/SR-5500-65429

Other Report Number

  • DOE/GO-102017-4829


  • Building America
  • combi space and water heating
  • high performance water heater
  • home heat with water heater
  • hydronic coil air handler
  • NorthernSTAR
  • residential
  • Residential Buildings


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