Aug 29, 2023

Case Study: Retrofitting for Electrification: Pairing a Cold Climate Heat Pump with an Existing Gas Furnace


Project Background 

Beginning in August 2022, CEE, MN ASHP Collaborative, and Twin Cities Habitat for Humanity (TCHFH) developed a plan for energy modeling and HVAC system design of an existing home. Based on TCHFH’s interest in retrofitting existing homes with ASHPs, CEE helped familiarize TCHFH with heat pumps and identified scenarios where they could use ASHPs to electrify and decarbonize their existing homes.

This case study shows how an electric ASHP can pair well with an existing natural gas furnace to increase heating efficiency, while also adding a source of efficient cooling. While TCHFH consistently builds and retrofits homes for efficiency and airtightness, they had not yet incorporated ASHPs into their home designs. Since the home highlighted in this study was relatively airtight, the heating and cooling systems could be more easily optimized for energy efficiency.

By performing in-depth analysis and testing of this TCHFH home, we were able to understand the up-front costs and ongoing operational costs of pairing an electric ccASHP with an existing natural gas furnace. In this dual fuel setup, a ccASHP can be used as the primary heating and cooling system throughout the year. Because ASHPs are much more energy efficient than natural gas furnaces, they can be used for heating on all but a few of the coldest days of the year. These gains in efficiency and electrification will result in substantial energy savings and opportunities for decarbonization. 


Case Summary

Energy Audit Profile 

  • House Details
    • Where: Rosemount, MN
    • Built: 1968
    • Size: 2240 ft sq
    • Number of Bedrooms: 3
    • House Type: Rambler
  • Insulation: R40 (attic), R11 (sidewalls), and R15 (rim joists)–weatherized by local CAP in 2011  

  • Primary Heating System: 80% efficient centrally ducted natural gas furnace with ECM motor (installed in 2020)  

  • Primary Cooling System: None  

  • Water Heating: Natural draft gas water heater (installed in 1996)  

  • House Tightness: 957 CFM50 (or 3.2 ACH50)  

  • HERS Model Score: 93 (TCHFH goal is 80 or below for rehabs) 

Audit Recommendations

  • Dual fuel setup pairing electric ccASHP (to use as primary heating and cooling) with existing two-year -old ducted gas furnace (to use as backup heat at 25°F or below)   

  • Replace 26-year-old water heater with heat pump water heater to add electrification and future decarbonization 

  • HERS score from 93 to 77 projected after ccASHP and heat pump water heater upgrades 

  • Projected 37% decrease in natural gas usage (from 848 to 642 therms) 

Air Source Heat Pump Specifications 

  • Variable-speed, 3 tons, 16 SEER2, 8.5 HSPF2, 2.26 rated COP @ 5°F  

  • Capacity Balance Point: 25°F  

  • Heating Design Load: 37k Btu/hour (at -11°F) *based off Manual J  

  • Cooling Design Load: 13k Btu/hour (at 89°F) *based off ASHRAE