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Water Heating & Radiant Heat Combo at Birch House

Case Study by Dan Welch [bundle]
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Water Heating & Radiant Heat Combo at Birch House
Heating, Ventilation & AC
Plumbing Systems
Water Heating
Superior equipment, not UL Listed
City of Bellingham, WA
James E. Tinner, City Of Bellingam, WA Building Official
Dan and Ashley Welch
Dan Welch, [bundle] design studio
Chris Tretwold, Tretwold Construction
NEEA Next Step Home


The state of Washington's residential building code requires domestic hot water heating system be tested and listed by a State-approved agency.  In the 1990s, Japanese engineers developed heat pump domestic hot water heaters using CO2, an abundant, natural refrigerant – more environmentally-friendly because they avoid conventional synthetic refrigerants which have high global warming potential (GWP). Although Sanden’s “Eco-Cute” system was not yet UL listed, the City’s Building Official allowed [bundle] design studio to install the system as an alternate material because of its superior performance and environmental attributes, once it met outlined conditions. The unit has been in operation since October 2014.

Permitting Process

In partnership with [bundle] design studio, The NW Energy Efficiency Alliance, Washington State University Energy Program, and Sanden, USA requested approval through the City of Bellingham, WA to use the environmentally superior Sanden CO2 refrigerant heat pump water heater system to provide both domestic hot water (DHW) and space heating in the Birch House, a custom single family residence.  Although the equipment was not yet UL listed, the City’s building code recognizes alternative technologies with equivalent or greater attributes may be approved. Building Official Jim Tinner appreciated the environmental superiority of Sanden’s system and upon satisfying certain conditions, allowed [bundle] to install the Heatpump system.

Code RequirementCompliance Path
2012 IRC R104.11 Alternative Materials & Methods and 2012 IMC M105 Approval Allows Alternative Materials & Methods and "authority to grant modifications for individual cases”
2012 IRC M2005 and 2012 IMC M1002.2Water Heaters "Domestic electric water heaters shall comply with UL 174... or UL 1453 Building official reviewed & provided conditional approval. Waiver of Liability from owner to City of Bellingham filed with County Auditor to remain with property Deed. Amended building & electrical permits upon conditional approval

Project Description

[bundle] design studio is a building design firm in Bellingham, WA focusing on high performance buildings built to the Passive House and Living Building standards. [bundle] designed and developed the Birch Case Study House ,a 2000 sqft single family residence, with the goal to educate designers, architects, contractors and building officials of innovative sustainable construction technologies. To satisfy this goal [bundle] partnered with Northwest Energy Efficiency Alliance (NEEA), Washington State University (WSU) and Sanden,USA to begin field testing the Sanden CO2 Heat pump water heater for a combined domestic hot water and radiant heating application. The field testing is a portion of Sanden's requirements for UL approval. Once UL listed and accepted outright by International Building Codes the product will have a much wider application and bring highly efficient water heating technology to the building industry.

The combined system (Sanden Combi) in Birch Case Study House was the first to be installed in North America.

The intent of Belingham's Residential Code is to ensure that materials and equipment such as water heaters are evaluated and certified by third-party organizations approved by the City.  Although Sanden had not yet completed that testing, the Bellingham building code recognizes that alternative technologies with equivalent or greater attributes (performance, safety, durability, etc) can be approved by the building officials.

The team supplied a proposal under, 2012 IRC section R104.5 Advanced materials and Methods, to Jim Tinner at the City of Bellingam. The proposal include an introduction letter, supporting specifications outlining the capabilities of the Sanden CO2 Heat pump, a Mechanical plan designed by Johnathan Heller PE at Ecotope in Seattle, and supporting documentation of approvals for the Sanden use in Portland, OR.

Jim Tinner reviewed the proposal and accepted the use of the Sanden system under the following conditions:

  1. System must provide back-up boiler should the Sanden fail. A tank water heater was provided as back-up and has recently been replaced with an electric on-demand DHW boiler.
  2. Heat tape must be provided exterior of the building at the supply piping of the unit to prevent freezing.
  3. A Waiver of Liability had to be written and filed with the Whatcom County Auditor absolving the City of Bellingham of any Liability associated with the Sanden Heat Pump.


[bundle] is constantly pushing to design projects to the Living Buidling and Passive House Standards. One of the major hurdles in this process is achieving Net-zero energy without using combustion appliances. As water and space heating are typically the major energy uses in buildings, highly efficient mechanical appliances such as the Sanden CO2 heatpump play an important role in achieving the Net-zero goal.

Typically [bundle] would specify a Minisplit heapump for space heating and a heat pump water heater for the DHW. The availability of the Sanden Combi system eliminated the need for two separate mechanical systems. A superior building envelope eliminated nearly all thermal bridges and air leakage and achieved a blower door test of .4 ACH50.  This drastically reduced the heating load and allows the building to easily operate within the specifications of the Sanden CO2 heat pump. On the very coldest days, if the Sanden system does not have sufficient heating capacity for both water and space heating [bundle] also installed a high-efficiency tankless electric DHW system as a back-up.

In the 1990s, Japanese engineers perfected high-pressure CO2 refrigerant heat pump technology (known as “Eco-Cute”).  Available in Japan since 2001, this technology has spread to Australia and Europe, but to date, it has not been available for North American homes.  Now, Sanden International is beginning to import its Eco-Cute system to North America. CO2 is an abundant, natural refrigerant. CO2 has a GWP of one, far below the GWP of conventional refrigerants (1,000 or 2,000).

Design / Build Process

[bundle] worked closely with the Northwest Energy Efficiency Allicance (NEEA), WSU, Ecotope (mechanical engineers) and Spectrum Plumbing subcontractors to design and install the system.

Cost / Benefit

Appliance Reduction: Using the Sanden CO2 Heatpump for both DHW and Radiant Heating allowed [bundle] to reduce the number of mechanical appliances within the project. This was a mild cost savings [CvD1] at the time of installation but may have lasting cost savings as the appliances age and mechanical maintenance is required.

Superior energy efficiency: Sanden CO2 has high COP and reliable heat capacity even in cold weather. Appliance is significantly reduces operating costs and will realize significant financial savings over other heating appliances for the life of the unit[CvD2] .

Environmental attributes: The use of CO2 refrigerant substantially lowers the global warming from refrigerant leaks into the atmosphere which occurs both slowly during operation and quickly either upon an accidental release or at the end of the equipment’s use if the refrigerant is not carefully captured and either reused or destroyed.

Project Contacts
Owner: Dan Welch [bundle] design studio 360.296.2657 Approving Offcial: James E. Tinner City of Bellingham (360) 778-8307 Other: John Miles Sanden International USA, Inc 734.738.5915
Builder: Chris Tretwold Tretwold Construction 360.201.1513 Subcontractor: Ben Fuller Spectrum Plumbing & Heating 360.325.5173
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