(360) 789-9669 codeinnovations@ecobuilding.org

CO2 Heat Pump Water Heater in Madrona Passive House

Seattle, WA, USA

Building Type




Maureen Traxler | City of Seattle, Department of Planning and Development
Richard Alford | City of Seattle, Department of Planning and Development

Dan Whitmore | owner
Richard Alford | approving-official
Maureen Traxler | approving-official
Maho Ito | Other

Ratings & Awards

PHIUS+2015 Certified on 9/13/2016

Seattle’s residential building code requires domestic hot water heating system be tested and listed by a testing agency approved by the State of Washington. in the 1990s, Japanese engineers developed heat pump domestic hot water heaters using CO2, an abundant, natural refrigerant. These systems are known as “Eco-Cute” which means environmentally-friendly hot water because using CO2 avoids conventional synthetic refrigerants which have high global warming potential (GWP). Although Sanden’s Eco-Cute is not yet tested by an approved agency, the City of Seattle’s Innovative Technology Advisory Committee (ITAC) recommends Hammer & Hand be permitted to install the CO2 system as an alternate material because of its superior performance and environmental attributes if it meets four conditions.

Hammer & Hand wants to use the environmentally superior Sanden CO2 refrigerant heat pump water heater system to provide both domestic hot water (DHW) and space heating in constructing a custom Seattle Passive House.  The intent of Seattle’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 the City.  Even though the City requires third-party testing to the UL standard, and Sanden had not yet completed that testing, the SRC recognizes that alternative technologies with equivalent or greater attributes (performance, safety, durability, etc) can be approved by the building officials.

Maureen Traxler, City staff supporting the ITAC, who worked with Hammer and Hand to assist and facilitate the proposal. The written application and subsequent oral powerpoint presentation to the Committee helped establish Sanden’s Eco-Cute’s superior performance and environmental attributes, based on a previous presentation and subsequent approval by to the City of Portland Alternative Technology Advisory Committee (see supporting documents.  With twelve (12) members of the ITAC present plus five (5) DPD officials, the meeting included lots of interest, questions and legitimate concerns. A 26-page report listing relevant codes and standards helped guide the 1-hour discussion.  The ITAC voted unanimously to recommend the Sanden system on the condition Hammer & Hand supplies the following information:  1.  Record of application and timeline for UL Approval; 2.  Until UL approval is confirmed, approval by a 3rd party testing agency approved by the State of Washington; and 3.  Confirmation of Lead-Free manufacturing and materials per code. ITAC’s recommendation is also conditioned on case-by-case review and approval in the permitting process for each specific building installation.

Code Requirement Compliance Path
2013 Seattle Residential Code (2012 SRC) section M2005.1:  “Domestic electric water heaters shall comply with UL 174.” Written application to City’s ITAC, ITAC in -person hearing, and ITAC’s conditional recommendation, followed by written building and electrical permit applications based on ITAC’s recommendation.
2012 SRC section R104.5  Allows alternate materials and methods
Written application to City’s ITAC, ITAC in -person hearing, and ITAC’s conditional recommendation, followed by written building and electrical permit applications based on ITAC’s recommendation. Allows alternate materials and methods
Square Footage: 4100ft2

Hammer & Hand has built passive houses for five years, and is currently under construction of a 4,100 square foot passive house in Seattle for a client who shares Hammer & Hand’s commitment to both superior home performance and lowest environmental impact, including the most energy-efficient water and space heating equipment, ideally electric to make use of renewable energy and avoid combustion of fossil fuels. Sanden’s CO2 refrigerant heat pump water provides an electric alternative to a gas water heater, which is more efficient than a resistance electric water heater.  Although there are several heat pump water heaters on the market, the low global warming potential of CO2 compared to conventional refrigerants makes Sanden’s system attractive to both Hammer & Hand and its client.

Although Hammer & Hand might ordinarily consider installing a mini-split space heating system in addition to the DHW system,  the home is designed to be cooled by passive measures (solar reflective materials, shading, and night-flush ventilation) and does not need air conditioning.  Relying on the Sanden system for both DHW and hydronic heating completely eliminates the need for an additional mechanical system.

With the passive house’ superior thermal enclosure, it will be an almost-airtight building enclosure with balanced, mechanical ventilation with heat recovery.  However, it will still need a small amount of “active” space heating and domestic hot water, which Sanden’s system should be more than sufficient to do.  If on coldest days the Sanden system does not have sufficient heating capacity for both water and space heating Hammer & Hand may also install a high-efficiency tankless electric DHW system for additional heating capacity.

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).

The Seattle Residential Code does not allow the installation of  a DHW system until it is tested to UL’s standard and listed by UL.  Although Sanden’s Eco-Cute is not yet tested by UL, the City of Seattle ITAC appreciated the environmental superiority of Sanden’s system and upon satisfying certain conditions, the ITAC has recommended Hammer & Hand be permitted to install the Sanden system.

Hammer & Hand is working closely with Sanden and its electrical and plumbing subcontractors to design and install the system

Benefits: Superior energy efficiency (high COP and reliable heat capacity even in cold weather) and environmental attributes (use of CO2 refrigerant substantially lowers the global warming from refrigerant leaks into the atmosphere).  Typically, refrigerants leak into the atmosphere 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.

Hammer & Hand is collecting information the City requested to meet the conditions of approval.  Sanden has made an application for testing and listing by UL, and it reported that it expects to complete the process by 2015.  The 2013 testing by Ecotope and Cascade Engineering for Washington State University and Bonneville Power Administration demonstrates that the Sanden system meets the first-hour test.   Sanden is providing proof that it meets the “lead-free” requirements in the Washington (State) plumbing code enforced by the King County Health Department (as the City of Seattle does not have its own residential plumbing code).

Dan Whitmore
Certified Passive House Consultant & Builder


Richard Alford
Electrical Official


Maureen Traxler
Code Development Manager


Maho Ito
CO2 Product Line Manager