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zHome First Rainwater Harvesting for Indoor Use in Issaquah

Case Study by Leah Missik, Chris van Daalen
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zHome First Rainwater Harvesting for Indoor Use in Issaquah
Plumbing Systems
Rainwater Harvesting
Rainwater Harvesting for Indoor Use
Issaquah, WA
, City of Issaquah Building Official
Ichijo USA
2020 Engineering
Ichijo USA
Built Green Emerald Star, US EPA Watersense for Homes Certification, Salmon Safe Certification, others see Updates below


zHome is a zero net energy, highly sustainable, ten-unit market-rate townhome development, part of a mixed-income planned development.  Led by the City of Issaquah Office of Sustainability as part of an innovative land deal, the builder received the land for no cost, and in exchange agreed to meet rigorous environmental benchmarks (e.g. net zero energy, 70% reduction in city water use).  zHome was the first project in Issaquah permitted to use rainwater indoors for toilet flushing and laundry, using the Alternative Engineered Design Section 301.4 of the Uniform Plumbing Code. As a result, the project helped change code and add flexibility for green building innovations, and set an example for other small municipalities in the region.

Permitting Process

Each of the 10 units in the townhome project was designed with a simple system, in which rainwater would flow from the roof, to the gutter and downspout through a “first flush” diversion filter, and into a unit’s cistern. Then, the rainwater would be pumped into the house, filtered, stored in a pressure tank, and used for flushing toilets and cold water supply for washing clothes. The City approved the system through a collaborative design process during which the project team met with code officials to consider issues of concern.  After determining appropriate design requirements to ensure sanitation and supply, and reviewing a site specific engineered design, the City approved the project as an alternative method under the 2006 Uniform Plumbing Code, Sec. 301.1-301.4.

Code RequirementCompliance Path
2006 Uniform Plumbing Code, Sec. 301.4 Alternative Engineered Design Rainwater harvesting for indoor use was approved under sec. 301.4 which provides for approval of an alternative engineered design, after site-specific design requirements were satisfied
City of Issaquah site-specific requirements for zHome City inspectors and other staff collaborated with design team to work through specific design requirements required to meet code official’s approval under Sec. 301.4

Compliance Details:


It wasn’t until the 2009 Uniform Plumbing Code (effective July 1, 2010) cycle that Washington State adopted section 16, a State amendment specifically recognizing and governing rainwater harvesting systems. Therefore the zHome water design team, led by 2020 Engineering, had to blaze a new compliance path for this project, in the process setting the stage for codes that would be adopted statewide.

Design Schematic

2020 Engineering wanted to make the zHome rainwater system as simple as possible, and encourage homeowner interaction with the sustainable features of their home. To maximize safety, the design team routed the potable and non-potable systems separately and required clear labelling in order to prevent confusion.  Inspectors were diligent to verify that every non-potable system was plumbed with “purple pipe” (indicating reclaimed water) and properly labeled every 12 inches, or in every joist bay. They also required all supply stubs on toilets and laundry connections to be labeled. Because the system is considered an unapproved auxiliary water source, it triggers a premise isolation requirement to protect the City’s water supply.  Therefore, each unit’s system was required to have three reduced pressure backflow prevention devices (one for the whole home installed at the water meter, and two specifically for the washing machine) so if the home were to be replumbed in the future and the potable and non-potable systems connected, the non-potable water would not reenter the City supply.

The designers proposed that if a cistern ran out during an especially dry summer, the resident could manually refill it with a hose.  However, the City of Issaquah was concerned that a homeowner might overfill the cistern, or that - however unlikely - rainwater could be syphoned back into the potable water system through the hose.  The permit team initially suggested an automated refill system, but the design team wanted as much homeowner interaction with the system as possible.  So together with code officials they synergistically developed a new idea: a manual refill port on the exterior of the home, plumbed with an air gap directly underneath the hose bib. This way, homeowners have more of a connection with the rainwater system, and there is no possibility of cross-contamination into the potable water system. Ultimately much of the design simplicity and homeowner interaction was retained.  In the process City established a few clear guidelines for approving such a system under the “Alternative Engineered Design” provisions of section 301.4.

Resources and Documents

“zHome: Setting a National Net Zero Energy and Green Building Precedent” by Leah Missik, Built Green. 2015. 2009 Uniform Plumbing Code Washington State Amendments, Section 1623-1628
zHome Blog on Rainwater Harvesting 2015 Uniform Plumbing Code Section 16
Rainwater Catchment System video

Project Description

In a unique and innovative land deal, the City of Issaquah and Port Blakely (master developer of the Issaquah Highlands) provided a prime site for mixed-income housing development in the Issaquah Highlands: the YWCA Family Village an affordable housing project, and the zHome project, with market-rate townhomes. The Highlands was a planned development that stipulated a 4:1 Transfer of Development Rights ratio, meaning that for every acre of land developed, four acres in another location would remain undeveloped in perpetuity. Under the agreement, the zHome project had to meet a set of very rigorous environmental benchmarks that would make the project an example of deep green building that was accessible to the average person.  Port Blakely also required all homes built in other parts of the Issaquah Highlands would have to achieve Built Green 4-Star certification or higher.  In this environment, a demonstration project on the scale of zHome supported the vision of both the City and Developer.

The land for zHome was provided to the builder at no cost, when the builder agreed to meet the development’s rigorous environmental benchmarks and provide for partnership project management costs, absorb market risk, and to reserve one unit as an education center for up to five years. Environmental benchmarks included: net zero energy use (measured over a year); a 70% reduction in utility water use compared to the average home; a 90% construction recycling rate; 75% of wood used must be FSC certified, reclaimed, or recycled; and the materials, paints, and finishes in the home must be low- or non-toxic. These benchmarks were contractual obligations under the land deal.

In order to hit the 70% water efficiency benchmark, it was clear the project needed to go beyond simply installing the most efficient fixtures (all WaterSense labeled) and having a smart plumbing layout. It was decided that rainwater would be captured and used not just for irrigation (due to the use of native plants, irrigation needs were minimized anyhow), but for non-potable use inside: namely, toilet flushing and laundry. Other technologies such as solar PV and geothermal heat pumps were used to meet other environmental criteria. None of the technologies utilized were new – but combining them all into an “average home” was innovative, and demonstrating this possibility was a goal contractually stipulated for the zHome project.

Homeowner interaction in the water system was facilitated in several ways.  If a cistern runs out during dry summer months, homeowners can refill it from a hose bib outside their home (plumbed with an air gap to refill port to prevent cross-contamination into the potable water system). Homeowners also interact with their system through routine filter changes (located in the home).  Each home also includes both a remote cistern water level monitor and a water consumption display that communicates daily water use and notifies homeowners of leaks (such as leaky toilets, sinks, showerheads, etc). Several homeowners have indicated that they check these displays and have adopted water conservation methods in their lives to keep their consumption numbers as low as possible. They also have somewhat of a feeling of defeat when their cistern runs out of rainwater, spurring further conservation. Overall, the system design appears to be successful from a homeowner’s perspective.


The zHome project originally came to life in 2006 when a group of the region’s leaders in green building (representing Built Green, King County GreenTools, the City of Issaquah, and the Washington State University Energy Program) coalesced with a vision to demonstrate that deep green homes can be accessible to the average person and also pushing the envelope on green building convention and market penetration at the time. The name zHome stems from zero, as in, net zero carbon and energy. Ultimately, the project would be an example of what 21st century homes should be like. This group created the project’s sustainability benchmarks and helped broker the land deal, which would help entice a developer to dive in on this project.

Due to the floundering market, there were some major stops and starts. The designer (David Vandervort Architects) had been selected, along with a builder, but the stock market crashed the very same day as groundbreaking. After a hiatus, the final builder, Ichijo USA, entered the team at the end of 2009, and then building began in earnest. Upon its completion, zHome – as the country’s first net zero energy townhome development! – received national media coverage and hosted tour groups from around the world. After all, part of the project’s conception revolved around demonstration and education.

Design / Build Process

Given the zHome project’s uniqueness, the number of project partners, and the economic crash, the design/build process was spread out over about five years. The strict environmental benchmarks were specified contractually as a part of the City to builder land transfer, with the City of Issaquah positioning itself as a municipality taking a lead on green building. The design process placed a heavy focus on eco-charrettes that involved many stakeholders. The design also evolved somewhat when Ichijo USA became the builder on the project; initially, the connection was made due to Ichijo’s Structural Insulated Panel manufacturing, and Ichijo also manufactures many of its own components, which were utilized later in zHome.

Cost / Benefit

The rainwater harvesting system (design, permitting, and installation) cost around $12,000 per unit. Clearly, the system was an investment. However, homeowners have indicated their satisfaction with the system. It is difficult to parse out the impact of the rainwater system relative to other environmentally friendly features. However, we do know that the average price per square foot for zHome was slightly higher than comparable homes in the area at the time of sale. The premiums ranged from an average of $30,942 (for a one-bedroom) to $54,040 (for a three-bedroom) to $54,519 (for a two-bedroom). zHome units, nonetheless, were relatively affordable for the location, and thanks to very large energy bill savings, monthly home-related payments are decreased. Because of water pricing structure and overall reduced water demands, there are very limited monetary savings associated with the rainwater catchment system specifically, but utility bills do not account for other positives for the homeowner: environmental well-being, satisfaction, and novelty being among them.


Happily, based on two years of post-occupancy data, zHome reached its goal of reducing baseline water usage by 70%. Built Green’s Emerald Star level uses 67 gallons per person per day as a baseline average, which was derived from the state’s average per capita household use – the number set as zHome’s baseline comparison point. On average, a zHome resident uses 16.07 gallons per day from the utility – a marked decrease that demonstrates the efficiency of its fixtures as well as the substitution of rainwater for common household uses. Unfortunately, there is no data on exactly how much water each zHome unit consumes from its cistern because these connections bypass the utility’s water meter. However, based on fixture ratings and studies on their use, it can be estimated that about 11.46 gallons of rainwater are used per person, per day.

Unfortunately, despite huge savings in utility water use, zHome residents see little impact on their bill (average bimonthly savings of $2.24) due to the billing structure. Much of the charge is a fixed connection rate, whereas the per unit cost of water is very small – not a good incentive for lowering use. Regardless, homeowners are overall satisfied with their cisterns. When surveyed, most homeowners “agreed” or “strongly agreed” that the cisterns work well, and none disagreed. The only complaints were that the water had an odor. It was discovered that birds were nesting under the solar panels on the roofs, right above the cisterns, and defecating in the water. Netting was installed to prevent nesting birds and more education on system maintenance was provided to homeowners, and this issue was resolved. zHome’s rainwater catchment system is not only a demonstration of success, but also of the importance of education and monitoring for unforeseen issues.

Additional Certifications and Awards

Living Building Challenge Petal Certified US Department of Energy HERS Rating 2011
FSC - 2011 Design and Build with FSC Award 2011 Development of the Year by NW ENERGY STAR Homes
2012 Build Green Hammer Award 2012 Excellence in Construction Award by the Assoc. Builders and Contractors of Washington
Japan's prestigious Good Design Award 2012 BASF Builders Challenge Award 2012
Project Contacts
Builder: Nick Nied Ichijo USA (425) 223-7482 Designer: Mark Buehrer 2020 Engineering (360) 671-2020 Plan Reviewer: David Fujimoto City of Issaquah (425) 837-3412
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