Bay View, Carmel: A Very Water- And Energy-Efficient Retrofit
It’s not every day that we get to work on our dream house… from a sustainability point of view. But that's what happened when we took on this remodel. On this project, we were fortunate that the homeowners were 100% committed to building in a way that significantly reduces water and energy consumption (and related emissions) and achieves excellent indoor air quality, all within a beautiful living space.
The project was an extensive remodel and total energy retrofit of an existing two-story, split-level house in Carmel, California. The 2,134 ft2 home has three bedrooms and three bathrooms. We started the planning and permitting processes in 2013 and completed the project in May 2015. Our firm served as both designer and builder.
Nexus eWater System
One of the most exciting opportunities this project afforded—in light of California’s continuing drought—was the opportunity to address water conservation. We put together a triad of technologies, used together for the first time in our area—a graywater-to-water heat pump (part of the first Nexus eWater system installed in a U.S. residence), demand-controlled recirculation, and structured plumbing. Together, this trio dramatically reduces both the amount of water wasted and the energy used to heat it. (See Figure 1 on page 6 for a schematic of the system.)
The technology that became the Nexus eWater system was developed and used successfully in Australia as a strategy to combat its decades-long drought. Nexus dramatically improves both water conservation and energy efficiency. Even with a low-flush toilet that uses 1.6 gallons per flush, the average person flushes 3,000 gallons of water a year. Because Nexus uses recycled graywater for flushing toilets (and landscape irrigation), the annual potable water savings is substantial.
The system includes the NEXheater energy-recycling graywater-to-water heat pump, the eWater Collector to collect graywater, the NEXtreater graywater treatment system, and the NEXservoir treated-water storage tank. The graywater treatment system is certified as an NSF 350 “on-site treatment system for non-potable graywater.” This certification is recognized by the County of Monterey Environmental Health Department.
Here’s how it works:
- The house is plumbed for separate graywater and black water drain systems, both terminating in the mechanical room.
- Each 10-gallon batch of graywater is treated to NSF 350 standards using the five-stage filtering system described later. It is then sent to the 200-gallon treated-water reservoir.
- The treated water stored in the reservoir is used for flushing toilets and for exterior landscaping.
- Before the graywater is treated and sent to the treated-water reservoir, a heat pump extracts the heat from the graywater and returns the heat to the water heater. This reduces the energy needed to heat water to only 25% of what an electric water heater would require.
- If not enough warm graywater is produced, the water heater will use an electric element to provide any additional heat needed. With normal water use in a two-person household, the heating element in the water heater may not be needed for weeks or months at a time.
- When you stop using the system at night, the cycle is completed, leaving the water in the water heater hot, the collector empty, and the treated water stored in a tank waiting to be used. The cycle starts over again the next time you use hot water.
The Nexus eWater’s filtering system is extremely thorough. It has five filters that process the water so that it can be stored and used for flushing toilets as well as on gardens, including vegetable gardens. Each of the filters is self-cleaning, back-charging the waste into the sewer system. The water is sent through the filters in the following order:
- The coarse-particle filter rids the water of elements such as hair and sand.
- The bubble chamber filter uses ozone in the form of bubbles. The bubbles attract substances like soap and shampoo from a shower and bring them to the top of the chamber in the form of foam, which is then discarded into the sewer system.
- The charcoal filter attracts things like VOCs and chlorine and removes them from the water.
- After going through the first three filters, the water is clear and ready for the UV filter. UV light can penetrate the water to kill bacteria.
- The fine filter comes last. This one is a redundant filter.
Once the water has been processed through the five filters, it’s transferred to the graywater storage tank. Every few hours, this water is recirculated through the UV and fine filters to keep it free of bacteria. It is completely free of chemicals and is safe for use in watering vegetables and other edible plants.
Where the Nexus system is an innovation that depends on buying hardware, structured plumbing is just good sense, and everyone should be doing it. Structured plumbing can be installed in both retrofits and new construction. If you have to pull up a lot of Sheetrock to get to the plumbing, you can ignore the current plumbing and add a new structured-plumbing system. You just have to determine where the demand points for hot water are and then figure out how to route the hot water pipe as close as possible to those points. To achieve the most efficient plumbing system possible, you would use an on-demand hot water recirculation pump with structured plumbing.
Studies show that the average home wastes more than 3,650 gallons a year waiting for the hot water to make it to the faucet. That’s an unnecessary burden on our area’s limited water supply. The structured plumbing system is meant to deliver hot water to any fixture in three to five seconds while limiting water waste to about one cup. And sure enough, if you go to any faucet and turn it on, the hot water is there in three to five seconds. Here are the basic parameters of our structured-plumbing system:
- There is a ¾-inch PEX recirculation loop that is routed as close as possible to all hot water fixtures.
- To keep hot and cooled water (already in the hot line) from mixing in the pipe due to turbulence, the main hot water line is plumbed using all sweeps, no elbows.
- Each hot water fixture is supplied by installing a tee and a short ½-inch supply line.
- All but one of the supply lines are less than 8 feet long and contain about 1 cup of water (~1 cup per 6 feet of ½-inch PEX). An extra foot was required for one
- When a demand button is pushed or a motion sensor activated, the pump sends hot water through the recirculation loop at 4–6 gpm and shuts off when the hot water arrives at the last fixture in the loop, which takes less than two minutes.
- Because the pipes are well insulated, the water in the loop will remain above 105°F for 30–40 minutes.
- A signal from another demand button will not trigger the pump until the water temperature falls below 105°F.
- There is nothing in this system that is outside the existing plumbing code.
In addition, a 3,500-gallon rainwater tank collects water drained from the roof. The rainwater is for irrigation only; this includes landscaping, watering the living wall inside the home’s main entrance, and watering the living roof on the carport.
Living Building Challenge, Zero Net Energy
From the beginning, the homeowners opted to pursue the Living Futures Institute’s Living Building Challenge (LBC) Zero Net Energy (ZNE) certification and use the LBC red list to guide materials selection. No fossil fuels are used—in fact, there is no gas line to the house.
To achieve ZNE certification we used a number of strategies to drive down overall energy demand. As a result, this house uses no combustion and only a fraction of the energy consumed in a conventionally built home. Its modest energy needs are offset with a small number of grid-tied PV panels on the roof.
Two of the key ways we reduced the home’s energy usage were by creating a continuous, airtight envelope from the foundation to the ceiling, and by super insulating the structure. To ensure a continuous supply of fresh filtered air and maintain a consistent and even temperature throughout the airtight house, we installed a Zehnder Comfoair 350 Luxe HRV and a ductless mini-split heating system.
The entire house is super insulated. Because foam is on the LBC red list, we challenged ourselves to avoid foam altogether. Instead we used blown-in cellulose (80% post-consumer waste) insulation in the walls, attic, and floor. We used Roxul rock wool board (a by-product of steel production) in place of rigid foam to insulate the exterior side of the walls. The home was on an existing slab. Rather than demolish this slab, we chose to install insulation between it and the living space. For this we used Aerogel blanket insulation (made from silica, one of the most effective insulating materials available) to achieve R-10.
The main floor of the original house was divided into a master bedroom and a small living room. We reconfigured the space into a single open great room that now includes the living and dining room as well as the kitchen. Fully capturing the spectacular ocean view required that the entire northern wall of the great room be specified as glass, logically triple pane. To complicate matters, triple-pane glass packages of that size were too heavy to carry through the home and up the stairs. Using a crane wasn’t feasible; there was not enough clearance, and it was too expensive.
We found a dual-pane solution that delivers insulation and heat gain performance comparable to some triple-pane solutions. The glass package from Cardinal is designated as Low-e 272/i89 and includes a low-emissivity coating on the inside of both the outer and inner panes. All the windows and doors are made using i89 double-pane insulated glass panels. The i89 glass uses a newer low-e coating that reflects more heat back into the living space than conventional low-e coatings, while allowing more natural light to shine through. Thanks to this coating, double-pane windows perform more like triple pane, but are lighter and less expensive and transmit light better. This also means that the windows can be big without sacrificing performance.
Of course, efficient lighting and appliances were used throughout. This house uses all LED fixtures, dramatically reducing the lighting load. All electric appliances were selected, in part for their energy-efficient performance, but also to eliminate combustion and the resulting CO inside the house.
The driveway and landscape paths are made from grid-like concrete block that has been planted with durable, drought-tolerant ground cover to maintain soil permeability. The carport has a living roof that also maintains permeability. Rainwater (and drip irrigation water) from the roof also flows into the rainwater catchment system. There is a living wall inside the house that consists of approximately 800 plants and covers about 100 square feet. It is irrigated with water from the rainwater catchment system. This wall oxygenates and beautifies the house.
The homeowners’ sense of style and passion for green living are reflected in their selection of fixtures and finishes. The living wall inside the main entrance, the finishes and built-in furnishings that emulate nature, and landscaping that is both beautiful and sustainable demonstrate the limitless possibilities for environmentally responsible living.
This project taught us that the products and processes needed to build a house that employs extreme water and energy efficiency and reduces the use of toxic materials is not a futuristic concept. They are here today.
As part of the requirements for the LBC Zero Net Energy certification, we will be thoroughly monitoring the home for a year. With the support of the homeowners, we will monitor well beyond what is required for the certification.
Find out more about the Nexus eWater system.
Learn more about the Living Building Challenge.
- Temperature and relative humidity sensors will be arrayed periodically throughout the building to measure temperature differentials when the house is occupied/unoccupied, ventilated/unventilated, and heated/air-conditioned. A CO2 meter will monitor the effectiveness of the ventilation system during high-occupancy events.
- All of the electrical circuits will be monitored by a PowerWise SiteSage energy-monitoring system.
- Because the Nexus eWater heater and graywater system is a new technology, we will be testing all parts of the system to determine how well it works compared to the specifications. Flow meters and temperature sensors are placed at key locations to monitor total city water, graywater produced, graywater reclaimed, graywater used for toilets, graywater used for irrigation, the performance of the structured-plumbing demand-activated hot-water recirculating system, and the efficiency of the graywater-to-water heat pump water heater.
Once we have collected at least 12 months of data, we’ll post an update on homeenergy.org.
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