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Does zero energy have to mean all-electric? Builders and designers across the country are achieving zero net energy performance with propane.

Architect Mary Ann Schicketanz strives to make all the custom homes she designs energy neutral. It’s part of her firm’s focus on sustainability — the large, luxury homes she builds in Big Sur and on the California coast are designed to be healthy for the occupant, for the surrounding ecology, and for the climate. But it’s also a necessity. Many of the homes she builds in the remote region are off the electric grid, necessitating that energy-neutral approach.

Like most designers and architects, Schicketanz, the principal architect at Studio Schicketanz in Carmel, California, relies on solar photovoltaic (PV) panels and battery storage as a source of renewable power. But despite the common perception that zero energy means all-electric, Schicketanz has learned that it isn’t practical to rely on electricity alone.

Instead, Schicketanz finds that specifying propane for energy-intensive systems such as space heating and water heating yields a particularly effective formula for zero energy homes. These propane systems offer ongoing utility bill savings but also the improved comfort of gas heating and performance of luxury amenities like gas cooking. For builders and designers seeking ambitious energy performance on their projects, propane offers a solution that can be tailored to a project’s unique needs.

What is zero net energy?

The energy-neutral approach Schicketanz takes is commonly called zero net energy,
or ZNE. In the simplest terms, a ZNE home produces as much energy as it uses, most commonly
using solar PV panels. But there are different ways to define a ZNE home, and these
homes are known by other terms as well. Here are a few ways a zero energy home might be measured.

Why source energy matters

Measuring zero energy performance by source energy captures the “upstream” efficiencies and losses of various energy sources. That might include the energy needed to extract, process, and distribute fuels like propane and natural gas, or the energy associated with using those fuels for power generation, along with the efficiency losses in the power generation process and distributing the electric power to the home.

Source energy provides a more complete picture of the resource efficiency of a home or building’s energy performance, which is one reason the EPA’s Energy Star for commercial buildings program uses source energy in scoring the energy performance of buildings. Source energy factors offer a way to compare the amount of source energy needed to deliver a unit of energy on site. For every unit of electricity used on site, 3.03 units of source energy in the form of a fuel like natural gas or coal must be extracted to generate and deliver that electricity. Propane required 1.15 units of energy at the source, accounting for losses from extracting, processing, and distribution.

Based on national averages. Source: GHG and Criteria Pollutant Emissions Analysis. Gas Technology Institute for PERC, 2017.

ZNE in the codes

Changes in building code to reward the use of home energy ratings are helping ZNE make inroads. Most states use the International Energy Conservation Code (IECC) as the model for their energy codes. The 2015 version of the IECC adds a compliance path called the Energy Rating Index (ERI), based on a project’s HERS (Home Energy Rating System) score, which is a measure of its energy performance compared with a reference model.

The lower the score, the more efficient the home. New homes typically score from the low 70s to 100, depending on the stringency of applicable energy codes, and existing homes score around 130 and up. So a home with a HERS score of 60 is 40 percent more efficient than a standard new home, while a home with a HERS score of 140 is 40 percent less efficient. ZNE homes score 0.

HERS scores in the mid- to low 50s are needed to comply with the IECC 2015’s ERI path. Only about 10 states used the 2015 version of the residential IECC as of March 2018, and some states and municipalities have implemented “stretch” or “reach” codes that raise the bar on energy efficiency beyond what the model codes call for, in some cases offering specific HERS targets. California, Oregon, New York, Vermont, Rhode Island, Massachusetts, and several municipalities have stretch energy codes. So far, California is the only state to call specifically for ZNE. Its 2020 home energy code is 50 percent tougher, moving closer to that ZNE goal.

 

What does a zero energy ready home look like?

 

Chris Trolle, co-founder of BPC Green Builders in Wilton, Connecticut, has built 11 zero energy projects to date, about half of which use propane.

One of those projects is a zero energy ready, LEED Platinum lakeside home in New Fairfield, Connecticut, completed in December 2012. The project features a highly insulated, airtight envelope featuring double-wall construction with R-33 blown cellulose insulation, an R-72 roof, and triple-pane low-e windows. An energy recovery ventilator brings in fresh air, while an air-source heat pump heats and cools the space with nearly zero duct leakage. To keep the home warm during Connecticut’s cold winters, a 90 percent AFUE propane boiler with a 40-gallon tank serves an in-floor radiant heating system on the first level. It also provides domestic hot water.

Those features help the home earn a HERS score of 39. At the time it was completed, adding a 9kW PV system would have brought it down to a HERS score of 2. “With each of the [ZNE and ZER] homes we do, we focus on a very robust thermal envelope,” Trolle says. “The fuel the client chooses to use is based on their budget and local availability.” Propane is a cleaner alternative to fuel oil and can be used nearly anywhere, including beyond the natural gas grid. Trolle says propane is generally used in these homes for domestic water heating, direct-vent fireplaces, and cooking ranges. Although ZNE construction relies heavily on the efficiency of the building envelope, the home’s performance can benefit from optimizing systems as well.

For example, instantaneous gas combination boilers are an energy-efficient option for space and water heating “with a very compact unit,” Trolle says. Because of their size, these tankless units can be located close to the point of use to reduce energy lost while transferring hot water throughout the home.

Strategies to achieve zero net energy

Mixed-Fuel Options

A common misconception is that ZNE and ZER homes are all-electric. In fact, mixed-fuel solutions are popular in this type of project because they give homeowners the opportunity to lower their monthly utility bills while still reaping the benefits of gas. Mixed-fuel systems typically offer lower first costs and reduced ongoing energy costs and emissions, and they allow for hybrid systems tailored to a project’s unique needs.

In high-performance projects, the building’s heating and cooling loads are greatly reduced through the highly insulated envelope and extensive air sealing. So overinvesting in heating system efficiency may not be optimal and could divert dollars that would have a greater impact elsewhere in the home’s design.

For instance, ground-source heat pumps are traditionally very efficient, but also have very high first costs. Based on a detailed heating systems analysis performed by the Propane Education & Research Council (PERC), a ground-source system for a typically sized home would cost almost $35,000, compared with around $11,000 for a high-efficiency propane furnace. So while the ground-source system was found to produce cost savings, the magnitude of these savings in a high-performance home with much lower loads would be smaller and extend the payback period significantly.

Another option would be a dual-fuel or hybrid heating system that combines an air-source heat pump with a high-efficiency propane furnace. The propane furnace can replace the inefficient electric resistance backup heat in cold outdoor temperatures, providing improved comfort and saving hundreds of dollars per year.

Energy savings is an important goal in all ZNE or ZER homes. In mixed-fuel homes, propane water heaters can offer significant energy cost and source energy savings. In a typical house in a mixed climate, a propane condensing tankless water heater offers the lowest annual energy cost of any system, providing savings of $120 a year over a standard electric storage tank and even offering savings compared with a heat pump water heater. Plus, condensing tankless water heaters are widely available, provide space savings due to their compact size, and offer affordable upfront costs.

The ZNE market is growing

We may be approaching a tipping point in zero energy home construction and design. More than 22,000 zero energy projects are in design, in construction, or completed, according to Team Zero. As high-performance technologies and solar PV systems become more cost-effective, energy codes require higher efficiency, and awareness increases, ZNE and ZER homes are likely to continue to increase in popularity.

A survey performed by Harris Insights & Analytics for PERC found that 83 percent of homebuyers and 89 percent of builders are likely to consider a ZNE home for their next purchase or build. And about the same number — 81 percent of homebuyers and 84 percent of builders — are very or somewhat willing to pay more for a ZNE home.

Defining what zero means for your project with elements such as site versus source energy is important, and a variety of factors including energy prices, net metering policies, solar resources, incentives, and your budget will influence the choice. But as you’ve seen, getting to zero doesn’t mean giving up desirable and high-performance gas systems. Find a strategy for getting to zero that works for your market — and thrills your clients.

Text and editing by: Jeffrey Lee

Interactive design by: Leslie Delzell and Rob Thornton

Interactive development by: Hanley Wood, Swanson Russell, and White Lion

Illustrations by: Leslie Delzell, Kelly Hume, and Swanson Russell

Project photos courtesy of Studio Schicketanz

Project managed by: Sarah Petzel