If you’re keeping tabs on industry news, you’ve probably heard the term “zero net energy” (ZNE). ZNE homes and other buildings are designed to produce as much energy as they consume. They do this through a combination of tight, well-insulated building envelopes, highly efficient mechanical systems, and on-site renewable energy.
The energy cost savings these homes promise is cause for their growing popularity. Navigant Research projected the count of ZNE homes to increase from 750 in 2015 to 27,000 in 2025. By 2037, the market value for ZNE homes could surpass $33 billion, according to the Rocky Mountain Institute.
Building to ZNE standards can be done progressively. In this article, we’ll explain how ZNE is addressed in building code today, the option for Zero Energy Ready (ZER) homes, and how propane can help.
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 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. Compare that with the average HERS score of 62 in 2017. 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. As of August 2017, California, Oregon, New York, Vermont, Rhode Island, and Massachusetts and several municipalities had stretch energy codes. So far, California is the only state to call specifically for ZNE, requiring all new residential construction to meet ZNE requirements by 2020.
Getting ZNE Ready
California leads U.S. states in the number of ZNE projects and builders. Oregon, Massachusetts, Vermont, New York, New Mexico, Texas, Mississippi, and Florida also have high ZNE project counts. Yet ZNE projects are still in the minority among new construction.
One way builders can pursue the ZNE trend while managing first costs and performance expectations for homeowners is by building ZER homes. The Department of Energy offers support for this building type through its voluntary national Zero Energy Ready Home program.
ZER homes meet project-specific energy efficiency thresholds that consider indoor air quality, thermal comfort, moisture management, and the efficiency of systems and appliances. While ZNE homes have a HERS score of 0, ZER homes score in the mid to low-50s. The homes are designed to be solar-ready, with the addition of photovoltaics bringing their HERS scores closer to 0.
A common misconception is that 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.
How propane can help
Chris Trolle, co-founder of BPC Green Builders in Wilton, Connecticut, is no stranger to net zero construction. His company has built 11 such projects to date, about half of which use propane.
One of those projects is a ZER, 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. “An instantaneous propane heater [that is more than 90 percent efficient and modulates] and that can do space heating and domestic water is a game-changer versus an old, inefficient oil-[powered] boiler that’s 70 percent or 80 percent efficient and probably considerably oversized,” he says.
To learn more about how propane can help achieve ZNE and ZER status, check out our course, “Introduction to Net Zero Energy Homes, And Opportunities to Leverage High Efficiency Propane Systems” on PropaneTrainingAcademy.com.