October 15, 2021 - January 15, 2022
One barrier to broader adoption of ZNE practice is the perception of ZNE as expensive and exotic. In this project we addressed this issue directly, demonstrating that ZNE renovation can be straightforward and practicable, providing qualitative improvements, significant savings in energy and carbon emissions, and excellent return on investment. We started with our small, simple, relatively efficient, sun-tempered home, our half of a cohousing duplex built in 1994. What would it take to make it ZNE? Answer: Much less than I had anticipated – and nothing fancy.
Our home was built “Energy Crafted”, with 7” dense-packed cellulose walls and tight construction. Our windows were reasonably good for the time – double-pane, argon-filled, low-e casements. A sealed-combustion propane-fired boiler provided heat and hot water. An exhaust-only ventilation system kept the air fresh. Most of the lighting was CFL. The design was compact and sunny, providing a comfortable living space with three bedrooms in about 1,000 square feet of finished area (1,596 SF conditioned space.) A solar hot water system, installed on our roof in 1999, was shared with our duplex-mates. Before our ZNE renovation, our electricity use averaged 300 kwh/month; yearly propane usage was about 300 gallons. A good starting point, but nothing spectacular.
The first step on our ZNE journey followed one of my favorite sustainable design maxims, “Don’t overlook the mundane”: we bought a new refrigerator, saving an estimated 40 kwh per month. Our path to a ZNE home included:
· Careful modeling to determine strategies
· Targeted air sealing (start with a sound envelope!)
· Reducing plug loads
· New air-source heat pump to replace propane boiler
· New HRV to replace exhaust-only ventilation
· Super-insulated electric water tank as new backup for solar hot water
· 5.9 kw PV array to cover all energy uses
With PVs now powering all systems – and in spite of a record-breaking winter – our home was net energy producing the first year: 430 kwh net annual production (1.2 kwh/day, 6.5% above load). With all of this in place, our household energy use declined by 23% (using kwh equivalents for the replaced propane).