This dataset describes a a viable approach to zero net energy (or "zero energy ready") in small, multifamily buildings-either for new construction or major rehabilitation. In Greenfield, Massachusetts, Olive Street Development transformed an old elementary school building into 12 high-end rental apartments. The developer aimed to get as close to net-zero performance as practical, installing: (1) R-30 high-density spray foam against the brick walls (2) R-50 roof assemblies (3) New triple-pane windows (4) Light-emitting diodes throughout (5) Efficient appliances (6) Ductless heat pumps for heating and cooling (7) A solar thermal system providing most water heating energy (8) A 30-kW photovoltaics (PV) system over the parking area.

With all these features, the developer was able to achieve Home Energy Rating System indices of 10-20 for the apartments and 72% source energy savings (50% not including PV). This translates to an annualized energy related cost (mortgage and utilities) savings of roughly $585 per apartment over a comparable code minimum built apartment. Although the building will not likely achieve zero net energy, residents will have very low energy costs-if any. Hot water (mostly provided by solar) is included in the rent, and each lease includes a specific amount of electricity (roughly 1/12 of the expected PV generation). If tenants stay within this budget, they'll pay nothing for energy; if they exceed it, the lease has provisions for them to pay for excess electricity. The building was completed April 1, 2014, and all apartments were rented before this completion date. The Consortium for Advanced Residential Buildings (CARB) has begun testing and monitoring of the building's domestic hot water systems. Even though a condensing, natural gas boiler is used for water heating, the developer chose to install several features to further reduce gas consumption. Researchers are monitoring performance of these systems: (1) Solar thermal (2) Drain water heat recovery (3) Demand-controlled hot water recirculation.

Many researchers have found that solar water heating systems are rarely cost effective in cold climate homes, especially when efficient natural gas systems are available. In a multifamily project such as this, however, total cost for the solar thermal system was approximately $31,000 (just less than $2,600 per dwelling unit). In CARB's experience, solar water heating systems on single-family homes in the Northeast cost approximately $9,000-$10,000. There is clearly a dramatic scale effect when a single solar thermal system serves several dwellings. Because the building was completed in spring 2014, performance data are available for late May through early October only. There have been interesting preliminary findings: (1) The solar thermal system has provided 88% of water heating energy. (2) The drain water recovery system has provided a modest 5% of water heating energy. (3) The demand-controlled recirculation system was installed incorrectly, and losses from recirculation were 38% of total water heating energy. As an interim solution, the recirculation system was shut off entirely. When shut off, heat required from the boiler dropped by 73% and the solar fraction rose from 73% to 91%.

Using some lessons learned from this project, Olive Street Development is currently planning another project in nearby Montague, Massachusetts. With more area available for PV, the developer intends that this next project will truly be zero net energy. As mentioned above, CARB believes this Conway Street project demonstrates a viable approach to zero net energy (or "zero energy ready") in small, multifamily buildings-either for new construction or major rehabilitation.