Page 68 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 8
STRATTON-LEE DIY HOUSE
5146
Zero Net Energy Case Study Homes: Volume 2
energy used by the gas water heater, which ended in August, 2019, when it was replaced by the heat pump water heater.
As the owners report, this energy use will be even lower in 2020 since the attic above the Phase 2 bedroom area was just recently insulated and sealed in 10/2019. The data in both charts re- flects the condition of an uninsulated attic space and the space conditioning loads are higher than what is expected in 2020.
Energy Production versus Energy Use: Zero Net Energy Performance
The charts on the following pages show the solar PV system performance over the course of the year starting in November, 2018. The charts of solar energy production versus energy use include the monthly energy use data for the house alone, making the adjustment to the recorded data by removing the estimated energy use for charging the EV. Two charts are shown: one shows the actual energy use with the gas water heater in use until August, 2019, when it was replaced by the more efficient heat pump water heater, The second one shows the hypothetical situation where that heat pump water heater is assumed installed throughout the period of data recording.
The second chart illustrates the effect of the water heater replacement on the overall energy per- formance of the house. The heat pump water heater is six times more efficient in producing hot water than the gas water heater (and produces no carbon on the site). Taking this efficiency ratio and applying it to the energy use data recorded for the gas water heater from 11/2018 through 7/2019, yields the expected energy use for the heat pump water heater were it in service during that period.
Basically, the DIY project falls short of ZNE performance in 10/2019 with the gas water heater but achieves ZNE if the owners had replaced this with the heat pump water heater in 11/2018, the start of the one-year of data recording. This is illustrated by the respective charts for the cumula- tive net energy production on page 56.
The cumulative net energy production is a chart that essentially shows the progression of the energy performance toward ZNE by adding each month’s net energy performance to the previ- ous month’s total—if, at the end of the 12-month period, the curve remains on the positive side of the zero axis, then the building is indeed performing better than ZNE, i.e., Net Positive. For this case study, there are two charts—one representing the actual recorded data including gas water heater and the second representing the hypothetical performance of the house at construction completion, derived from the recorded data and substituting the heat pump water heater for the gas water heater for the entire period.
The second chart shows that the house is expected to be ZNE with the heat pump water heater instead of the gas water heater. As the first chart shows, the energy performance falls short of ZNE with the gas water heater in operation.