ASHRAE Journal - June 2021 - 49

TECHNICAL FEATURE
Windows are dual pane. Original
appliances included a standard
effi ciency liquefi ed propane
(LP) gas furnace, LP gas hot
water heater, LP gas dryer, an
electric range/oven and no AC.
The home also includes a fi replace
with maximum capacity
of 35 kBtu/h (10 kW) and rated
at 63% effi ciency with outdoor
intake air. However, the fi replace
is mostly decorative. The thermostat
setting for heat is 62°F (17°C)
while unoccupied.
This second home is occupied
approximately 25%
of the time, mostly during
spring/summer/fall, but also
some during winter, with the
number of occupants ranging
from six to 25. In 2020 , the occupancy
increased to nearly 45%
of the time due to the COVID-19
pandemic.
Five-Year Net Positive Energy Timeline
An ideal net zero energy (NZE)
(i.e., the home generates as much as
it consumes) or net positive energy
(NPE) new construction includes
a structured four-step sequential
process of planning, load reduction,
high-effi ciency equipment and
renewables.5 Retrofi t projects may
include all these elements, but not
necessarily in a structured way.
In this case study, the retrofi t
features were not planned ahead
of time, but are organized in a fi veyear
timeline (Figure 2) that can be
loosely split into three main phases:
1) load reduction; 2) primary energy
source shift; and 3) phased down use
of fossil fuels.
The baseline began in 2006 by
installing CFL to replace incandescent
light bulbs. The fi rst phase
FIGURE 1 Home before (left) and after (right) renovations.
FIGURE 2 NPE retrofit timeline.
LED Replacing CFL
Bulbs Early 2014
First Set of PV Panels
October 2015
1 in. R-5 Insulation
June 2017
Phase 1
GSHP Installation
June 2014
Second Set of
PV Panels May 2018
started in 2014, with LED replacing
the CFL light bulbs and the installation
of a 3 ton (11 kW), R-410A
ground source heat pump (GSHP)
that uses four, 140 ft (43 m) wells.
The GSHP is mainly used for heating;
cooling mode use is negligible
due to comfortable breezes off the
lake during the warmest part of the
summer.
Heat pumps have a coeffi cient of
performance (COP) of at least 3,
which means they are at least three
times more effi cient than gas or
electric heating, since the latter two
have a conversion effi ciency less
than 1. That results in substantial
load reduction, as is discussed in the
next section.
During the second phase, between
2015 and 2018, 6.7 kW of solar
photovoltaic (PV) panels were
installed in October 2015, with
Condenser
Evaporator
Phase 2
HP Dryer
July 2019
Phase 3
2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Condenser
Evaporator
HP Water Heater
January 2019
an additional 4.9 kW of solar PV
installed in May 2018. In June 2017
1 in. (25 mm) R-5 insulation was
applied to the aboveground portion
of the concrete foundation when
siding was replaced. The new insulation
covered approximately 40% of
the exposed foundation.
The last phase began in January
2019 when a 50 gallon (189 L) heat
pump water heater (HPHW) was
installed in the basement. Finally, in
July 2019, a heat pump dryer (HPD)
was installed, eliminating the need
for LP gas.
In 2019, the home achieved NPE
and became fossil fuel free after
all heating systems migrated to
heat pumps. The total investment,
including deductions from state and
federal incentives, was approximately
$41,000 as summarized in
Table 1.
J U N E 2 0 2 1 ashrae .o rg ASHRAE JOURNAL
49
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