early estimates indicate that when
implemented at scale in the test
region of east tennessee, the
[foundation heat exchanger]
may be feasible at $1,000 per ton
— a fraction of the cost of the
outdoor portion of traditional
gshP systems.
The loops of the FHX are placed in the foundation
excavation before being buried by backfill. Preliminary data shows that this method of installing
heat exchangers in the construction excavations
rather than in separate boreholes or trenches
could significantly reduce the cost premiums
associated with ground-source heat pumps, without sacrificing their efficiency.
trenches received six-pipe loops (three circuits
of one-inch-diameter high-density polyethylene
pipe, out and back) with a minimum 1 foot of
spacing between pipes.
To obtain data for validating the overcut
loop-sizing method and performance-simulation
model, it is important that the overall loop be
sufficiently sized so that loop operating temperatures are in the design range. To accomplish this,
for the purposes of this experiment, our team
used horizontal loops installed in all the utility trenches plus some additional trenching to
provide adequate capacity, rather than installing
loops below the basement floor. However, calculations indicate that if the loop were installed
below the basement floor, no additional trenching would have been necessary.
Left, in the foundation heat exchanger (FHX) concept, heat-exchange loops are installed only in
the construction excavations, without extra drilling or digging. But unlike conventional horizontal
ground heat exchangers, with FHX the mass of
soil around the house’s foundation is subject to
heat transfer from the foundation itself (qw) and
from the heat-exchanger pipes by way of conduction within the soil (qp). Moisture flow takes
place within the soil as well. At the surface, heat
is transferred through convection, radiation and
evapotranspiration (qs). Researchers are developing tools to engineer loops in the overcut.
comparing standard,
integrated heat Pumps
Construction of houses 1 and 2 was com-
pleted in November 2009, and data collection
began in December. Baseline data was collect-
ed during the first year, when each home used
one water-to-air heat pump for space condi-
tioning (the two-stage ClimateMaster model
TTV026) and a separate water-to-water heat
pump for water heating. The three-circuit (six-
pipe) ground loop was “headered” into a single
supply and return in the basement, allowing
the two heat pumps to operate in parallel, con-
nected to the common supply and return. The
baseline data documents the performance of
the FHX−GSHP system using the best water-
source heat-pump equipment commercially
available, from industry partner ClimateMas-
ter; the equipment fully satisfies the space-con-
