2009
GREEN MECHANICAL
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★ NELSON MECHANICAL DESIGN, INC. ★
CLEVER APPROACH FOR NEW DIGS
Creative solutions and the use of multiple technologies
make this newly constructed home on Martha’s Vineyard an
innovative standout. ;e 5,000-sq.;. timber frame, net zero
residence is powered by a 10 k W wind turbine. Annually, the
wind turbine output is greater than the power used by the
house and the geothermal system.
;e project location on a hill made a horizontal geothermal installation impossible. The solution was to install a
6-ton vertical direct exchange geothermal system inside the
basement mechanical room using a compact drill rig. Direct
exchange geothermal was chosen due to the extremely dif-;cult drilling conditions; each of the six loops is only 100-;.
long.
Other project highlights include:
★ ;e refrigerant-to-water heat pump provides hot and
chilled water to fan coils and radiant ;oors via bu;er
tanks. Domestic hot water (DHW) preheat is provided
by the heat pump with ;nal heat provided by an electric
water heater.
★ Tekmar controls provide three separate mix temperatures for the six radiant zones.
LEFT: The geology of
the site made for a
challenging geothermal
installation.
RIGHT: Six, 100-ft.
geothermal loops
were drilled through
the basement.
★ A direct digital control (DDC) system coordinates the
equipment operation and provides dew point control
and mixing; in the future, fan coils will be used for dehumidification using water below the room dew point,
then the fan coil return water will be mixed with the
water in the radiant ;oors to provide sensible radiant
cooling.
“One of the primary goals for this mechanical system
was sustainability — we had to ensure that the wind turbine
would provide enough energy to run the home year-round,”
says Brian Nelson, president, Nelson Mechanical Design,
Inc., Vineyard Haven, Mass. “While ;rst cost was important,
long-term energy e;ciency and reliability were paramount.
The very low maintenance requirements of a geothermal
system and low energy use made this installation attractive
to the homeowner.”
A horizontal ;eld was impossible due to the shape of the
site, so a vertical — actually diagonal— installation was
approved. ;e direct exchange system was chosen to minimize drilling costs and borehole depths.
“Drilling was extremely di;cult due to the geology of the
site. The crew encountered 20 ft. of sand, then enormous
20-;. boulders le; over from the glacier that formed Martha’s Vineyard at the end of the last Ice Age, then layers of
sand, then more boulders,” Nelson says. “;e pounding the
drill rig endured was tremendous. Fortunately, the direct
exchange borehole diameter was only 4 in. instead of 6 or 8
in., as is common with closed-loop glycol systems. ;e e;ort
was worth it as the entire manifold system for the vertical
borehole ;eld is directly under the mechanical room ;oor
with little chance for damage from future landscaping or
construction projects.”
Timber frame made the installation of ductwork di;cult,
so radiant space heating (and future space cooling) and three
fan coils for heating, cooling, and dehumidification were
installed in various zones throughout the residence.
“Plastic water heaters were chosen to greatly reduce long-term maintenance as the water quality was fairly aggressive,”
Nelson continues. “The 2-in. foam insulation also greatly
reduced standby loss, which allowed for more e;cient loading of the geothermal heat pump.”
;e sophisticated DDC schemes allow for optimized system operation and enabled Nelson to reduce the size and
cost of the mechanical plant. He estimates the homeowner
will save 40% in annual energy costs as compared to a traditional fossil fuel-burning residence.
