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West Tisbury Net Zero Residence - description
This mixed use residence has a large yoga studio on the first floor
and living quarters above. Built around the frame of a reclaimed
barn (originally built in 1852), this structure features an extremely
tight and energy efficient building shell.
As one of Martha’s Vineyard’s two net-zero buildings
powered by wind turbines, a 5 kW wind turbine provides enough
power annually to meet the entire energy load.
Walls (R50), ceilings, and floors (R60) were insulated to reduce
the size of the mechanical system. It was carefully designed to
meet the projected load of 20 people practicing yoga.
A horizontal 6 ton direct exchange geothermal field was selected
to serving radiant space heating loads through a buffer tank.
The heat pump uses R407 as an improvement over R22’s ozone
depletion potential.
An ERV is connected to a duct system designed for a future dedicated
outdoor air
system.
DHW preheat is accomplished using a flat plate heat exchanger
powered by the heat pump with final hot water heating from an
electric water heater.
A separate outside reset on the buffer tank optimizes heat pump
operation; the other two areas each have separate reset temperatures
as the different rooms have differing loads.
West Tisbury Net Zero Residence - project details
This project demonstrates the value of careful design and building
shell optimization. By focusing on long term energy use (as driven
by the selection of a 5 kW wind turbine and its projected output)
and the goal of annual net zero operation, the building shell
was improved and the mechanical system reduced until energy goals
were met.
Walls, ceilings, and floors were much thicker and better insulated
than the norm – this worked well to reduce the sensible
loads but increased the need for mechanical ventilation. We installed
an ERV large enough for the nominal load of 5 people exercising
– we understood that the occasional larger classes would
present an intermittent load that could be met over a longer period
of time by a smaller ERV.
The small site footprint precluded the use of a closed loop horizontal
glycol geothermal system so we opted for the direct exchange approach.
This reduced our field size by half which reduced excavation costs
as well. Use of a buffer tank with the six ton heat pump ensured
that more of the operating time of the heat pump would be closer
to full load, especially when a smaller radiant zone was the only
active zone.
We set up an outside reset scheme for the buffer tank to make
sure heat pump energy use was held to a minimum – the buffer
tank setpoint typically rides about 5F hotter than the outside
reset setpoint of the other two radiant zones.
Domestic hot water is preheated using the geothermal heat pump
through a flat plate heat exchanger with final heat provided by
a plastic electric water heater.
Schematic:
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