The project location on a hill made a horizontal geothermal installation impossible. The solution was to install a six 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 difficult drilling conditions; each of the six loops is only 100’ long.

A six ton refrigerant to water heat pump provides hot and chilled water to fan coils and radiant floors via buffer tanks. DHW preheat is provided by the heat pump with final heat provided by an electric water heater. Tekmar controls provide three separate mix temperatures for the six radiant zones. A direct digital control system is used to coordinate the system and provide dew point control and mixing - in the future, fan coils will be used for dehumidification using water below the room dewpoint; then the fan coil return water will be mixed with the water in the radiant floors to provide sensible radiant cooling.

Net Zero Residence, Longview - project details

One of the primary goals of the mechanical system at the Longview Residence was sustainability – we had to ensure that our annual power use of the mechanical system (combined with the power use of the whole house) would be met and/or exceeded by the on-site wind turbine. While first cost was important, long term energy efficiency and reliability was paramount. The very low maintenance requirements of a geothermal system and low energy use made this installation attractive to the homeowner.

Numerous features help to make this system green and more energy efficient: Because the homeowner did not want to see outside condensing units, wanted an all electric mechanical system, and wanted high efficiency with low maintenance, we proposed the use of a geothermal heat pump. A horizontal field was impossible due the shape of the site so a vertical (actually diagonal) installation was approved. To minimize drilling costs and borehole depths we chose to use a direct exchange refrigerant based system.

After a careful cost/benefit analysis of comparing building shell improvements (to the insulation and windows) with the reduction in mechanical equipment size (and consequent lifetime energy savings), the geothermal installation was sized at six 100’ diagonal boreholes.

Drilling was exceedingly difficult due to the geology of the site – the drill crew encountered twenty feet of sand, then enormous 20’ boulders left over from the glacier that formed Martha’s Vineyard at the end of the last Ice Age, then layers of sand, then more boulders. Needless to say, the pounding the drill rig endured was tremendous. Fortunately, the direct exchange borehole diameter was only 4” instead of 6” or 8” as is common with closed loop glycol systems.

The effort was worth it as the entire manifold system for the vertical borehole field is directly under the mechanical room floor with little chance for damage from future landscaping or construction projects.

Timber frame made the installation of ductwork difficult 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. Their 2” foam insulation also greatly reduced standby loss
which allowed for more efficient loading of the geothermal heat pump.

The direct digital control (DDC) system allowed us to optimize system operation and reduce the size and cost of the mechanical plant.

The DDC evaluates outside temperature, dominant mode of operation (heating or cooling), current chilled water and hot water buffer tank temperatures, and the current setpoint (based on heating and cooling outset temperature schemes) to determine how best to use the output of the heat pump.

During the shoulder seasons where cooling and heating may be required, the DDC controls heat pump output to cool the chilled water buffer tank and heat the hot water buffer tank through a diverting valve. Domestic hot water preheat is giving priority and the heat pump, via a second diverting valve, heats up the preheat tank through a flat plate heat exchanger.

By decoupling the distribution system (fan coils and their respective circulators and the radiant floors and their circulators) from the generation/storage system (the heat pump and its buffer tanks), this system can maximize the full load (and hence most efficient) run time of the heat pump.
By using the DDC to control heating, cooling, and DHW preheat functions, one heat pump can perform the three tasks, thereby reducing equipment size and cost.

As the shoulder seasons change into winter or summer, the DDC switches priority of hot or chilled water operation based on outside temp and dominant mode of heating or cooling.

Each fan coil unit has two sets of coils, one that receives chilled water and is equipped with a condensation pan and the other that uses hot water for heating. The DDC will allow us to heat a space with radiant floor heating and dehumidify with the chilled water coil and then reheat with the hot water coil – a scenario that often occurs in the shoulder seasons due the proximity to the ocean.

During the coldest parts of winter, the DDC can also change the chilled buffer tank into a second hot buffer tank and both coils in the fan coil can be used for heating. As the heat load is dominant in our locale, this strategy allows us to shrink the size of the fan coils and their subsequent equipment and energy cost.

The DDC system is capable of providing dew point control for radiant cooling – the fan coils will be used for dehumidification using water below the room dewpoint (generated using a cooling reset temperature scheme in the chilled water buffer tank). The return water from the fan coils will be mixed as necessary with the water in the radiant floors to provide sensible radiant cooling at temperatures above the room dewpoint.