Planning Geothermal - Unit Selection and Loop Design

WaterFurnace has software for dealers to help select the needed geothermal equipment and design the loop.  The software is called GeoLink Design Studio.  This software’s input is very simple, the heat loss/gain figures from whatever load calculation method the dealer used.  I used ACCA’s Manual J and entered the heating load of 36,948 Btuh and the cooling load of 21,232 Btuh.  From this data I decided to pick the WaterFurnace Envision dual capacity 3-ton unit with hot water assist.

My existing Trane unit is a 2.5-ton unit, and the duct work I have will fit a 3-ton unit.  I need an extra outlet in my finished family room, so the extra size will help.  The Envision dual capacity compressor will run in low speed which is 69% of the capacity of the 3-ton total. To put this in a different way my Envision’s low speed is roughly equivalent to 2 tons. The advantage here is that the unit will produce cooling and heating Btuhs that are closer to what my home needs on typical summer or winter days, and will not use unnecessary energy as a 3-ton unit would use during the same period.

The software produces a BIN Chart which show how many Btuh my home needs at different temperature conditions.  The chart show how many hours in Richmond is typical (based on 20-yr weather data), and how much capacity the 3-ton has at each temperature BIN has. The chart is interesting in that it predicts when the house Btuh needs exceeds the unit’s capacity and electric strip heat is turned on.  On the chart this is at 17 degrees.  A typical conventional heat pump will start to need electric back-up at 32-37 degrees.


BIN Chart -

The typical loop design has ¾” PE pipe circuits, and 1 ¼” PE header pipe to carry the water from the circuits to the circulating pumps inside.  (Although there are many other design approaches depending upon field conditions and other factors). The pumps are often set next to the geothermal unit where pipes are then connected from the pumps to the unit’s heat exchanger. The recommended number of circuits is 1 per ton; odd tons either add or subtract one circuit.  My system will have 4 circuits and two wells.  The GeoLink software calculated 375 total feet of bore for my unit given my Manual J load and soil conditions (average rock for wells).  So I rounded up to 200 feet for each bore.

I had my geothermal loop installed on December 9 & 10, 2010, see my photos on this Blog. My area in Charlottesville, VA has a strong aquifer. My well driller's experience in this city helped him plan a water containment strategy to minimize the risk of flooding my neighbor's yard.  He took the extra precaution to build a containment pit and trench.  In most vertical geothermal projects, this is not needed.  However due to close quarters with my neighbors, this was well worth the extra effort. 

For more information about geothermal loop design see the FAQ section of the International Ground Source Heat Pump Association's (IGSHPA) website.  For additional information about geothermal heat pump technologies start at their home page.

For additional information about aquifers visit the USGS website.