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Open Loop's Hidden Agenda
While at first blush the open loop geothermal solution seems to be more efficient, cheaper to operate and DIY friendly, the pump aspect of this statement could not be further from the truth.Being an educator and a professional, I can tell you the sad fact that the quickest way to clear a room full of pump and well contractors is by saying three things:
1. Pump curve
2. Feet of head
3. Shut off pressure
When we look at pumping water through a heat pump, we find out very quickly that heat pumps require, for the sake of discussion:
1. 1.5 to 3 gallons per minute per ton of load;
2. An operating pressure of as low as 10 to 15 psi on the inlet side of the exchanger.
If these two factors were the only parameters we have to meet with water supply it would require a pump with very low HP and, depending on the tonnage load, a very small gpm flow rate. Unfortunately, problems start to arise when we have multiple water usages at varying pressure demands.The best way to address the needs of a heat pump would be through a dedicated water system to support the system; however that takes away from the inexpensive aspect of the design by adding more wells, pumps and systems.When we first look at upsizing pumping equipment to meet the demand of the heat pump, it is to simply add the needed gallons to the existing pumps capacity and we have a winner. Unfortunately that leads to the most common complaints involved with owners of these systems, which are:
1. Pump cycling
2. Excessive electric consumption
Pump cycling is a function of any operating system. The down side, though, is that every time you turn an electric motor on and off, you are reducing its life expectancy. Water pump manufacturers have recommended run times for their products to maximize the life of the pump, however they all die a little bit each day like running up mileage on your vehicle. Excessive electric consumption occurs because the pump is now sized to meet the maximum demand placed on it by the distribution system. If you are only using a portion of your pumps capacity, and it is cycling on and off, you are paying for electric consumption to meet a demand that does not exist.
A common complaint is that a homeowner’s water pump runs nonstop. This should not be a complaint but a blessing as to the life of your pumping equipment. Today’s submersible pumps were designed for continuous duty= nonstop operation. If your pump can run nonstop to support your geo and you suffer no deficiencies in regard to the rest of the system when the geo is running, then you have harpooned the white whale.The correct sizing of a systems size is paramount to its longevity and cost effective operation. If you have any of the aforementioned maladies, be advised that there is no magic solution, nor is a solution inexpensive or quick to achieve. Unfortunately, it is our demands that make this such an interesting puzzle.We demand low cost, multiple usages at differing pressures, and instant gratification with the unwillingness to put up with life style choices. The addition of more tanks, valves, pressure reducing devices, or variable speed pumps will never meet all of our goals.In summary, while we can take steps to address one or more of the maladies it all comes at a cost. There is no solution that meets all our expectations, just a combination of the ones we are willing to pay for and accept in terms of shortfalls in other areas. I hope this sheds some light on a reoccurring hot button of issues surrounding the pumping requirements of an open loop system.
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