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Bangor Daily News Op Ed 2/9/06
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Summary and Conclusions

After researching Geothermal Heat Pumps I determined a 2.5 ton model could meet my heating needs assuming I kept my oil furnace intact for heating on colder days below 35 degrees. The actual heat output is about 23500 BTUs, equivalent to about 4 or 5 portable electric heaters. The heat pump can output 32100 BTUs in cooling mode. After selecting a vertical standing heat pump, I needed to find a convenient and functional place in my house for the heat pump. Since noise is a factor, I opted for a remote but centralized basement location where I could access the kitchen, living room and bathrooms with ductwork. From this location I could also add on a branch to the upstairs bedrooms later. The cold air return was located halfway down the stairs to the basement and a vent was installed in the basement door. This low spot ensured that the coldest air from the house would go into the heat pump for heating.

heatpump.jpg

I then had to decide whether to use a trench with buried pipe or to use a drilled well. A trench system costs less to install but a drilled well is more efficient since it goes deeper into the ground where the ground is warmer in the winter months. I bored a 4 foot hole and discovered I had very dense wet clay below 3 feet. If I had found a light dry soil I would have had to use a much longer trench or a drilled well since heat from the ground would not conduct very well to the pipes.  Trench length is a function of heat pump size, soil type, number of pipes in trench, ground temperature and soil moisture. There was no way of being certain of the soil type or soil moisture  at 5 to 7 feet until I had actually dug the trench. Ultimately I decided to put 6 parallel pipes in the longest trench, 300 feet, I could and then adjust the heat pump duty cycle, how long it can be on per hour, empirically.  I had this flexibility since this is a hybrid system where my oil furnace could provide heat when the heat pump was off. I think designing a trench to heat an entire building with just a heat pump drastically increases the cost of a project because designers must overdesign trenches and equipment to ensure the system works on the coldest days. A useful paper for design information is GSHP Design Recommendations developed by the Tennessee Valley Authority in conjunction with a TVA Ground Source Heat Pumps promotion:

 

             http://www.geokiss.com/res-design/GSHPDesignRec2.pdf

 

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