Physics Guy

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

Right now, I'd recommend a hybrid heat pump system coupled with gas heat to new home builders. My heat pump is rated at 30000 BTU and I use an oil furnace on colder days. I ordered the heat pump from a company in Florida that I found on line. 


Make sure you get a heat pump rated for geothermal, that can utilize heat from water down to 32oF. The lowest cost way to go is hire an excavator to dig your trenches but a geothermal well is more efficient since water temperatures are higher as you go deeper in the ground in winter. The trenches should be dug as deep as possible, hopefully down to and below the water line. Your excavator contractor will/should have a lot of useful information you can use to determine how deep you can go and how quickly you'll need to put in the pipe before the trench collapses. The wider the excavator bucket used, the better. The excavator can dig around obstacles more easily with a wider bucket. Also pipes can be spaced wider apart in a larger trench. Note temporary placement of the extracted earth may be a problem that limits how wide your trench can be and how deep. My trenches are 5-7 feet deep. Wet soil conducts heat to the pipes better. Note your soil type is important. A light dry soil is bad whereas a very dense wet soil is the best. You may want to dig a test hole before doing anything else.


I used 6 rolls of 300 foot 3/4" black poly pipe. The trench was a little under 300 feet and all joints are inside my basement. Make sure the pipes are tightly sealed where they enter the basement to avoid ground water leaking into the basement. I protected the pipes from scraping from thermal expansion and contraction by inserting them in 1 inch plastic pipe where they went into the basement wall. I used 1.25" white plastic pipe as headers for loops. White plastic is a better insulator than metal and costs less.


I used a circulator pump to circulate water through the loops and heat pump. One of my mistakes was using too small a pump.  I was in a hurry and even though a larger pump was specified in the literature, I could only get a 1/25 HP pump locally. I recently ordered a 1/8 HP pump on line. I've got pressure gauges and a temperature probe on the input and output water pipes of the heat pump. If I run the heat pump for too long with the small pump, the pressures increase and the output temperature goes down to 34F.  If I run it longer the water freezes in the heat exchanger and the system shuts down even though I have about 4 gallons of antifreeze in the water. The type of antifreeze I use turns slushy at low temperatures so I am considering using another type of antifreeze. February is the worse month to run a heat pump in Maine because the ground temperature is the lowest.


Even in February the heat pump works fine as long as I keep track of the water temperatures coming out of the heat pump. If the temperature goes below 35F, I'll shut off the heat pump and leave the circulator pump running. The water temperature usually recovers within an hour or so. At least for now, I recommend a hybrid system for Maine rather than using a heat pump as the sole source of heat. There could be times on the coldest days when the heat pump shuts down with extended use.