SOCM Energy Efficiency Stories: Franz’s Story

By Franz Raetzer

You probably have a heat pump in your home, which keeps you warm in the winter and cool in the summer. Usually a heat pump uses the outside air to warm or cool the air inside your home going through the ducts into each room. But if you have a water well, you can have a heat pump installed that uses the well water to extract heat from it in the winter and dump heat into it in the summer. This is a geo-thermal system.

The well water temperature is usually 55 – 65°F year-round in Tennessee, which is fairly well in between the desired duct air temperatures of around 40°F for cooling and 90 – 100°F for heating. In comparison, the average temperature in Tennessee for February is about 35°F and 88°F in July, a 50° swing. A single stage heat pump can make the duct air about 40°F warmer or about 30°F cooler than the outside medium (water or air). So, the well water system is much more efficient in providing sufficiently warm (or cool) duct air to keep your home comfortable. A conventional heat pump has to use the AUX heat in very cold weather to boost the duct air so the house stays comfortable. AUX heat is usually pure electric heat and thus very expensive.

I considered these circumstances for about 10 years before I decided to have my 30-year-old central heating/cooling system replaced in 2005. The old one consisted of a compressor for the cooling and 20 kW pure electric heating coils for heat. Since I did not have a water well, I decided to use a closed water pipe system laid in trenches 6 – 7 feet deep in my backyard. But I could not find a local heating/cooling systems dealer willing to install the water pump and field lines. I finally found one who was willing to install the heating and air conditioning unit, but I was responsible for all the “water works” and hook it up to the unit.

I started to look for more data concerning the water circuit. I found that I should use about an area of half the square footage (900 ft2) of my single floor house (1,800 ft2heated). But it did not tell me how much footage I would get with a 10 ft straight  ¾ in. copper pipe laid at the bottom of the 6 ft deep trench. I decided to calculate with 1 ft2/linear foot of pipe (6 inches on each side).  The a/c unit required a water flow of 5 – 9 gallons/minute, which a ½ hp shallow well pump provides nicely. I told a backhoe operator to make four 3ft wide trenches, for a total of 200 ft. I bent the soft  ¾ in. cooper pipes into a serpentine pattern 3ft wide and laid them on the floor of the trenches. This way, I ended up covering  800 ft2, and connecting the four lines in parallel gave me a flow of  9 gal/minute with 30 psi at the pump.

This system saves me about $500/year on electricity for heating and cooling, even though it does not operate optimally. The water temperature gets close to 32 °F in January and February (although not this year!) and 95 °F in July and August. What I did not know when I planned the system was how much energy I’m wasting because of the very low energy efficiency of my house. It still has only 6” of insulation in the attic, aluminum frame windows and leaky doors. The “half your floor area” rule is probably right for a well insulated and sealed home, with energy efficient windows. So my next steps this summer are: putting an additional 12 inches of insulation in the attic, replacing my windows, and sealing the leaky spots. I estimate that the remodeling will save me another $500/year.

 

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