Published Thursday February 20, 2014: Updated June 9, 2014
It’s been an entire year since we had the Ground Loop Heat Pump installed.
While the ground loops themselves are only 1” in diameter, the earth surrounding them (and affected by them) is much larger. Three earth columns 300 feet deep and 15’ in diameter yield a volume of 3 columns x 300 feet deep x (7.5 foot radius)² x 3.14 = 158962 cubic feet of soil weighing over 11 million pounds.
Raising the temperature of 11 million pounds of moist, lake bed soil by only 10 °F would take 8.34 BTU/lb x 11M /3412 BTU/kWh = 26877 kWh
That is an enormous thermal mass
capable of storing a tremendous amount of heat energy. It's down there not doing anything. Why not make some good use of it?
In the short time span of a single year, the actual amount of Earth that is directly influenced by the ground loop is probably much smaller than that. But that influence (though heavily attenuated) will extend out indefinitely given enough time.
Amazingly, only 1800 kWh of electrical energy was required to heat our 5000 sqft house and supplement much of our hot water needs this past winter. An additional estimated 9000 kWh was provided for free from the ambient heat in the earth and 3600 kWh from passive solar heating, (average of 2.5 full sun hours per day and average of 12 kW of solar radiation from all the south-facing windows).
In February 2013, the temperature of the ground loop (and surrounding soil) was initially 56 °F. After finishing out the rest of a really cold winter, ground loop temperatures dropped a little but. I don’t know by how much because no active temperature monitoring was taking place at that time.
In the spring, we switched the heat pump over to cooling mode a little early just to try it out. Summer was a scorcher and by the end of summer, after dumping waste heat, day and night into the ground all season long, the temperature of the ground loop (and surrounding soil columns) had risen to almost 67 °F.
Only 2400 kWh of electrical energy was required to cool our 5000 sqft house and supplement most of our hot water needs last summer.
On September 18, 2013, the heat pump was switched from cooling mode over to heating mode. While I imagine some of the summer heat reserves were dissipated into the surrounding soils, much of it was recovered and used to heat our house. By October 29, 2013, the heat pump was running pretty regularly in heating mode each night. Late November was very mild but winter came in with a vengeance in December with unusually cold, single digit and below zero °F temperatures.
By December 2, 2013, the ground loop temperature had dropped to 57 °F.
On or around December 12, 2013, the summer ground heat reserves were finally depleted. At the end of the day, ambient ground loop temperature was at 55.8 °F.
By February 8, 2014, ground loop temperature was 52.7 °F.
By February 19th, after a couple weeks of mild winter temperatures, (causing the heat pump to not draw heat as vigorously), the ground loop temperatures drifted back up to 55.0 °F.
I am amazed that by using the Earth as a heat storage medium, great energy efficiency and energy security can be had by anyone.
Why should we perpetually pollute the air by burning fossil fuels when you can access free energy under your own backyard for free, forever?
Besides the terrific energy savings, there are other benefits of tying a ground-loop heat-pump into your home’s air handler.
Health Benefits of a ground-loop heat-pump:
Reduced dryness and cracking of the skin. Thanks to the humidifier option, we no longer suffer from dry-cracking hands/skin during dry air winter months. Our daughter’s perpetual eczema is totally gone.
Reduced sickness caused by bad air quality. Thanks to a MERV 13 air filter in the air-handler, the air in our house is far cleaner than the dirty, polluted, inversion air outside. MERV 13 air filters yield hospital grade air cleanliness. Our family did not get sick this year like we usually do each year. I attribute this to better air quality in the home, though we have been eating healthier this year too. Time will tell if this single data point can reproduce into conclusive evidence.
I estimate the indirect medical savings are greater than $140/month ($1680/year) for our family of 7.
Other non-energy Benefits of a ground-loop heat-pump:
Odors dissipate more quickly. Thanks to the constant ventilation (currently set to 195 cubic feet per minute), all the air in the house is filtered and re-circulated several times each day.
More constant temperatures throughout the house.
Quiet operation. With continuously variable fan speeds, you can hardly hear the air flowing through the vents.
Worry free operation. Just change out the air filter every 90 days. BTW, the large MERV 11 filters are $16 each in a 2-pack on Amazon. MERV 13 filters are $18 each in a 2-pack.
No risk of CO poisoning or gas explosions. Every year we hear on the news sad stories about families dying from carbon-monoxide poisoning and at least one home blowing up because of a gas leak/explosion. This just isn’t possible with an electric heat pump and electric water-heater.
Emergency preparedness. In the event of a power outage, since the ground-loop heat pump uses so little energy, a 2800 watt inverter and battery bank could provide all the backup power needed to keep the house warm during an emergency. I’m not there yet, but this is something I would like to peruse in the future.
I estimate the indirect value added to our lifestyle at greater than $85/month ($1020/year).