Published Sunday March 3, 2013: Updated Tuesday August 19, 2014
Why We went with A Geothermal HVAC
Options for Sustainable Home Heating
After so many efficiency improvements to my house, appliances and commuter car, and after nearly a year of not using a single net kilo-watt-hour of electricity from the utility, I began looking into ways to attack and eventually eliminate my home's natural gas usage.
One option was the brute force method. Get more solar panels and use an electric furnace. With the cost of solar panels dropping below 75 cents per watt, this is a very simple and cost effective solution. My friend Jared Campbell proved this method out in his own super efficient, net-zero, solar powered home.
Jared's amazing house.
By using an 8kW solar array (on his house and garage), he is able to power his entire home, (appliances, lighting, space and water heating).
He even has enough energy left over to drive an electric car to work and back each day.
Jared proved that it is possible to use photoelectric solar panels to build up a massive credit with the electric utility company, which in turn supplies all his home's energy needs regardless of available sunlight throughout the year. It isn't complicated, it's clean energy and over the long term, it doesn't cost anywhere near the price of paying utility bills month after month, year after year.
It's Absolutely Brilliant!!
While I loved the idea, and even considered it for our own home, I still wanted to have the option of running our entire home from a battery bank in an emergency.
Resistive electric heating requires an enormous peak energy demand. Supplying all this energy from a battery bank (and other emergency equipment) would require sizing that equipment accordingly. For our large home, it would be utterly massive and expensive to have full battery backup power.
Why settle for an 80% - 100% efficient appliance when you can have one that is 480% efficient?
Heating appliances vary in efficiency. Our original natural gas furnace was only 80% efficient. Currently there are super high-end, efficient gas furnaces that are 97% efficient.
Electric furnaces are at best, 100% efficient. But the efficiency of a heat pump, can be more than 100%. "More than 100% efficient" you say? How can that be?
Just as in Homer Simpson's house, in my home we also obey the laws of Thermodynamics.
Heat pumps can not create energy out of nothing. They simply extract heat that is already available in the environment and pump it to where it can serve our needs.
| Heating Technology||Efficiency||Initial Costs||Energy Costs|
|Natural Gas Furnace||80%||Low||High|
|High Efficiency Gas Furnace||95-97%||High||Less High|
|Electric Furnace||100%||Low||Very High|
|Geothermal Heat Pump||480% +||High||Super Low|
In a ground source heat pump, the ground temperature never drops below freezing (not even close) and the efficiency remains very high.
In theory, with a ground loop temperature of 54 °F and room temperature of 68 °F, the Carnot COP of a geothermal heat pump is 37.7, in effect giving it an efficiency of 3770%. In reality, there are physical limitations in even the most state-of-the-art heat exchanger, compressor and refrigerant technologies. These degrade performance well below what is theoretically possible (A COP of 5 is what is available in equipment today-2013).
We ended up going with a super efficient model of heat pump called the Tranquility 30. It has a 3.8 ton cooling capacity and a COP of 4.8. This means that with 1 unit of energy in, it will deliver up to 4.8 units of heat out. That is as if it was 480% efficient. It also has a SEER of 28 for super efficient summer cooling.
In the past 2 years, We have made a lot of efficiency improvements to our home. When the HVAC company Utah Geothermal ran the heat-load calculations for my home, (because of my home's efficiency), the size of the required ground loop was relatively small. Only 900 vertical feet of ground loop. I opted for vertical loops so our yard and landscaping could be preserved. I also liked the idea of a shorter vertical loop vs. a super long, coiled up horizontal loop. Less length = less energy required to circulate the fluid? Perhaps.
But Wait There's More!
A side benefit of the ground loop heat-pump is the extra heat can be used to supplement the heating of domestic hot-water. Water is drawn out of the input of the hot water heater where it is circulated through a magically named device called the “super-deheater” inside the heat-pump. This device delivers up to 18700 BTUs of heat into the water. In theory, that's enough energy to raise the temperature of 20 gallons of water by 72 °F per hour. This now hotter water is then back-fed into the domestic hot water heater via its drain line. Any time the heat-pump is operating,(whether it be in heating or cooling mode) hot water is being produced. In cooling mode the heat is a waste product, so putting it toward heating water is a great benefit and energy savings. In heating mode the available heat is shared between heating the home and heating the water.
The forced air heat pump system also has the option of circulating air throughout the house continuously. This equalizes the temperature across the entire house. For the first time, the bedroom on the north side of the house is no longer 10 degrees cooler than the rest of the house. The normally cooler basement is also significantly warmer during the winter time.
Our home has lots of south-facing windows (about 250 square feet) that collect tons of passive solar energy during the winter time.
Even on a cold, 28 °F winter day, if it is sunny outside, the rooms on the south side of the house will easily reach 74-76 °F.
At the cost of only 36 watts, the blower on the heat pump delivers a continuous, silent 500 cfm of air-circulation. This ventilation effectively distributes all that passive solar heating energy throughout the entire house and into much larger thermal masses, (like the basement), instead of just remaining in the rooms with south-facing windows.