Sunday, May 8, 2011

Heating and Cooling with Geothermal

So I figure, it’s about time I start contributing to Chelsea’s house blog. As everyone knows from reading one of Chelsea’s first posts, we will be using Structural Insulated Panels (SIPs) for the exterior of the house. If you haven’t read it you should definitely check out "How Our Green Home Works".

Once we decided on our exterior shell of our house we then had to decide on how we would heat and cool our house. Natural gas is not available in our area so that option was immediately out. We thought maybe propane may be a good idea due to the air tightness of our house along with an air conditioner for the summer.

On a side note: we have been living in apartments since 2006 and have always been leery of the ever increasing gas prices. Midwest propane prices are $2.15/gallon in 2011. This is up from $1/gallon back in 2001. In the summer time we would use the air conditioner just enough to keep our apartments below 85 degrees in the summer and use the furnace in the winter just enough to keep it around 62-63 degrees.
We also considered an air source heat pump which is essentially an air conditioner but can operate in reverse to provide heat to the house during the cold months. Unfortunately, if it gets too cold outside the air source pump cannot adequately heat the house and would require a supplemental heat source such as propane or electric. The consideration of the air source heat pump led us to the ground source heat pump or geothermal system.

We learned that not only could geothermal heat and cool our house for less than any other option but it would also supplement the hot water heater for us. Trying to take a green yet economical approach to our new house we figured we should probably look into this and find out the pay back, the efficiency, the longevity, and how the heck this more expensive type of system works.

Through reading various literature and talking to multiple geothermal companies we learned that the geothermal systems can provide us with approximately 70% savings over a propane furnace and air conditioner. The higher gas prices go, the shorter our payback becomes. We also learned that the geothermal furnace produces 5 units of heating and cooling into the home for everyone 1 unit of electricity used. This equates to around 500% efficiency while high efficiency gas or propane furnaces typically run around 94%.

So how does a geothermal system work? Surprisingly, it is quite simple. The Earth’s surface down to 10 feet or so maintains a temperature of 45-75 degrees depending on your latitude. In the Midwest it is closer to 50-60 degrees year round.

During the heating cycle, the heat pump uses the earth loop to extract heat from the ground. As the system pulls heat from the loop it distributes it through the duct system as warm air. In the cooling mode, the heating process is reversed - creating cool, conditioned air throughout the home. Instead of extracting heat from the ground, heat is extracted from the air in your home and either moved back into the earth loop. Both cycles provide heating for the water tank. See the picture below.

There are multiple variations of geothermal systems to use. We wanted one that utilized a forced air system rather than a radiant heat system. The next choice was the type of system. There are two main types, the open or the closed loop. The closed loops can be horizontal, vertical, or a pond/lake system. See the pictures below.

Originally, we had considered the open loop system where water is used and pulled from a well and then disbursed into another well or water source. Due to the high iron content of the water in our area, it would cause our system to be less efficient as the contents in the water would build up in the system. This is similar to clogging of arteries in the human body. So although this system had the lowest upfront cost, it would lead to a long term higher cost of maintenance and operations.

The system we will be having installed is the horizontal closed loop. This can be used when there is adequate land available. Instead of using water, polyethylene pipes are filled with a water antifreeze mix and laid in trenches. The trenches typically it require 1/4 – 3/4 of an acre. These systems generally have 50 year warranties on the loops.

The vertical loop is designed for people who live on small lots of land where space is limited. This is where the loops are bored down 200-300 feet. The vertical loops contain the same mixture as the horizontal loops.

The last loop is a pond/lake loop. The loops are sunk at the bottom of a lake or pond. This needs about a half acre and water depth of at least 8 feet.

So we are not experts on geothermal but we are learning. If you already have your land for building and are considering geothermal, it is pretty easy to have a heating and cooling company that does geo, come and view your property to see what kind will best suit your land, and your needs. It will be an exciting day when the system finally goes in!

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