Installing a Geothermal Heat Pump in Cary, NC

In the fall of 2009 all three units comprising the heating and air conditioning system in our home went out.   Faced with replacing two gas/electric central units and an electric window unit, we looked into options that could reduce our energy costs.  We learned of geothermal heat pump systems and liked what we saw.   The 30% federal tax credit and 35% credit offered by North Carolina as well as rebates offered by Progress Energy enticed us to take the plunge and give geothermal a try.

If you have a basement, you've probably noticed that even in summer it remains cooler than the rest of your house. Thats partly because heat rises, but also partly due to the fact that six feet below the surface of the earth the temperature remains about 55 to 59 degrees Fahrenheit throughout the year.  A geothermal heat pump is a central heating and cooling system that leverages the constant temperature just below the surface of the earth to efficiently heat and cool a building.    

A traditional heat pump heating and air conditioning system uses outside air as its heat source in the winter and heat sink in the summer.  A geothermal heat pump, also called a ground source heat pump, instead uses water circulated through a ground loop as its heat source and sink.  On a cold winter day the system extracts heat from 57 degree water instead of 32 degree air and therefore it can extract that heat using less electricity.   We focus here on our experience.  For more information on the underlying technology here are some nice sources:  Department of Energy, Wikipedia, HGTV Video.

We selected AllenKelly & Co. as our primary contractor and after several meetings signed the contract for the system in late December, 2009.  The initial time frame given for the install was end of January.  The heat on our second floor was working and we used portable units on the first floor.  These were adequate but made for some chilly breakfasts during the cold winter months.  AllenKelly hired a subcontractor to drill the wells for the ground loop.   Due to scheduling difficulty with the driller, work did not start until March 9, 2010 and was completed on March, 25.  

Our new system consists of two units, two pumps, and a hot water heater in the crawlspace and one unit in the attic.  Existing duct work was used everywhere except a third floor office which required ducts, vents, and a return installed.  Two outside units and the window unit from the old system were removed. The ground loop exits the crawlspace underground, proceeds into our front yard and then into three 300-foot deep wells.  This type of ground loop is called closed-loop since the same water just continuously circulates through the tubing; after installation no water leaves or enters the process.  The loop is also termed a vertical installation since it goes down into the ground (in the three wells) instead of just remaining parallel to the surface over a larger area.  

The first and most intrusive step of the installation is the drilling of the three "wells" down which the piping was inserted.  This was done by driving the huge drill shown below into our front yard.  The tall tower folds up and down somewhat like the ladder on a hook-and-ladder fire truck.  Each well has a diameter of about six inches and a depth of 300 feet.  The drill works by repeatedly attaching a twenty foot segment of drill shaft, drilling that down into the ground and then attaching another twenty foot segment.  

As you can see, the truck is so heavy it pretty much destroys the portion of the yard it is in.  The drilling is also extremely loud.  I was watching TV in a room on the back of our house (the opposite side from the wells) one morning when they fired up the drill.  It was so loud that I could not hear the TV.  Needless to say, the drilling in an established neighborhood is quite a spectacle, much more so than we anticipated.  The truck was in our front yard working on the wells for a little over a week.  

More pics of the drill in action

Right. The most drama of the project occurred when this silt fence broke. Some of the mud got into a storm drain. The Town of Cary showed up and threatened to fine us $5,000 per day. So I got to scoop mud out of the creek bed until 10pm one evening with the driller employees. In the end no fines were assessed.
Right.  The trenches that connect the wells and bring the loop into the crawlspace.   More trench pics.
Right. The trenches are filled in.
We underestimated the cost and effort that would be required to repair the yard. We had envisioned just reseeding where the trenches were dug. It turned out that the lawn in a 2000 square foot portion of our yard was destroyed and clay unsuitable for growing grass was left on the surface. We had to bring in 14 yards of topsoil/compost mix and lay new sod. Even though I did all the work myself this still ended up costing us about $1,500 (not covered by tax credits).

The new system works very well, easily keeping the temperature at whatever level we set.  The hot water heater also performs perfectly.  When you are on the first floor you can barely hear a slight hum of the pumps when the system is running, but I would argue that this is less intrusive than the sound of a normal exterior unit.  AllenKelly took care that vibrations from the pumps did not come into the house and we do not notice any.    

Of course the most important question to ask about the project is how much energy does the system save? That will in turn tell us how much money we will save and how much good karma we build up by going green and reducing our carbon footprint. We have lived in the house for a little over two years. It was built in 1995 and includes a little over 3000 square feet of living space. To estimate the savings I plot below total energy costs (gas and electricity) for the two years prior to installing the new system and I will post the new data as it comes in. April was the first month in which the systems was in operation. Neither the heat nor air conditioning was used much during the month, so the savings over last year comes in at an underwhelming five dollars. Hopefully the savings will pick up in the coming summer months or it will take me twenty five years just to pay for the lawn repairs.  

Update: I have updated the numbers through July 2010.  Some good savings are starting to show up.  I also show below average monthly temperatures for the period in question.