GEOTHERMAL CONVERSION

DAVID R. OLSON, PE, LEED-AP
In the summer of 2009 my wife and I purchased a home in a small community outside Boulder, Colorado named Niwot. Niwot was named after a 19th century Arapaho Indian chief by that name. It means “left hand”. Consequently, there are a lot of local “left hand” this and “left hand” that. However, fortunately, you don’t have to be left handed to live here though. The house we bought is a modest one-story ranch with a full basement. It was built in the late 1980’s and generally was ready for some tender loving care.

We had a home inspector perform his job, and proved what we had seen during the initial walkthrough of the residence. The windows were worn out wood framed, double-clear glazed, and the baby-poop colored lap siding exterior looked like its days were definitely numbered. I noticed that the domestic water piping was an early generation of polyethylene with crimped fittings and joints. It turned out that both the windows and water piping had been the subject of a class action lawsuit, perhaps the previous owner got to take advantage of? Who knows, but we got help on the price tag due to the condition of the windows and the history of this sort of water piping developing major leaks without warning (can you say flooding discovered after returning from recent dive trip?). We replaced the water piping with Type L copper and the windows with 3-pane Low-E window units with composite frames. I am very pleased with both of those decisions. The three pane windows save energy and cut down the train noise from the tracks that are a mile and a half away.

The house was heated and cooled by a 20 year old ± 80% efficient 120 mbh natural gas furnace and a 3 ½ ton SEER-10 air cooled condensing unit installed on grade in the backyard. The inspector was careful to avoid certifying the condition of the furnace’s heat exchanger. I’m very aware that ASHRAE says in its Applications Handbook that gas fired furnaces have an 18 years. The average service life of an air cooled condensing unit is 15 years.

In both cases, I felt that we were rolling the dice to keep relying upon this aged equipment. It was tired, had worked hard, and had earned a rest. I wanted to upgrade to more efficient heating and cooling equipment. There were condensing units on the market in 2010 that had SEER ratings reaching into the high-teens. Condensing gas furnaces could get 12 to 15% more efficiency than the nameplate of the existing furnace in my house. The gas piping and electric capacity were in place, and would be easy modifications. However, I wanted to do something more.

I looked into installing a ground source heat pump system for the house. This is an all-electric system that utilizes the grounds constant temperature as a source of heat and a sink for cooling energy year around. The heat pump technology has been around for years, and I am comfortable with it. The system employs a refrigeration system. In cooling mode, it works just like a conventional direct expansion air conditioner – except without the high cost and just as importantly, without the noise in my backyard. It circulates the closed ground loop as condenser fluid, condensing refrigerant hot gas with its steady 50°F± water an internal heat exchanger. In the heating mode, the internal reversing valve allows the compressed high pressure – high temperature refrigerant discharge to bypass the internal heat exchanger and instead it is piped directly to an internal heating coil where it can heat the circulating airflow. The evaporator becomes the condenser – and the condenser becomes the evaporator.

There were several factors that appealed to me. I was worried about converting my 80% atmospheric furnace to a high efficient direct vent appliance and having the two unsightly 3” diameter flues sticking through my exterior wall. It would have been costly to remove the flue chase up to the existing roof vent and convert it to plastic vent and combustion air. Plus – I still have a natural gas fired water heater that needs a Type B vent (admittedly smaller than what was installed – after removing the furnace load off the existing combined vent). The new indoor heat pump would fit right in the same location departed by the existing gas furnace. The modifications to the supply and return ductwork would be simple, and the existing supply and return air distribution system would still work fine.

We have a relatively large front yard. The existing landscaping featured nothing but grass and two deciduous trees. We wanted to do something to reduce the grass area in the yard in an effort to save some summertime water expense. I spoke to a colleague in the geothermal Figure 3 – GROUND SOURCE HEAT PUMP IN MECH ROOM
drilling business and he had recently done a project a few blocks from my lot. I felt comfortable with the similarity in ground composition, and used the advice I got to size the loop field. Following the completion of a heat gain-heat loss calculation for my house, I felt the home would cool better with a 5-ton air conditioner. The existing 3 ½-ton condensing unit was a little small. So the loop field needed to be sized appropriately.

There are a few different ways to layout the ground exchange loop. Some people use a horizontal arrangement, with the piping residing about 6 feet below grade, in a larger area. This is usually a little less expensive of a manner of installing the loop field, due to drilling expense – but it takes a great deal of excavating. While we had a nice sized front yard, it was not large enough for this. Lacking a pond or ocean front to drop coiled piping into, we opted for a vertical drilled well field. Our field consists of four 300 foot deep wells, connected by a common header at about 6 foot below grade. The loop capacity is more than sufficient to accommodate the single 5-ton heat pump, and is sufficiently large to allow me to use heat pump technology to heat domestic hot water when our gas fired water heater eventually fails.

The ground loops are circulated into the basement mechanical room of my house. The drilling company core drilled my concrete basement wall beneath my front door, and bored the piping into the room. There is a simple dual pump package with built in pressure gauge and expansion tank mounted on the wall, with 1 ¼” diameter polyethylene piping that runs to the indoor heat pump a few feet away. All in all, it is a very simple system. A special heat pump thermostat controls the equipment. It is mounted on the wall on the main level right where the old furnace/air conditioner thermostat was positioned.

The Type B water heater flue had to be partially replaced in order to be sized properly. The former 6” inside diameter combined water heater and furnace flue was too large for just the water heater alone. The sheet metal installers I hired were able to route a new 3” diameter Type B flue up the inside of the old flue. This avoided the need to open up the flue chase through the main level of my house. I warn readers not to be tempted to keep an oversized flue. The excessive diameter will slow the velocity of the products of combustion within the flue. This will lead to enhanced cooling of the flue gas, with the resultant condensation leading to corrosion within the flue. Obviously, this is something that must be avoided – for life safety considerations.

I was very fortunate to find an outstanding drilling service – Colorado Geothermal Drilling, owned by Dan Rau, PE, LEED AP. Dan is a very honorable man, who overcame some challenges encountered while drilling my well field. Prior to starting his drilling he laid out the planned well field. He mapped out the locations of underground utilities, with a utility locating company being used to mark the locations of water, electric and natural gas lines onto the property. He was responsible for locating the sewer line. On the third day of drilling, his bore hole accidentally clipped the side of my PVC building sewer, breaking out about an 18” long section of the sewer. While flow-filling the bore hole with thermally conductive grout, he pumped this into both open ends of the sewer – effectively sealing the building sewer off to flow. The next morning, his crew was drilling the fourth bore hole when I discovered wastewater flowing out of a floor drain in the mechanical room following the flushing of a toilet. I knew something was wrong.

Following some initial resistance to the possibility that the drillers had damaged the sewer somehow, the condition was uncovered and quickly repaired to my satisfaction by a local plumbing contractor – Kerwin Plumbing & Heating. It turns out the sewer is routed a little differently than Dan had determined prior to commencing the drilling. I called the local wastewater company – Niwot Sanitation District, and within 45 minutes they had brought over a hand sketch site plan drawn by the original plumber- showing the actual routing of the sewer out of the house and offsetting across the yard before connecting to the utility main in the street. Before accepting the sewer repair as complete and satisfactory, I insisted that it be jetted out and filmed to insure that all remnants of the previous PVC sewer pipe section were ejected out of my sewer line and into the utility main in the street. I alerted the sewer company that a short section of my building sewer would be in their lateral. They appreciated my notification, but thought that this would not impact their operations. They clean the sewer mains periodically, and would remove it from the next downstream manhole at the time of their next service.

Our front yard became a construction site during the drilling exercise. The drillers created a pond out of one section, and all of the area around the four bore holes was completely excavated. This is not a real clean process – there is mud everywhere. There is a lot of water used in the process, and of course, there is 6” diameter by 300’ cores of rock and dirt that must be removed from the ground and spread about the property. When the drilling was finished, it took about one week, the driller’s re-leveled the front yard, and we were left with a dirt yard – ready for new landscaping. The formerly installed front yard irrigation system was destroyed in the process of drilling the ground loops, a reality that we had been warned about prior to the experience. We ended up reducing the amount of grass landscaping in our front yard by about 50% – opting for gravel, rocks and flower and perennial gardens. There is no evidence of the geothermal loop field beneath the landscaped front yard.

I hired some good friends in the sheet metal business, ZIA Metal Systems, owned by long-time Denver area tinner – Bobby Heber and assisted by my buddy Steve Schlosser. Steve has been in the business a long time and I consider him the most knowledgeable HVAC technician I have ever met. They evacuated the old refrigerant, removed the furnace and condensing unit, and maneuvered the new ClimateMaster heat pump into place in the mechanical room of my house. Then they quickly and efficiently modified the short section of supply and return ductwork to connect the new appliance to my existing supply and return mains. They made quick work running a new wire up to the thermostat location, and programmed the thermostat so that it was ready to run after the condenser water piping was hooked up and filled with a 20% methanol-water solution.

The electrician – Dave Martinez of EZ Electric made quick work of installing a new disconnect and wiring the heat pump and circulating pump package. This may have been the simplest and easiest part of the project.
The entire project was permitted and inspected by the Building Safety and Inspection Division of the Boulder County Land Use Department. The final cost of the installation was $19,438.64, including all drilling, equipment, permit and installation costs. I enjoyed professional discounts from Colorado Geothermal Drilling and Ace Mechanical Equipment – the supplier of the ClimateMaster 5-ton water source heat pump. We were granted a 30% tax credit from the federal government the following year, saving us almost $6,000 of the installed cost. The local utility company – Xcel Energy also advertised a rebate for the installation of geothermal systems. Unfortunately, they were not
honorable about this incentive. When we applied for the rebate following the completion of all work, we were told that because Zia Metal Systems was not an approved Xcel contractor, we were not eligible for the rebate. I am still miffed by this.

The system has been up and running for a little over 5 years now. It works great. There was no need for any sort of back-up heating or cooling equipment. The water source heat pump and ground loop provides all the heating and cooling we need. About 2 years ago, we noticed that the heat pump was running for very long periods of time, and seemed to be having trouble bringing the house up to daytime temperatures after undergoing night setback the day before. Ace Mechanical equipment enlisted their factory trained technician to diagnose the apparent problem. It turned out that there was a small leak in the refrigerant system somewhere. He recharged the refrigerant and the system worked great again. After losing the charge once again about 9 months later, Ace Mechanical and ClimateMaster very honorably replaced the heat pump with a newer model of equal capacity. They covered all installation and equipment costs as part of an unwritten extended warranty. I believe that the occurrence of a refrigerant leak in this factory tested equipment is quite unusual.

I compared the energy cost of the geothermal system to the old gas fired furnace – air conditioner during the first year of operation. After adjusting for weather conditions, I estimate that we used about 35% less energy with the heat pump system than the gas furnace. Unfortunately, in the summer of 2011, Xcel Energy started charging residential electric customers an enhanced electric rate for what they considered excessive energy use. Unfortunately, the heat pump system is an all- electric system. Therefore, we fell into this rate category and have been paying the surcharge ever since. This makes the investment a little harder to justify, but in my mind I know that we did the right thing. My house is cool in the summer, and warm in the winter. My backyard is quiet on a hot summer day – except for the lingering sound of my neighbor’s air conditioners less-efficiently spinning and churning and a lawnmower or two working nearby.