Geothermal systems can significantly reduce energy consumption in a traditionally built house, but when installed in conjunction with a tight thermal envelope, they can help a homeowner procure the coveted net zero energy label.

Keeping Cool In Texas

Helping a homeowner achieve net zero energy status was the goal of Chris Wieboldt, Tradesman Services Ltd., Elm Mott, Texas. In 2008, he was asked to install the HVAC system for a 1,877-square-foot house that the homeowner hoped would be classified as a net zero energy structure. To achieve that goal, numerous energy consumption-reducing features were utilized, including open cell spray foam insulation; a sealed attic assembly; low-E double-glazed, argon-filled windows; insulated slab perimeters; a 6-kW solar PV system on the roof to generate power for the entire home; and a high-efficiency geothermal heat pump.

The 24,000-Btu, 2-ton EarthLinked® geothermal heat pump chosen for the home included a desuperheater for water heating and an auxiliary cooling module (ACM) to further increase the system’s efficiency. The ACM is a refrigerant condenser designed to exhaust refrigerant superheat to the ambient air through an air coil; the remaining heat is then discharged by the earth loops. The proprietary refrigerant management controls enable the use of the dual condensers in a single system, without the complexity of multiple valves and electronic controllers.

Another reason why the EarthLinked heat pump was chosen was the fact that it had a small earth-coupling footprint, which was perfect for the homeowner’s urban lot. “The ability to go into a backyard with a small skid steer-mounted drilling rig and be able to drill in a 10-foot-diameter circle was attractive to the homeowner,” said Wieboldt.

The home’s net zero energy status was put to the test in the summer of 2011, which set a record for being the hottest in any state since record keeping began in 1895. For 70 days, the thermometer exceeded 100˚F, and while many were suffering from both discomfort and high energy costs, the homeowner had no electric bills. In fact, thanks to proper insulation and building practices, the highly efficient geothermal heat pump, and the PV system, the home achieved true net zero operation.

As Wieboldt noted, “Building science is a big part of what we do. So we’re HVAC contractors, but we’re picky ones. We are more like HVAC design engineers without having a P.E. on staff. We love to help the homeowner and the builder from the architect drawing phase clear through to occupancy, make the best building material, insulation type, window component, building style and HVAC system selection, possible, to create the lowest energy footprint possible.”

Net Zero, Sir!

Daryl Pater, owner, Mainstream Heating and Cooling, Clarksville, Tenn., started installing geothermal systems in 2008, due to the buzz surrounding the generous federal tax credits. Becoming a certified geothermal installer also gave him the inside track to becoming involved with a net zero energy duplex that was to be constructed at a large military housing project at Fort Campbell, Ky.

“We had already built about 600 homes at the Fort Campbell military installation when the contractor came to us and said they wanted to do the first net zero energy installation on an army post in the United States,” said Pater. “So we joined the team and started planning how the duplex should be constructed.”

This preparation stage was crucial, noted Pater, as it brought together all the trades to determine how best to implement a net zero energy strategy. “It was basically a partnership concept. We went through the plans from A to Z in order to determine where we could find net zero energy. It literally took about a year to work through all the issues, such as how do we achieve a low HERS score? What do we do about fresh air? Where should we install the ductwork? Do we integrate with solar hot water or geothermal hot water?”

However, before any other decisions could be made, the solar electric system needed to be designed, as it was not certain there was enough roof space available to handle the number of PV cells that would make the house net zero energy. In the end, each 1,900-square-foot home was outfitted with a 12 panel, 7.4 kW PV solar array on its south-facing roof, and during the day, the panels produce enough energy to power the geothermal units and appliances. A separate, three panel solar array provides domestic hot water.

A 2-ton ClimateMaster Tranquility 27 water-to-air geothermal heat pump was installed in each home, as well as an energy recovery ventilator, which left Pater with one more challenge to face. “The geothermal units were highly efficient, but then we had to consider the pumps that pump water through the loop, and they are typically energy hogs. How do you manage that? We ended up finding an adjustable-speed pump, which was the most energy efficient one that we could get our hands on.”

The installation of the geothermal heat pumps was similar to those placed in traditionally built homes, with the exception of the duct systems, which required special attention in the net zero energy duplex. “We had to make sure the duct systems were essentially leak free, so we had to do a lot more sealing than we would normally do in a home,” said Pater. “Every bit of the ductwork was sealed, we taped every joint of the metal elbows, and we caulked the boots through the floor, then sealed the grilles on top of them to make sure everything was airtight.”

While Pater has not worked on another net zero energy home since the completion of the duplex, he would certainly consider doing so in the future. “We learned quite a bit about what to do and what not to do, including the process of just how to determine usage and where the energy drivers are. Overall, it helped our discipline, as we learned a lot about the installation parameters in this type of environment, where there were very tight performance specifications.”

From Solar To Geothermal

Jeff Irish, PE, president, Hudson Solar, Rhinebeck, N.Y., started installing solar electric PV systems in 2002, then added geothermal to his offerings in 2005. When constructing Hudson Solar’s new 5,500-square-foot office building/warehouse, he wanted to prove that it was possible to construct a low cost, net zero energy building.

“We chose a high energy performance building design with traditional pole barn and steel construction. We insulated around it, underneath it, and foamed the walls and ceilings in order to get the heating and cooling loads down,” said Irish. “We acted as the prime contractor, designing the thermal envelope, doing the heat load calculations, putting in heat recovery ventilators, and then installing a PV system on the roof.”

Hudson Solar also designed its own geothermal system, which consisted of a WaterFurnace water-to-water unit that is used for the in-floor radiant heating system and a Climate Master water-to-air unit that is used to heat and cool the office spaces. Three 500-foot wells were installed, as well as 1,500 feet of vertical ground loop, and by July 2008, the year-old building achieved net zero energy status.

It cost about $150,000 to include the energy-saving features, which is approximately $105,000 more than it would have cost to utilize conventional materials, such as fiberglass insulation, an oil furnace, central air conditioning, and ductwork. The additional cost was rolled into the mortgage, which increased the monthly payment by $702, but the building’s energy costs decreased by $841 per month.

“We saved $139 a month out the gate, and we have complete immunity to energy inflation,” said Irish. “This last May, we generated 23.4 megawatt-hours and we used 20.4 megawatt-hours. Five years in, and we still have no electric bill, no oil bill, and no gas bill.”

Soon after completing his company’s new building, Irish started working with builders and architects in the area on constructing net zero energy homes. For these homes, Hudson Solar designed the thermal envelope and geothermal systems, performed the heating and cooling load calculations, then sized the PV system to cover the geothermal load, plus the other loads of the building.

Irish was involved with about 15 net zero homes, and he said taking a whole house approach from the very beginning was the key to being successful. “There is a tendency for the trades to work independently, and it is not possible to cost effectively achieve a net zero energy home that way. When we worked with builders that understood thermal envelopes and what we were doing, it worked well. But sometimes, the builders and contractors did not really understand that part, and then it would not work well.”

Unfortunately, when the housing bubble collapsed, Irish found there was virtually no demand for net zero energy homes in his area. While he got out of the geothermal business in 2010 and now focuses solely on designing and installing solar electric energy systems, he predicted that when oil prices rise, there will be more interest in net zero energy homes. “The cost of heating oil last winter was still high, at $3.80 a gallon. But it was a mild winter, so homeowners didn’t panic. When the global economy picks up again, it won’t take long for oil to hit $4.20 or $4.50 a gallon. Combine that with one lousy winter, and I think there’s going to be a lot of interest in net zero energy homes again.”

Publication date: 7/23/2012