“Typically boreholes require 20-foot spacing, and with the Btu requirements for the HVAC system we’d designed, the backyard space was technically not quite big enough. However, using the RAUGEO™ system from REHAU, including the double U-bends that allow for collection of up to 30 percent more energy per borehole, we only needed to drill five wells, which the backyard could accommodate."
-- Rich Abernathy, Air Cool & Heating Systems
Old Town Alexandria, Virginia Home
New technologies make historical residence retrofit possible
When Harry and Maria Hopper purchased their 1830 Federal-style Old Town Alexandria home in 1999, they were excited to take ownership of a piece of architectural history just miles from the nation’s capital. Over the years, the Hoppers would learn that owning a home on the historic register presented some notable challenges in upgrading for energy efficiency while remaining aligned with preservation society guidelines.
“After 150 plus years, even the most well-crafted windows are just not going to be top-performers anymore,” said Maria Hopper. “Nonetheless, we had to keep the window and door openings looking as they did when the house was originally built. We realized in the initial years of living in our Old Town home that this was going to be a challenge for us when it came to maintaining indoor comfort, and that ultimately we would need to compensate for lost heating and cooling with a highly efficient HVAC system upgrade.”
Because the Hoppers have a geothermal heat pump system in their farmhouse in rural Virginia, they were already familiar with the benefits and had hoped to retrofit a geothermal-driven HVAC system into their Old Town home as well.
"When we decided to dig out a basement last year, it seemed to make sense to also take the plunge in upgrading to geothermal,” Hopper said. “We knew from our previous experience that this type of system could significantly reduce our energy bills, as well as provide a much more comfortable and consistent internal temperature, particularly in times of severe D.C. summer heat and humidity.
According to Hopper, the couple was initially unsure as to whether the size of their backyard might impede drilling the necessary amount of boreholes for the loop field portion of the project. Also, the limited 17 by 30-foot space was framed by a 6.5-foot brick wall, included a gazebo, and could only be accessed via a narrow 15-foot wide alley behind it.
“We weren’t even sure if this was going to be possible,” she said, “but we were hoping that with the right people and equipment involved it would be.”
With help from regional architect Adams and Associates, the Hoppers identified Alexandria-based Air Cool & Heating Systems, Inc. as the mechanical contractor for the job.
“Our specialty is in HVAC retrofit projects, which are in high demand with the type of existing building stock here in Alexandria,” said Rich Abernathy, president of Air Cool & Heating Systems. “We’ve also been in the geothermal business for about 15 years, which has given us a considerable leg up as this type of system has come into greater demand thanks to tax incentives and overall increased awareness. Air Cool oversaw the first residential geothermal retrofit in Old Town, and since that time we’ve had several more projects come on board in that area, including at the Hoppers’.”
According to Abernathy, a combination of experience, creativity and the latest technology made it possible to move forward with a geothermal retrofit at the Hoppers’ home.
“Typically boreholes require 20-foot spacing, and with the Btu requirements for the HVAC system we’d designed, the backyard space was technically not quite big enough,” Abernathy said. “However, using the RAUGEO™ PEX-based ground source heat exchange system from REHAU, including the double U-bends that allow for collection of up to 30 percent more energy per borehole, we only needed to drill five wells, which the backyard could accommodate. After that it became about getting the drilling equipment into the backyard, which we achieved by lifting it 50 feet into the air with a crane.”
The system was designed using a home-run loop field configuration, with five 260-foot loops of RAUGEO PEXa pipe run from each of the five 230-foot deep boreholes. In addition, two 5-port REHAU PRO-BALANCE® XL balancing manifolds were used to accommodate 10 supply and 10 return runs from the borehole field.
“One of the main advantages of a PEXa-based system like RAUGEO is that each U-bend in the borehole can be controlled and isolated individually,” said Mike Maher, sales manager, renewable energies at REHAU. “Unlike the typical HDPE reverse-return header system, the REHAU system with a balancing manifold allows for the flushing and purging of the system to be done on a circuit-by-circuit basis. Also, an individual and adjustable flow rate can be set for each borehole, which provides notable flexibility to the overall system. It’s additionally possible to isolate an individual borehole for maintenance or troubleshooting without shutting down the entire system, which is incredibly advantageous for both the mechanical contractor and the homeowner.”
According to Maher, a PEXa-based system like RAUGEO provides an additional level of security as there are no joints or fusions inside the borehole itself. “In HDPE applications, numerous joints are made during installation, including those within the U-bend at the bottom of the borehole,” he said. “This can present potential risk if one or more of the joints were to fail, particularly in a reverse-return header configuration when the entire system is linked together. With a double U-bend PEXa-based system, there are no joints to fail inside the borehole. Knowing this can bring a lot of relief, particularly to the homeowners themselves.”
REHAU provided Air Cool & Heating Systems with the loop field design to deliver nine tons of heating and cooling to a hybrid water-to-water heat pump and water-to-air system. “That was a major plus for a company like mine, which doesn’t have in-house engineers,” said Abernathy. “You just can’t put a price tag on being able to sleep at night knowing your loop field calculations are reliable, as I was confident was the case with REHAU’s design.”
Abernathy and Air Cool & Heating Systems removed two existing rooftop units, and retrofitted three Unico high-velocity units inside the home with new air handlers and environmentally friendly 410A refrigerant. One additional air handler was installed to create a four-zone system throughout the home. The company was also able to use an existing boiler as backup heat for the geothermal system, as well as repurpose all existing HVAC ductwork in the house.
"We really enjoy the period architecture of our home, and had hoped that upgrading our HVAC system wouldn’t mean having to add ‘bulk’ to ceilings and other areas of the house. We wanted as clean a look as possible, and as close to what the original interior looked like as could be achieved."
"It was exciting when we were told that the existing ductwork could be repurposed thanks to the latest in air handler integration technology," Hopper said. "It was also such a relief not to have the old units on the roof blocking light and the general view from the upstairs windows.”
Rich Abernathy shares that the design of the PEXa-based geothermal loop field allowed for integration of other systems in the home beyond just heating and cooling.
“This has been a great example of how cutting-edge technology and intelligent system design can maximize a geothermal system’s output,” Abernathy said. “Using a multi-circuit system like this notably elevates its energy efficiency ratio, or EER, and makes it possible to, for example, heat domestic water and run the air conditioner at the same time without stressing the overall load. We’re expecting about a 35 to 40-percent overall energy savings when compared with the old system.”
According to Gregg Drunagel, the project’s geothermal contractor and chief operations manager at Green Hill Mechanical in Warrenton, Va., the RAUGEO PEXa-based ground loop geothermal system provides a new level of opportunity for retrofit applications.
“Getting significantly more Btus out of a borehole like this opens up so many project doors,” Drunagel said. “Once you understand this type of a system, I wouldn’t see why you’d want to use anything else. We are experiencing more interest in retrofit versus new construction projects these days, and while homeowners are becoming more educated on why geothermal is a superior option, it helps to be able to say with assurance that we can deliver the required amount of energy no matter what kind of tight space we’re faced with.”
"We’re looking forward to a summer of that consistent, dry, cool temperature that only geothermal can provide. With our experience in our other home, we’re also confident that the utility bills are going to be minimal compared to what we’ve seen here over the past 12 years. The system is also so pleasantly quiet, friends who visit always seem to wonder if it’s even on!"
Maria Hopper additionally shares that the project has created “viral” interest in geothermal among her friends and neighbors.
“Several of the neighbors commented that we should have gone in on a multi-house system installation, which would have made the project even more cost-effective from the drilling perspective,” said Hopper. “I’ve already given Rich’s contact information to a few people in the neighborhood and some friends in the Alexandria area.”
As for Rich Abernathy, having deemed his first job with the REHAU RAUGEO system a success, he is looking forward to more in the near future. “I wouldn’t have been able to do this job without the RAUGEO system from REHAU,” he said, “and I’m more confident than ever that Air Cool & Heating Systems will remain a leader in HVAC retrofits by incorporating it into future projects.”
|Project:||Hopper Residence, Old Town Alexandria, VA|
|Type of construction:||Residential, retrofit|
|Scope of project:||1,300 ft (396 m) of pipe; 5 geothermal boreholes|
|Mechanical contractor:||Air Cool & Heating Systems, Inc.|
|Geothermal contractor:||Green Hill Mechanical, LLC|
|Architect:||Adams and Associates|
|REHAU systems used:||RAUGEOTM ground loop heat exchange|