Passive house strategies reduce or eliminate the need for supplemental heating and cooling beyond what mother nature provides.
BY SEAN O’KEEFE
ARCHITECTURE IS AN OPPORTUNITY, an almost inex¬haustible array of choices that combine to produce a sum greater than the parts. Just as the decisions we make in our lives have costs and consequences, the decisions made in residential design make a difference in the place we call home and how we live in it. For Kati Jenista, Associate Principal and Design Director at Architectural Workshop, sustainable architecture is more than a matter of bells and whistles but a holistic approach to integrating design in balance with nature.
“Living in alignment with nature means advantageously siting a home to be in harmony with the sun and prevailing winds and incorporating technologies that harness what the earth has to offer.”
Kati Jenista
Associate Principal and Design Director
Architectural Workshop
“In architecture, there is a constant stream of new products and technologies meant to make our lives easier,” says Jenista who was born, raised and educated in Germany and immigrated to the United States in the early 2000s. As the Design Director for a Denver-based design practice that straddles the line between residential and commercial design, Jenista and her team strive to infuse sustainable sensibilities into every project they touch. “Whether we’re talking about photovoltaics, Energy Star appliances, or low-flow toilets, all of those choices still require resources. Passive design strategies offer another perspective on sustainability where the focus is on lowering baseline energy consumption rather than taking advantage of the latest gadgets and gizmos.”
Defined as a design that works with the local climate to maintain a comfortable temperature in the home, passive house strategies intend to reduce or eliminate the need for supplemental heating and cooling beyond what mother nature provides. The key component of the equation is thermal comfort, the indispensable attribute of what it means to be cozy.
“Living in alignment with nature means advantageously siting a home to be in harmony with the sun and prevailing winds and incorporating technologies that harness what the earth has to offer. A geothermal heating and cooling system can be instrumental in a home’s ability to break away from supplemental energy reliance that makes a lot of sense for designers and homeowners looking beyond first costs.”
Known by several names, a geothermal heat pump or a ground source heat pump is a highly efficient renewable energy technology that significantly increases energy independence by letting mother nature absorb the long-term burden of thermal comfort. Rather than creating heat using purchased energy, the way a traditional gas-powered furnace or electric heat pump does, a geothermal heat pump does not create heat. Instead, geothermal systems simply transfer heat either from the earth to the home, or from the home to the earth depending on the thermal conditions inside the house. In terms of efficiency, for every unit of energy used to power a geothermal system, roughly four units of heat energy are supplied, a four hundred percent return on energy investment.
“Typically, a geothermal system will provide all of the heating and cooling needs, which represent 50 and 60 percent of the total energy needed to operate an American home, virtually eliminating the need for natural gas or propane,” continues Jenista. “Geothermal is not a standalone net zero approach, but it certainly goes a long way toward energy autonomy.”
The way geothermal works is a well field is dug adjacent to the home and one-inch PVC pipes encased in concrete are used to circulate a mix of water, antifreeze and refrigerant through a ground loop. The liquid mixture circulating within the pipes is completely non-toxic and environmentally safe. By digging a well field adjacent to the residence, homeowners leverage the earth’s natural ability to thermally regulate our living environments through a controlled source that will never send you a utility bill. Depending on site considerations the well field can be either vertical or horizontal.
“Primarily, the decision to do vertical wells or horizontal wells depends on the space available on the property,” shares Jenista. If the home has open acreage without any trees, a horizontal well field can be placed about 10 to 12 feet underground. More commonly boreholes are dug vertically and can go as deep as 300 to 500 feet. A horizontal field may require 10,000 square feet of open area, while the vertical bores only require 10 to 20 feet of space and go straight down to achieve the required surface area. “Geothermal technology can be easily integrated into homes virtually anywhere in the world and is suitable for both new construction and renovation projects. Architectural Workshop has done it in both.”
Beyond where the wells are placed, the surface area of the well system depends on the volume of space that needs to be thermally regulated and the homeowner’s expectations on how the system is used. At the Cornell House in Denver, Architectural Workshop retrofit a 2,025 sq. ft. mid-century modern house in an urban neighborhood with a geothermal system by placing three vertical closed loops 300 feet-deep apiece in the front yard. Likewise, the firm designed a 5,060 sq. ft. showcase home for a sustainably focused new construction community Southeast of Denver where the goal was net zero. Here a geothermal system was chosen for its ability to provide 110 percent of the home’s heating and cooling as well as domestic hot water. In that case, four vertical closed loops at 400 feet deep each proved to be the most efficient configuration. At the Murphy residence, an upper-end mountain home currently on the boards, plans call for five vertical closed loops at 250 feet deep apiece for a 4,250 sq. ft. home.
With geothermal, a well field is dug adjacent to the home and one-inch PVC pipes encased in concrete are used to circulate a mix of water, antifreeze and refrigerant through a ground loop.
“In terms of selecting and sizing the system, the design team needs to understand the homeowner’s lifestyle expectations,” says Jenista. “Geothermal heat pumps work with both ducted forced-air systems and in-floor radiant heat coils. In terms of functionality, the controls are identical to a gas-based system, so users can heat specific zones, at specific times, or the whole house simultaneously. The newest controls are programmable on a smartphone and can even draw data from local weather conditions and self-adjust the system for you.”
Though geothermal makes a lot of sense financially for homeowners, in the long run, the challenge for system selection lies in first costs. Typically, a geothermal furnace is between 25 to 40 percent more expensive than a standard gas furnace depending on the energy needed. There is also the cost of boring the required wells which could be another 20 percent. In total, a geothermal system might cost as much as 75 percent more than a gas system to install. However, once it is in place, there is no utility bill, and the payback period is estimated at just five years.
“Unlike solar, which only provides renewable energy for 10 to 12 hours at a time on sunny days, a geothermal system provides cost-free heating and cooling 24-hours a day,” shares Jenista. “Once you get past the cost of installation, the cost of operating a geothermal heat pump is roughly the same as the electrical load of a hairdryer with zero maintenance required on the wells.”
In total, a geothermal system might cost as much as 75 percent more than a gas system to install. However, once it is in place, there is no utility bill, and the payback period is estimated at just five years.
It is important to appreciate that geothermal systems are ideally paired with other passive house strategies if net zero is the goal. Orienting the home to maximize solar gains, daylighting and views are an obvious first step. Super-insulating walls, using triple-pane glazing for South and West-facing windows, and passive ventilation strategies all go a long way toward energy independence. In most cases in North America, the geothermal system will be designed around heating loads, so when paired with a ducted system, the ability to condition air for cooling is a bonus byproduct.
“Climatically, geothermal systems work almost anywhere,” says Jenista. “The choice between one system manufacturer or another is largely dependent on the system the mechanical contractor is aligned with, what is available and what is most cost-effective.”
As for any drawbacks, Jenista shares that other than first costs being total costs, geothermal’s only shortcoming is in its ability to heat a large volume of domestic hot water.
“A geothermal heat pump cannot keep up with six people taking a hot shower at the same time, so in larger homes, we would either add in a larger water storage tank or use a dual system incorporating an electric booster to preheat the water to the desired temperature.”
Though geothermal systems have been available for at least 50 years, they have not been widely used in residential design primarily because of the exorbitant first costs. In commercial architecture, where a building is planned as a 50-year asset, the first cost is easier to reconcile against the building’s operational costs. With greater use, equipment costs have become more manageable, and today, geothermal systems are now readily available for residential-scale spaces.
