Heat Pumps Used in Super-Efficient Home
Home Built to be as Energy- and Water-Efficient as Possible
When a homeowner approached Rob Smith, president, e2 Homes Inc., Maitland, Fla., about building a net-zero home, Smith jumped at the prospect. As a residential contractor who primarily builds custom homes, Smith specializes in environmentally responsible, cost-effective building solutions, so he was excited to tackle this new challenge.
The result is a newly built 4,300-square-foot home, located in Winter Park, Fla., that features a whole host of energy-efficient features, including a 13.4-kW solar array system, autoclaved aerated concrete (AAC) block construction, a supply only ventilation system, high-efficiency air-source heat pumps, ducted and ductless mini-split systems, tankless, propane-fired water heaters, and a 7,000-gallon cistern that collects and supplies rain water to all toilets and urinals.
With projected total annual energy gain (with solar) of $123, the home falls safely into the zero net-energy usage category.
The homeowner involved in this project, Rick Wilson, was definitely hands-on and had a large wish list for his new home. Smith’s job was to help Wilson prioritize that list, so that the home could be as energy- and water-efficient as possible, while still coming in on budget. At the top of the list was Wilson’s desire to have no drywall in his home; instead, he wanted AAC block construction.
As described by the U.S. Department of Energy (DOE), AAC is made of most of the ingredients found in concrete but put together in a process something like bread making: sand is ground like flour and mixed with water, cement, and a little bit of aluminum powder. The mixture is poured into railroad car-sized molds and allowed to sit for four hours while the aluminum reacts with the cement forming millions of tiny bubbles in the concrete, causing it to rise and more than double in size. Rebar is set into the foam-like concrete, which is cut into blocks or slabs that are baked in ovens. The hardened, aerated concrete is then ready to be used to build walls and foundations.
Like concrete block, AAC is mold-resistant, fire-resistant, and not penetrable by termites or pests. Primarily used in commercial construction, AAC is lightweight (one-fifth the weight of concrete), sound resistant, and features a high insulation value (R-8 for an 8-inch block). While it has all these wonderful features, it was not a material that Smith had used before, which posed the first challenge on this project.
“Because it is not usually used residentially, my masons had to learn how to use it,” said Smith. “We had to bring in a consultant to teach us how to build with this product, which was an interesting process. We were all very excited about the product and hope to use it again soon.”
Next on the list was deciding which windows to use. After analyzing window placement and overhangs, Smith decided to use Energy Star-certified low-E windows that block 95 percent of ultraviolent and infrared light with a solar heat gain coefficient of 0.24.
Downsizing the HVAC
Thanks to the AAC block construction, combined with energy-efficient windows, Smith was able to downsize the heating and cooling equipment by approximately 30 percent. “Sizing the equipment properly was very important because I wanted to make sure we weren’t oversized. I had my mechanical contractor and the energy rater both do load calculations to make sure everyone was comfortable with the sizing. We ended up with about 6 tons of cooling for the whole house.”
The HVAC equipment includes two Carrier air-source heat pumps (18 SEER and 16.5 SEER) that condition most of the house, a Mitsubishi 16 SEER ducted mini-split heat pump, which conditions the master bedroom, bathroom, and closet, and a ductless mini-split that Wilson brought from his prior residence to condition his fish room, which houses multiple aquariums and corresponding equipment.
This variety of HVAC equipment may seem somewhat unconventional, but it was necessary due to the layout of the house, said Smith.
“A large great room separates the master bedroom from the rest of the house, so we couldn’t bring ductwork over to the master bedroom. In addition, the master is on the southwest corner of the house, which is usually the warmest spot, whereas the rest of the house is shielded by the second floor. This area also only required 1 ton of cooling — smaller than a traditional heat pump — which is why it made sense to have a dedicated ducted mini-split for the master bedroom.”
The design of the home also made it more challenging to install the ductwork, so with the exception of one section of exposed metal spiral duct in the great room, the rest of the ductwork is housed in a conditioned attic space. During construction, the ducts were sealed with duct board and the boots were taped shut to prevent contamination. A smoke test was performed after installation, to make sure there was no leakage.
Because the house was built to be airtight, fresh air was needed to ensure proper air quality. To that end, Smith specified a supply only ventilation system, which creates a slight positive pressure in the house to help control infiltration of outdoor humidity. This low-cost ventilation system includes a fresh-air duct to the outside of the home with an electric damper regulated by the thermostat to meet ASHRAE ventilation standards. If needed, the system can be upgraded to an energy recovery ventilator (ERV) or augmented with a supplemental dehumidification system.
One of the keys to just about any zero net-energy home is the prodigious use of solar panels, and in that respect, the home’s 13.4-kW solar array is no different. The home’s 69 solar panels don’t sit on top of the roof; they are the roof over the rear lanai. The completely watertight structure allows about 15 percent of natural light to filter through the panels, lighting the space below, while the 195-W solar panels are bifacial, meaning they can generate some electricity from reflected light that hits the bottom side of the panels. All wiring is hidden within the canopy’s aluminum support beams.
To conserve water, a 7,000-gallon rainwater cistern was installed, allowing reclaimed water to be used for all toilets, as well as the drought-tolerant landscaping. Unfortunately, city officials were not familiar with this technology, which led to some problems during the permitting process.
“We were the first ones to construct a rainwater cistern, and while we put it on the permit package, the building and utility departments didn’t have a system in place to deal with it,” said Smith. “With the cistern, I double-piped the house, so the rainwater could supply the toilets, hose bibs, fish tanks, and irrigation. The use of non-potable water created a potential issue, however, as unmetered water could be introduced into the sewer system, which is not allowed. As a result, metering of the gray water is required, so the city can measure how much water is used and bill for the sewer appropriately.”
After construction, the home’s energy efficiency was tested, and without solar power, it scored a HERS 57, which is well below the HERS 100 for a standard home built to code. With its photovoltaic system, the home produces better than zero net-energy performance, with a score of HERS -7. Thanks to its energy-efficient features, the home became the first DOE Challenge Home certified in the U.S. and was also certified as LEED-Platinum, NAHB Green — Emerald, Florida Green Building Coalition Platinum, and Florida Water Star Gold.
Wilson has been in his home for almost a year and is very happy with the outcome. In fact, he recently posted on Facebook that his electric bill was 25 cents last month.
Smith is happy, too, noting that he achieved his goal of building an energy-efficient home that used the homeowner’s money wisely. “We were able to design a home that produces more energy than it uses and creatively utilized different construction techniques to meet the client’s goals, while minimizing construction costs. I’d say that’s a win-win.”
Publication date: 5/27/2013