Chuck Reiss, a builder in northwest Vermont, had a bold plan in 2007: he wanted to build a cluster of six superinsulated homes on a 24-arce site in Hinesburg. Reiss planned to install a roof-mounted PV array on each house, with the goal of making the homes net-zero energy, or close to it.
The homes would occupy about 10 acres of the site; the remaining 14 acres would remain agricultural. For anyone interested in passive solar design, the acreage was extremely attractive; architect Rolf Kielman (TruexCullins Architects) describes the sloping site as “a south-facing bowl.” The site is within easy walking distance (via a pedestrian path) of the shops in Hinesburg village.
The project, known as South Farm, now has five homes; the sixth will be built soon. It’s been five years since the first homeowners, David and Carrie Fenn, moved in, so it’s a good time to find out how well the homes have been performing.
Cellulose insulation and roof-mounted PV
Each of the homes that Reiss built is a custom home; however, the homes share many similar characteristics and specifications:
- Area: Between 1,600 and 2,200 square feet
- Foundation: Poured concrete basements (backfilled on the north side only, with windows on the south side)
- Foundation wall insulation: Interior closed-cell spray foam installed after 2×4 walls were framed (with a gap between the 2x4s and the concrete)
- Wall framing: Double 2×4 walls on 9.25-inch-wide (2×10) plates
- Wall insulation: Dense-packed cellulose
- Windows: Double-glazed low-e windows on the south, triple-glazed low-e elsewhere
- Ceiling insulation: R-45 cellulose
- Siding: Fiber-cement
- Roof slope: 12 in 12
- Roofing: Standing-seam painted steel roofing
- Space heating: Econar ground-source heat pump using an open ground loop (a drilled well); one house has a pellet boiler
- Heat distribution: Hydronic in-floor tubing
- Domestic hot water: Preheating using GSHP desuperheater, with electric resistance backup
- Mechanical ventilation: A heat-recovery ventilator (HRV)
- Renewable energy: Roof-mounted 3.9-kW photovoltaic array (peel-and-stick Uni-Solar amorphous PV modules).
According to Reiss, the size of the PV arrays was a compromise. “I think people are willing to put money into PV, up to a point,” Reiss told me. “But we didn’t size the systems to be at net zero.” According to energy modeling projections, the owners were likely to use about 5,500 kWh of electricity per year — more than the 4,000 kWh per year that the 3.9-kW PV arrays were expected to produce.
All of the homes sold for more than $425,000.
Favorable feed-in tariffs
Fortunately for the owners of homes at South Farm, the local utility, Green Mountain Power, has a very favorable rate schedule for homeowners with grid-tied PV systems:
Here’s an example of how this works: Let’s say you use 500 kWh during the month of April. Your PV array produced only 410 kWh that month — much less than you used. Here’s what the bill looks like: you are billed $70 for the electricity you used, plus a service charge of $11.25. That’s a total of $81.25. However, you get a credit of $82 for your energy production, so you end up ahead — even though you used more electricity than you produced.
Most of their electricity is produced on site
The first homeowners to move to South Farm were David and Carrie Fenn. David Fenn kindly shared his electric bills with me:
- In 2008, they used 5,164 kWh, while the PV system generated 4,796 kWh (93% of their usage).
- In 2009, they used 4,945 kWh, while the PV system generated 4,669 kWh (94% of their usage).
- In 2010, they used 5,308 kWh, while the PV system generated 4,516 kWh (85% of their usage).
- In 2011, they used 5,404 kWh, while the PV system generated 4,544 kWh (84% of their usage).
David Fenn provided a few details about his heating system. “We have a ground-source heat pump,” he told me. “There have been some troubles with the heat pumps in some of the houses, but we’ve had fewer problems. If lightning strikes nearby, it passes through the ground, and it can affect things. A couple of houses have had to replace equipment.”
The Fenns try to conserve. “We have propane for the stove in the kitchen and for the dryer,” David said. “The domestic hot water is heated by the heat pump. We set the water temperature lower than some of our neighbors. Little things like that add up over time, so our house has uses less energy.”
She received 12 monthly bills, but only had to pay one of them
The second completed house at South Farm is owned by Johanna White, a retired woman who lives alone. She moved into her new 1,625-square-foot home in June 2007. “I cook with gas and have a gas-fired dryer, but I dry my laundry outside in good weather,” she told me. Over the last 63 months (a little over 5 years), she has spent $594 on propane ($113 per year). Her per-unit cost for propane is $4.99 per gallon, so she uses about 23 gallons of propane per year.
Johanna told me that when it comes to energy use, “I’m not frugal, but I’m not cavalier. I have a bathtub that is massage tub. It’s a luxury thing. I don’t use it a lot, but I do use it. I have a backup wood stove, a Hearthstone, that I use only half a dozen times a year — only for ambiance on very gloomy days. I don’t watch a lot of TV because I only get one channel here. I like to have lights on in the house at night, because I like good light. But if I don’t need it, I turn it off. I don’t have a lot of hard-wired fluorescent lighting, but I do use the CFLs.”
Johanna sent me a year’s worth of electric bills — from May 2011 to April 2012 — and they make for fun reading. Her May 2011 bill accounts for the electricity she used and produced during the previous month, when she generated $22.62 of electricity but used only $12.64 worth (including the monthly service charge). So she ended up with a credit of $9.98. That meant that she didn’t have to pay anything that month.
For the next six months, her surplus kept growing. Here’s a tally of the credits she received from the utility: June 2011, $42.79; July 2011, $102.32; August 2011, $162.40; September 2011, $208.14; October 2011, $224.28; and November 2011, $230.20. (Each bill tallies the usage and PV production from the month before the bill was issued.)
From then on, Johanna started drawing down her accumulated credits. On her December 2011 bill, her credit dropped to $193.12; on her January 2012 bill, her credit was $106.87; and on her February 2012 bill, her credit was down to only $4.21.
Finally, in March 2012, she received a bill that she actually had to pay. The amount she owed that month was $40.12. The next month, however, she accumulated a surplus of $29.62, so her total electricity bill for the 12 months from May 2011 to April 2012 was only $10.50. Remember, that covered almost all of her space heating and domestic hot water as well as appliances, lighting, and plug loads.
Problems with the ground-source heat pumps
Four of the five homes at South Farm have ground-source heat pumps with open loops. Each house has a single drilled well; the same well that supplies the ground-source heat pump also supplies drinking water.
“The Econar systems cost us about $25,000 per house, not including the well,” Reiss told me. “They are 7-inch diameter wells, drilled to 350 feet.” The heating systems were designed by energy consultant Andy Shapiro. “We went with the standing column well systems because they already had to drill a well anyway,” Shapiro told me. “We just needed some additional depth to the well for the additional surface area.”
The heat pumps have had a few problems. “There have been some bugs with the Econar heat pumps — some manufacturing issues,” said Reiss. “There have been problems with a plastic component called the Bleimo valve. I had to replace one heat pump. Econar eventually paid for the equipment but not for the labor to pull it out and install the new one.”
Going forward, Reiss will be changing his specifications for heating systems. “We won’t be using ground-source heat pumps in the future,” he said. “We’re going with air-to-air heat pumps — Mitsubishi ductless minisplits.”
Last week’s blog: “A New Passivhaus Standard for North America.”