I’ve always been a strong proponent of the combined use of solar PV and geothermal heat pumps to achieve residential zero net energy status. As I began to draft this article for Green Energy Times. It struck me that what better way to sing its praises than to provide a real-world example from a climate zone that experiences both hot, dry summers and snowy winters? I reached out to my good friend, Bill Martin in Quincy, California. I hope you enjoy his story.
—Joe Parsons, Senior Product Manager for ClimateMaster, Inc.
Getting to Carbonless Zero Net Energy in the Sierra
I’ve now lived for 51 years at 3,500 feet in the northern Sierra Nevada. The challenges of heating (and more recently cooling) have been easiest since completing our last home in 2014 that I call the Quincy Zero Net Energy House. It has achieved “cost zero” for six out of its seven years of operation and “net zero” for three—meaning it consumed less power than it pushed back to the grid. Electricity from the sky and heat from under my feet has helped achieve my dream of thermal comfort on-the-cheap.
Our worst year came with 180% of normal rain and snowfall, costing us $6.34 per month. We are all-electric, with a net energy metered intertie to the utility. Our 7.4 KW A/C solar array of 31 panels rests on our roof (at a 22.5° tilt from a 6/12 pitch). The utility is our battery, and we try to consume power off-peak when possible, because the rate is incentivized for that.
Higher expense is part of the rural mountain experience. I’ve heated with propane, kerosene, firewood, and for seven years with an air-source heat pump. Much of that was an increasingly painful challenge (especially fighting for firewood and getting-keeping it dry). We built a code-qualified home in 1998 that heated water and space with kerosene, but the initial 90¢ price per gallon zoomed to $4.78 by 2010. I had been a believer in heat pump technology since the mid-70s, so it was easy to understand the advantages of heat exchange from more stable temperatures underground. I also had enough construction experience (including solar since the 80s) to know I could design a very robust thermal envelope.
Our current home may be a somewhat rare performer but rest assured, it’s fully conventional without exotic equipment. The basic fundamentals still mater the most, and I kept those in mind because a great thermal envelope surrounding conditioned space means less heating and cooling needed. That means smaller equipment, costing less. And with geo heat pumps, it translates to less underground heat exchange piping, again costing less. Put another way, not building with excessive windows and maximizing R-value cuts HVAC installation and operating costs. And if you’re trying to include solar PV to reach for a certain level of performance—that means fewer panels. Strong envelopes provide multiple benefits!
Our walls are a double offset stud system on 16-inch centers on a two-by-eight plate. Thermal transfer inbound or outbound dies in the insulation and even where framing meets doors and windows, aligned studs still have a thermal break between their edges. Our ceiling, walls, and floor are R-49, 30, and 30. All windows equate to R-3.5 and our doors to R-6.5.
I like building with an attic space that’s well-ventilated with low soffit intakes and continuous ridge venting. A spacious attic space like this buys time as does insulation—they both get you past the high highs and the low lows until the outside air improves. After hot days when my attic thermistor reads 130°F, it drops to be equal to outside ambient by sunrise. I benefit from a naturally dry environment that helps, particularly on clear nights.
One always sizes HVAC equipment by the dominant load. Mine is heating, and although our ASHRAE winter design temperature is now only rarely achieved (+10°F) our house should lose no more than 18,000 Btu per hour at that temperature. I chose a three-ton, two-speed geo heat pump because at low output and incoming loop water at close to 40°F, I would receive 22,000 Btu per hour, some of which is diverted to hot water pre-heat. Even with two circulating ground loop pumps, my total electric consumption on low speed is 1,750 watts (and as I like to say—less than Susie’s hairdryer).
My attached garage contains our HVAC equipment in an 1,800 square foot space with good insulation and insulated doors. It is a good buffer against outside winter temperatures and has never dropped below 49°F. I chose a water-to-air heat pump for filtering the air (electronically) dehumidify it in summer (condensate removal) and humidify it in winter (with electric-steam) for a relative humidity of 43%. Our thermostats are set to 69° and we’re still comfortable.
The mechanical area also has a heat recovery ventilator into our duct system and a 50-gallon tank for hot water pre-heat, ahead of a 65-gallon air source hp water heater driving a circulating loop. Our heat pump delivers only 850 cfm at low speed, so three zoned thermostats serve inside space to keep air velocity up.
A geo heat pump doesn’t struggle when outside air is hottest or coldest because it has a steady thermal battery underground. In 104°F summer heat, my heat exchanger rejects heat to dirt that is 41.5°F cooler. Winter nights at 22°F concentrate heat from 50°F dirt (28°F warmer).
A more detailed white paper on this project is available at https://bit.ly/californiageo-QuincyZNEPerformance.
Bill Martin is an IGSHPA-certified ground source heat pump installer and currently serves as president of CaliforniaGeo.
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