Net-Zero Performance: Page 3 of 5

Space Cooling

Chimney Effect. The Colorado climate provides cool, dry nights through the summer, so we had no trouble meeting 100% of our cooling load through passive means, even with a week of record-high temperatures (in the mid-90s). The house includes a central open staircase that serves as a thermal chimney. At night, we open three small lower-floor windows on the north and south faces, and two windows at the highest point at the top of the stairwell. The convective chimney effect, plus north-south prevailing winds, pull cool air across the concrete floor and exhaust hot air through the upper windows. If we properly time closing the windows in the morning, the house does not get hotter than 76°F, since the thermal mass floor moderates the temperature. On the couple of occasions that we delayed closing the windows until mid-morning, temperatures reached 80°F inside. With the low humidity and the occasional use of an Energy Star-rated ceiling fan, however, even these temperatures feel very comfortable.

Energy Recovery Ventilator. The RecoupAerator ERV has an “econo-cool” mode that shuts off energy transfer between incoming and exhaust air. Combined with the earth tube inlet, the ERV supplied comfortable, cool air during days that were too hot to ventilate the home via the windows.

Overhangs. Roof overhangs were designed to block most of the high summer sun, but allow maximum heat gain in the winter. We were in the second week of August before the sun started to appear on the windowsills, and into September before the sun reached the concrete floor at midday.

Domestic Hot Water

The goal of producing 100% of our hot water demand year-round with SHW was met. The backup electric element has not been used. The 120-gallon Vaughn storage tank with three SunEarth EC-40 collectors kept the water at an average temperature of 165°F in the winter, peaking at 170°+ in January and February. The average summer temperature was between 140°F and 150°F due to collector high-temperature limits. The lowest temperature at the top of the tank was 128°F during a rare three-day period of cloud cover and fog.

The SHW system is an unpressurized, indirect drainback design with distilled water as the heat-transfer fluid. Three roof-mounted (39° pitch) SunEarth collectors form the heart of the system. A 15-gallon drainback tank sits in a conditioned attic, 10 feet below the top of the collectors. A Vaughn 120-gallon, dual-heat exchanger tank provides storage and backup electric water heating. A Caleffi iSolar Plus controls the system.

Hot water production is only one side of the efficiency equation—distribution and demand is the other. Every inch of hot water line in the house is insulated with 3/8-inch-thick foam pipe insulation, and we installed high-efficiency water appliances and plumbing fixtures (see “Water Conservation” sidebar).

Solar Electricity

The grid-tied PV system has 20 Sharp 224-watt modules, with each connected to an Enphase 190-watt microinverter. Total rated output, given the nominal limits (see below) of the microinverters, is 3.8 kW. We mounted the modules on our standing-seam metal roof using nonpenetrating SolarMount S5! clamps to secure the rails.

A key element in the design process was to estimate the system’s energy production. The graph on the following page shows the system’s month-by-month predicted production (generated by NREL’s PVWatts program) and actual production to date. I used a 0.82 total DC-to-AC derate factor instead of the default 0.77 derate, due to the better efficiency of the microinverters compared to a central inverter. With the exception of October 2011, the actual energy production exceeded the predicted values. October’s lower production values were due to a failed roof-mounted disconnect switch, which took out 10 modules’ production over 10 days.