Heliospiti at 5 Years: Lessons Learned

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Jim and his boys today. After five years in their passive solar, solar thermal, and PV-powered home, the Riggins family is assessing their home’s systems and their solar lifestyle.
Jim and his boys 5 years ago.
Twelve-inch-thick walls slightly block solar gain at the south-facing windows.
Pulling back a throw rug reveals the degree of fading on the cork flooring due to ultraviolet light exposure.
At winter sun angles, most of the thermal mass floor is exposed to sunlight.
Three SunEarth 4-by-10-foot solar thermal collectors provide a substantial portion of domestic hot water needs, even during cold and cloudy periods.
The substantial temperature difference between the 40°F incoming well water and 120 gallons of 170°F stored solar-heated water required replacing the original 4.4-gallon pressure tank with a 10.3-gallon unit.
The solar thermal differential controller’s high-temperature setpoint was lowered from 205°F to 190°F to prevent vaporization at the 7,000-foot elevation.
Before the addition of a second electric car (purchased in August 2016), the 20-module PV array provided an average surplus of 4.8 kWh per day.
The Enphase Envoy monitoring screenshot shows the lower performance of three modules (dark blue) and complete failure of three microinverters (gray). Possible solutions include a piecemeal or full-array upgrade to more reliable microinverters or switching to a string inverter.
A water-to-air heat exchanger was planned to move heat from the solar thermal system to the input of the HRV air handler. However, passive-solar direct-gain performance is so good that the system was never hooked up.
A water-to-air heat exchanger was planned to move heat from the solar thermal system to the input of the HRV air handler. However, passive-solar direct-gain performance is so good that the system was never hooked up.
A push-button switch in the bathroom activates a pump that circulates hot water through the distribution system to avoid wasting water.
Even on cold days (here, the weather station shows 18.5°F outside), the passive solar gain can raise interior temperatures to comfortable levels (77°F).
At high altitude, a lower boiling point increases cook time and thus energy use. Using a pressure cooker on the induction range shortens cooking time, saving electrical energy.

With the December temperature plunging to -11°F and snow outside, our central Colorado home’s interior temperature was a comfortable 71°F, and rose to 76°F as the storm passed and sun came out in the afternoon. Even after five years in our net-zero energy Heliospiti (“Sun House”), my family and I still marvel at achieving these temperatures—with no mechanical heating.

In two previous Home Power articles, I described the design (HP141) and one-year performance (HP150) of Heliospiti. With five years of experience, this article answers the common question I’m asked when showing the house: “Knowing what you do now, what would you have done differently, if building today?”

The house was built with the Passive House focus of extreme airtightness and super-insulation. Passive solar gain satisfies the bulk of the space-heating load, while a combination of extreme efficiency, plus active PV and solar water heating (SWH) systems meet the full energy requirements for this all-electric house and our electric vehicle.

An all-electric net-metered house makes it easy to determine net consumption and production. Based on the initial meter reading upon moving into the house, and the reading on the five-year anniversary in May 2016, the net excess production of our 4.5 kW PV system was 8,829 kWh, an average of 4.8 kWh of surplus electricity per day. This includes powering the house—plus refueling our  Nissan Leaf since April 2012. The house did indeed perform as a net-zero energy home—with energy to spare.

Passive Solar Resolutions

The passive solar plus Passive House design has met or exceeded our computer modeling results. Even with temperatures as low as -25°F, we put less than five hours on the air-source heat pump during the first five years. It’s clear that the three concepts of ultra airtightness, super-insulation, and passive solar design (which includes ample interior thermal mass) will achieve high heating and cooling performance. All three elements work in harmony to capture winter solar gain, hold it within the house at night and on cloudy days, and provide comfortable, even temperatures throughout the house without the typical cold spots or drafty rooms.

The first passive solar lesson learned was failing to account for 12-inch-thick walls limiting the effective aperture of the south-facing, solar-collecting windows. The thick window wells narrow our solar exposure, reducing the amount of sunlight reaching our concrete slab floor and 1 1/4-inch-thick gypsum wall thermal mass. This is especially a factor with low sun angles early in the morning and late in the afternoon. This solar energy is not lost but it strikes the window well material, not the high thermal mass areas. Building today, I would replace the oak window trim with drywall. Although oak has an 85% greater specific heat value than gypsum drywall, the gypsum is 116% more dense, giving it greater thermal mass value.

Another advantage to removing the wood window trim relates to our choice to use low-iron glazing in the south-facing windows. Although this glazing results in a higher solar heat gain coefficient (SHGC), which is fantastic for improved solar gain, it also increases ultraviolet (UV) transmission by approximately 25%. This UV transmittance created significant fading on the wood trim, cabinets, and second-floor’s cork flooring. The fading was so significant on the window trim after just one year that I had to sand and refinish the trim using a UV-protecting exterior polyurethane.

Water Heating Solutions

Our nonpressurized, drainback SWH system has worked well in spite of reluctance and warnings from the local installers we spoke to during our design phase. When properly installed, nonglycol-based drainback systems do indeed have a very low risk of freezing in cold climates. We like the simplicity and lower maintenance of the simple drainback system, and avoiding the need to build a dump circuit to extract excess summer transfer-fluid heat.

Our key problem with the SWH system, which was easily remedied, stemmed from my lapse in basic high-school-level science: At 7,000 feet elevation, unpressurized water does not boil at 212°F—it boils at 198°F. We initially programmed our Caleffi iSolar Plus controller to shut down the circulation pump when the collector temperature reached 205°F. But this allowed the heat-transfer fluid—distilled water—to flash boil, creating violent vibration in the copper pipes. It sounded as if the system was ripping itself apart in our attic. Air was getting trapped near the circulation pump and bubbles were moving up to the roof. As the transfer fluid heated and expanded, it compressed the air pockets so that when they shot into the drainback tank, it was at high pressure and vibrated the pipes. The key was to bleed the system both high (at the drainback tank in attic) and low (inlet to circulation pump) after the distilled water was partially heated from the sun. Bleeding air from the system and setting the upper shutoff temperature to 190°F solved this problem. The lower shutoff temperature reduced the efficiency of our SWH system by sometimes shutting down on clear days before the storage-tank water reached its maximum temperature. However, it was still an acceptable trade-off for us to have the benefits of a simple drainback system.

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