passive-1 To help heat the house in the winter we stole an idea that’s been used for thousands of years. Shade the south side of the house with an overhang so that the Sun is excluded during the hot months, but allowed to shine in during the cold months. Early examples of this architectural technique, such as Montezuma’s Castle, abound throughout the southwest. What makes this system work is the changing elevation of the Sun as it goes from one season to the next.

At solar noon at our location, the elevation of the Sun is 33° on the Winter Solstice (Dec 21) and 80° on the Summer Solstice (Jun 20). This is a regular and predictable cycle brought about by Earth’s tilted axis as we orbit the Sun. The people who built Montezuma’s Castle knew about this cycle, even if they didn’t understand what really caused it. Knowing the elevation angles allows you to calculate the correct length of overhang to achieve optimal shading. We decided we wanted to start letting the sunlight into the house around September, and start excluding it again in March (which correlates roughly with our heating season). From there, it was simple geometry to calculate the correct overhang length. For our floor-to-ceiling south windows, this worked out to around 48 inches.

This architectural technique is called passive solar design because there are no moving parts (other than the Sun). Once it is built into a structure, the shading and unshading is fully automatic. Of course, we need to open the blinds on sunny winter days, and to close them after sunset to trap the heat inside. The picture here shows the shade line at noon on the Summer Solstice.

passive-2 Here, in an early photo taken before the windows or floor tile were installed, you see the penetration of sunlight under the overhang and into the house at noon on the Winter Solstice. We estimate that, during the heating season, the Sun provides almost 2/3 of our heating requirements. The heat soaks into the (now) dark-tiled floor during the day, and is released slowly back into the house over a period of 3-4 hours after sunset. We still need auxiliary heating on the longer cold and cloudy stretches, and our energy efficient air-coupled heat pump easily provides that warmth.
passive-3 In another early photo, we see the maximum penetration of sunlight that occurs during winter sunsets. This time of year the Sun sets around azimuth 240° (WSW). The curved section of windows, better visible in the preceding photo, is aimed in exactly that direction. So not only does it bring in as much Sun as possible when you really need it, it also provides a fine view of winter sunsets from the comfort of what is now the dining area.
passive-4 In between the Solstices are the Equinoxes, near the dates we chose to start and end the passive solar heating cycle. This image shows the system in transition. The yellow lines represent rays of sunlight. You can see how the shade line is right at the bottom of the windows. The date is September 12th, about a week before the Equinox. By next week the sunlight will be starting to enter at the bottom of the windows, and the free solar heating season will be underway again.