|Wood burning stove|
The literature suggests that, for passive solar purists, wood-, corn- or wood chip-burning stoves and masonry heaters are commonly used to raise the ambient temperature a few degrees to a comfortable level, particularly in earth sheltered structures. My guess is that most conventional homes embracing passive solar have conventional heating and air conditioning but down-sized to fit their passive solar capabilities.
What makes Annualized GeoSolar conditioning (AGS) a significant upgrade from classic passive solar is that it maintains the same comfortable year- round temperatures that conventional HVAC systems provide. (For details on AGS, click on "Featured Post" in the left column.) And it does so by first increasing the size of the thermal mass then using the summer sun to heat it. According to Hiat and Stephens, it takes a couple of years of solar input to reach the desired room temperature, during which auxiliary heat will probably be necessary. Accordingly, we plan to use wall- or ceiling-mounted infrared heaters in tandem with wintertime passive solar as our secondary heat sources for the first couple of winters while the AGS system is heating the thermal mass.
Eventually, passive solar alone should be sufficient to supplement the AGS system except possibly on below-zero cloudy days. Then we will resort to infrared heaters. Also not to be forgotten is the heat generated by people living in a structure. The amount of waste heat from cooking, lighting, water heating and from human bodies is not inconsequential. In fact, there are case studies in the literature in which waste heat provides half of the necessary supplemental heat for well-insulated passive solar installations.
Mazria discusses something that I have not seen in the other sources with which I am familiar -- what he calls the "thermal environment". The topic is somewhat hard to grasp at first, much less explain, but here's a go at it.
There is a relationship between the air temperature and something called the mean radiant temperature (mrt) which is the average temperature of all of the surrounding surfaces. Both mrt and air temperature influence the feeling of comfort but not equally. Mrt has a 40% greater impact on comfort than air temperature which means that, for the same feeling of comfort, the air temperature can be reduced by 1.4 degrees for each degree mrt is raised. The following examples come from a chart in Mazria (p. 64)
- Mrt of 65 degrees means the air temp has to be 77 degrees for a comfortable feeling of 70 degrees
- Mrt of 70 degrees means the air temp can be 70 degrees for a 70 degree comfort level
- Mrt of 75 degrees means the air temp can be 63 degrees for a 70 degree comfort level
- Mrt of 80 degrees means the air temp can be 56 degrees for a 70 degree comfort level
Dark colors absorb more sun energy than light colors so the color selection for exterior surfaces of a passive solar home might vary with climate. Up north, darker colors could be a good balance between heating assistance in winter without impacting cooling in summer. In temperate and warm climates, light colors can be used to reflect solar energy as part of the thermal barrier for the envelope. Radiant barriers can also be used in the attic to intercept solar energy before it has a chance to challenge the insulation.
At a latitude of 39 degrees and heating degree days of just south of 5,000, we have the potential for getting around 65% of our heating from passive solar during the winter. However, we consider this option a bonus. The AGS system will meet all of our conditioning needs because of the design of the thermal mass : (a) it is considerably enlarged, (b) it is well insulated and (c) most importantly, it is heated from the earth side by the heat from the summer sun instead of from the house side from solar gain through the windows. Our goal is a mrt that gives an average comfort level of 74 degrees year-round with the expectation that the mrt might drop a couple of degrees by the end of the winter and rise by a couple of degrees by the end of summer.
Once we get past needing supplemental heat (2-3 years), it remains to be seen whether the combination of AGS, wintertime passive solar and waste heat produces more heat than we need. Already, we are optimistic enough to postpone thermal shades until the need for them is apparent. And we are prepared to mothball some of the AGS conduits should there be overheating from the summer sun. Too much passive solar heat? What wonderful problem to ponder.
As to exterior colors, our plan is to use white cladding and light colored roofing. As explained in a prior post, we will have a well-ventilated "mini-attic" between the cathedral ceilings and the roof itself. The sheathing to support the roofing will be OSB with foil backing as a radiant barrier to keep the roof cooler in summer.