Passive Solar Home Heating
By Paul Shippee
Introduction
The design of passive solar buildings is an imaginative art. It is a process that brings together knowledge from many fields, demands a common sense approach, and quietly resists complication. It tries to create a living space that is comfortable, attractive, and genuinely responsive to climate while eliminating most of the need for heating fuel.
The results are simple, yet somehow quite sophisticated. It has already been demonstrated in several passive solar homes that these goals can be successfully met. My purpose here is to present general principles and methods for reaching these design goals. Passive solar, along with energy conscious construction, is attractive not only because it works, but because it may now be the cheapest heating system for houses and other buildings.
A passive solar home heating system is one in which the flow of heat is by natural means; i.e., conduction, convection, and radiation. Loosely speaking, it also includes good insulation techniques, siting, building shape, etc.; these are necessary to any energy conscious design. Passive solar heating introduces an opportunity for new architectural forms because the first principle is the use of the house as a solar collector,
The principles of passive solar can be summarized according to four simple categories. They are: south-facing glass, thermal mass, building insulation, and heat distribution. These four comprise the complete day and night operation of a passive solar home. Other aspects of building a passive solar home, such as architectural appearance, initial cost, financing and appraisal, heating fraction met by solar, lifetime operating cost, choice of building materials, and the sizing of passive solar components can be addressed when some preliminary design decisions are made according to sound passive solar principles. The four functional categories mentioned above are universal and can be applied to the various types of passive solar design styles, such as direct gain (including clerestory), trombe wall, sunspace/greenhouse, roof pond, and convective loop.
South-facing Glass
The most basic aspect to understand about a passive solar home is that the house itself and the living space are the solar collector. This may sound strange because it is different from the conventional mechanical approach to home heating. It has been said that the amount of sunshine falling on a house in sunny climates can provide all the energy needs of the home.
The first things to consider are the house siting and orientation, the aspect ratio (relation of length to width), and sizing the glass areas. It is best if the shape of the house footprint is a rectangle with the long side facing solar south (or a few degrees east of south to favor the morning sun). The recommended aspect ration of 2 is when the length is twice the width.
How much area of this long side facing south is going to be glass, and what kind of glass? The least expensive double glass insulating fixed units to buy are patio door replacement units. The most popular mass-produced sizes, in increasing order of cost per square foot, are (in inches) 34 x 76, 48 x 76, and 28 x 76. Look for a replacement warranty of at least 10 years. Although the better insulating double glass units, called low-E (low emissivity), lose less heat on long, cold winter nights, they also block enough sunshine during the daytime to result in a net energy loss over most winter day-night cycles. So, don’t use the more expensive low-E double glass units on the south side. But, by all means, always use low-E glass on the north, east, and west windows.
Many passive solar elements are estimated early in the home design process by applying various rule-of-thumb sizing methods. Since the whole house and the living area is the solar collector, careful attention must be applied to recognize and integrate the interdependent nature of all the solar, architectural design, and comfort elements. These can be designed to function harmoniously for both comfort and performance. For example, the size of the solar collector (i.e., net south glass area) depends on the size of the building heating load, and the percentage of heating desired from the sun. In turn, the comfort factor depends on how these elements relate to the desired interior temperature swings. This comfort factor is mediated by the size and distribution of the interior thermal mass elements; more on this in the next article.
A useful rule-of-thumb that can be applied to the south-facing glass area is to allow 60-85% of the south wall to be glass. The higher percentage of glass used means more direct sun is allowed into the house. Winter sun can reach into the house at a distance of twice the window height. The comfort factor, related to overheating the house on winter days, can be managed by strategic placement of interior thermal mass materials of the appropriate volume and distributed area.
This thermal mass aspect of passive solar home heating will be discussed in the next article in this solar design series. More detailed treatment of all these design factors will be covered in a local Solar School presented by Paul Shippee this summer. Call 719-256-4656 for registration details.