2 A SURVEY OF AIR-HEATER OPTIONS Solar-heated air can be used in a wide Technically speaking, the heat generated variety of applications, but it is a limited re- by a solar air heater is a relatively "low-grade" source. Only so much sunshine strikes each heat. That's not a disparaging term, for it simply square foot of the earth, and a well-built col- refers to the relatively low temperatures (80 to lector system will typically deliver about half 140°F) at which air heaters operate. If the of this energy to a house in the form of heat. airflow rate were reduced, a collector would The way this heat is used often means the heat air above this range, but in terms of Btu's difference between a useful, cost-effective delivered, there is a lot more heat in a strong system and one that never seems to perform blast of 90°F air than there is in a tiny trickle of properly. Effective use of collected solar heat is 140°F air from the same collector. Why? A especially important in retrofitted systems collector operating at 90°F has less heat loss which, because of space limitations, are often than one operating at 140°F and therefore undersized for the houses they are serving. Yet operates more efficiently. In a hotter-running even a small collector used as a supplemental collector, the increased heat losses through the heat source can make a substantial difference in glazing, sides and back of the collector mean a house's fuel consumption if the heat from it is that less heat is actually delivered to the living used effectively. There are also limits to cost- space. This is a very important concept to keep effective collector sizing, which are discussed in mind in the design and operation of both in chapter 3. Even where there is room for a small and large systems. This low-grade solar large collector, it is usually not cost-effective to heat is indeed very usable, but it must be build one so large that it provides 100 percent handled differently from the 140 to 160°F "high- of a house's space- or water-heating needs. So grade" heat produced by a forced-air furnace. A before diving into the actual design of your strong blast of 150°F air from a furnace will feel system, it is important to look at the best ways warm to the occupants of a home, whereas a of making full use of the heat a collector strong blast of 90°F air from an efficient delivers. There are a number of factors collector or from rock storage can feel drafty to involved in determining the best ways to the occupants, even though it is heating the "manage" your solar heat, including the house. Thus for solar air to heat a living space temperature of the heated air, the type and size without drafty discomfort, it must enter the of the collector and the size and layout of your living space slowly and continuously and from house. These are discussed in the following several different points. This rule doesn't hold pages. when 9 Made available with generous permission of the authors www.BuildItSolar.com DIY Solar Projects 10 THE COMPLETE HANDBOOK OF SOLAR AIR HEATING SYSTEMS the collector is very small, in which case it the output temperature in these collectors will would be impractical and unnecessary to create typically be close to 110°F. This low-grade heat more than one outlet. We can look again at the enters the house very slowly through large different types of air heaters, this time in terms openings, and the air movement is hardly of their specific air handling requirements. noticeable, thereby heating the home in a very comfortable way. Convective air heaters dump the heat they produce directly into the adjacent room and Convective Air Heaters therefore don't require a heat distribution Maintaining a continuous flow of low- component. If the south-facing rooms in a grade heat is the goal of any convective air- house are often occupied by day and thus heater design. Since a properly built passive air require a lot of daytime heating, a simple pas- heater will raise the temperature of the air sive collector is an appropriate choice. If these moving through it by about 30 to 40°F, rooms are seldom used and require little heat, Figure 2-1: Above and on opposite page. Window box collectors and TAPs are easy-to-build collectors that deliver their heat by natural convection. They are best used to heat rooms that are frequently occupied and that require daytime heating. Window box collectors are self-damping at night. TAPs require lightweight backdraft dampers to prevent reverse thermo siphoning when the sun isn't shining. Made available with generous permission of the authors www.BuildItSolar.com DIY Solar Projects A SURVEY OF AIR-HEATER OPTIONS 11 a more involved active collection and distri- devices in that no dampers of any kind are bution system is needed to satisfy the daytime needed to prevent nighttime heat losses. Their heating needs of non-south rooms. There's an best application is on houses with wide, south in-between wrinkle, too: A large passive heater facing, double-hung windows that are 4 feet or can provide too much heat to adjacent rooms, more aboveground level. They can be installed which necessitates additional passive (vents in on other types of windows, but the walls or floors) or active (blowers and ducting) modifications required are more difficult and heat distribution. often expensive and unattractive. If the win- dows are less than 4 feet from the ground, the Window Box Collectors natural convection that moves air through them Window box collectors are one of the and into the house will be weak, and the simplest solar heating devices you can build, collectors won't perform as well as they could. but since they are quite small in relation to the Since window box heaters are fairly small and size of the room they are heating, they don't act as a supplemental heat source, no heat provide a great percentage of a house's total distribution or storage is incorporated into their heating needs unless several are used. These design. They don't involve any major heaters provide a slow, continuous flow of modifications to the house and don't sacrifice heated air into the adjoining rooms. They are any south-facing exposure that later may be self-operating and, when properly built, have desired for a larger collector installation. an advantage over other solar heating Made available with generous permission of the authors www.BuildItSolar.com DIY Solar Projects 12 THE COMPLETE HANDBOOK OF SOLAR AIR HEATING SYSTEMS Thermosiphoning Air Panels sign of thermosiphoning air panels on new Thermosiphoning air panels (TAPs) are houses. This involves building a large collector also a good choice for buildings where the below the house or underneath a massive floor available south-facing wall area is small and (concrete slab) so that solar-heated air rises by where the rooms behind this wall can use natural convection into and through the rock supplemental daytime heat. Day-use spaces, box. Storage in retrofitted convective air such as small workshops and offices, are a good collectors is a tricky and expensive operation choice for TAPs because these collectors will that is almost never justified unless the collector deliver heat when the rooms are occupied. is very large and Like window box systems, TAPs require a very free flow of inlet and outlet ~air, so the vent openings to the adjoining rooms need to be large (relative to vent requirements for active systems). When you install a TAP system on a frame structure, you can easily cut these vent openings between wall studs at the top and bottom of the wall. A Trombe wall, first cousin to a TAP, is usually a better retrofit choice for a masonry wall, especially since it incorporates the added feature of heat storage in the masonry. Heat distribution in most TAP systems simply involves a convective flow of air (con- vective loop) inside the room behind the col- lector. This steady circulation of warmed air provides comfortable heat but can result in an overheated room if the TAP is large relative to the size of the room (where the collector area is more than 20 percent of the room's floor area). In single-family dwellings it is often de- sirable to direct solar-heated air from a large TAP to rooms that aren't adjacent to the TAP. This can be accomplished with a small blower and ductwork, but if the retrofit project calls for extensive heat distribution, it is a better idea to design and build an active solar system to work with an active distribution system. Even though passive air heaters are gene- Photo 2-1: This 8-by-14-foot TAP was retrofitted erally regarded and used as daytime heaters, onto the Hitchcock Nature Center in Amherst, Massachusetts. It heats the second-floor office space heat storage can be incorporated into the de- and uses an open stairwell as the cool-air return. This collector experiences some morning shading, which indicates that a little tree trimming is in order at this site. Made available with generous permission of the authors www.BuildItSolar.com DIY Solar Projects A SURVEY OF AIR-HEATER OPTIONS 13 located well below the rooms to be heated. solar heat. Because the flow of solar-heated air (Chapter 11 goes more deeply into heat is isolated from the living area, 80 or 90°F air storage.) from a collector can be used without creating Both window box heaters and thermo- chilly drafts.