Abstract:
A structure and method of solar energy collection use a building roof acting as a heat collector, conductor, and convector passing thermal energy to air in a volume below. The air may stratify according to temperature, with flow controls maximizing temperatures by operating dampers, valves, or both in the air and a liquid working fluid, respectively. A finned heat exchanger may be a hydronic baseboard-type heating unit operating to transfer heat in the opposite direction, into water inside the central pipe. A thermo-mechanical actuator, such as a wax motor may provide passive control, and heat may be collected in an open circuit of culinary water, a closed loop, or a double loop system. Ducting aids stratification, temperature maximization, and heat exchange to enforce pre-selected temperatures in the air and water working fluids.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of co-pending U.S. Provisional Patent Application Ser. No. 61/295,062, filed on Jan. 14, 2010 for FREE-CONVECTION, PASSIVE, SOLAR-COLLECTION, CONTROL APPARATUS AND METHOD. 
     
    
     BACKGROUND 
       [0002]    1. The Field of the Invention 
         [0003]    This invention relates to heat exchange and, more particularly, to novel systems and methods for passive solar energy collection. 
         [0004]    2. The Background Art 
         [0005]    Attic and roof structures are struck by solar energy as radiation, largely as a nuisance, damaging roofs, overheating attics, and generally achieving little good. Solar collectors from the 1970s found support on rooftops, at some considerable capital expense, ongoing maintenance costs, reliability problems, unsightly structures, and hard-earned tax credit with a headache. Solar radiation occurs in greatest concentrations in places that need the least space heating. Mainly attic ventilation is about passing enough air through an attic to keep moisture evaporated, while encouraging ambient air to cool the attic to about ambient temperature through small openings and screens designed to keep out birds and insects. 
         [0006]    What is needed is an apparatus and method to modify roof structures to improve heat transfer, protect solar heat collection devices inside the roof structure instead of on top thereof, maximize the temperature of working fluids in heat transfer, and minimize costs while improving reliability with minimum maintenance. Minimizing risk by simplifying the fluid handling systems and using well adapted, readily available hardware would be a plus. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In view of the foregoing, in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention as including a structure and method of solar energy collection using a building. A roof acting as a heat conductor has a top surface converting solar radiation to thermal energy, which is then conducted down through to the bottom surface. The bottom surface of the roofing buildup convects thermal energy to air in a volume below. 
         [0008]    The air may be stratified within the volume of an attic or other space according to a temperature thereof. Controlling a maximum temperature in the air by operating a damper relies on controlling a rate of a flow of air in the volume. The air flow passes through an exchanger extracting heat from the flow at substantially the maximum temperature. 
         [0009]    The heat exchanger uses a fluid cooler than the maximum temperature of the air, and may use an open cycle of culinary water, a closed loop of any suitable fluid, or the like. The heat exchanger may comprise fins extending into the air flow, including a hydronic baseboard-type heating unit operating to transfer heat in the opposite direction, into water inside the central pipe. 
         [0010]    A damper may operate by any suitable means, such as a thermostat, and may use a wax actuator dependent on local temperature near the heat exchanger or under the roof layer to control air flow heating water in the heat exchanger. A thermo-mechanical actuator, such as a wax motor provides passive control, but may be replaced by conventional controls if desired. 
         [0011]    Suitable collectors may evacuate condensate from the heat exchanger as water is condensed from the humidity in the air being cooled. The condensate may be conducted in a channel, pipe, or the like to a location outside the volume used as the solar heat collection region so as to not increase humidity by re-evaporation, which could also present heat loss for the collection process otherwise. 
         [0012]    Air in the volume (e.g., attic, ducts, ventilation spaces, etc.) may be freely movable therewithin by free convection, and may be substantially unobstructed in at least one direction. Ducting or some other director may be positioned in the volume to conduct air through at least a portion of the lower and upper ends, directing the heated air through the exchanger or banks of exchangers in series, parallel, or both. A channel may substantially enclose a heat exchanger against substantial escape of air, except in the direction of flow. The flow may transfer heat in the air flow, collected from the roof&#39;s under surface, through the heat exchanger, which extracts the heat from the air into a transfer fluid in the heat exchanger. 
         [0013]    A damper controlling the flow of air, and valving controlling the flow of water may both be passive or active, or one of each. In any event, the controls may support limiting air flow to obtain maximum temperature in the air from the roof, and maximum temperature in the water from the air. Air and water flows may be restricted to maximize temperatures, and thus, the thermodynamic availability of the heat collected. Contrary to much of solar collection technology, solar radiation need not be concentrated to raise the thermodynamic availability of the fluid stream collecting the heat. 
         [0014]    An actuator, such as a wax actuator or wax motor may operate as a thermostat to selectively open and close the dampers, valves, and the like to enforce pre-selected temperatures in the air and water working fluids. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
           [0016]      FIG. 1  is a schematic block diagram of a passive solar heat exchange system in accordance with the invention, controlling temperature rise for increased thermodynamic availability by promoting stratification and restricting air flow; 
           [0017]      FIG. 2  is an end view of one embodiment of a dampered, stratified, solar collection system under a roof, using the roof for solar energy absorption; 
           [0018]      FIG. 3  is a schematic diagram of the air flows associated with stratified heating within an attic space of a building such as a house; 
           [0019]      FIG. 4  is an end view of a heat recovery system in accordance with the invention installed in a trussed attic space; 
           [0020]      FIG. 5  is an end view of a trussed attic space containing a substantially closed circulation system down drafting cooled air in accordance with the invention; 
           [0021]      FIG. 6  is a partial, cutaway, end view of the system of  FIG. 5  showing the open air path through the soffits and lower vents, along the underside of the roofing materials, through the heat exchange elements, and ultimately through the top damper and exiting out the ducting created by a vent cap; 
           [0022]      FIG. 7  is a perspective view of the apparatus of  FIGS. 5 and 6 , illustrating alternative locations for storage tanks, one high and suitable for free convection heating, and the other set low and thus remaining cooler than the exchanger absent forced convection; 
           [0023]      FIG. 8  is an end view of the apparatus of  FIG. 7 , relying on a larger tank located principally above the heat exchangers in order to maintain maximum temperature by free convection of the liquid, heat-exchanging fluid; 
           [0024]      FIG. 9  is an end view of one embodiment of an alternative heating system relying on passive solar collection through windows provided with curtains as absorbent receptors of solar radiation and directors of (e.g., effectively ducting) heated air toward the heat exchange element in accordance with the invention; 
           [0025]      FIG. 10  is a perspective view of one embodiment of a dry, rooftop, solar collector relying on transparent, insulating cover over a duct provided with an absorbent top surface to collect heat without requiring large expanses of conventional, liquid, carrier lines; 
           [0026]      FIG. 11  is a side elevation view of one embodiment of a damper controlling horizontal or vertical flows, depending on orientation of the wall element, of incoming or outgoing air heated in accordance with the invention; 
           [0027]      FIG. 12  is a side, cutaway, elevation view of a duct, illustrating various alternative embodiments of dampers to control the flow of air through the heat exchange elements thereof; 
           [0028]      FIG. 13  is a partial, cutaway, end view of a heat recovery system collecting passive solar energy from a roof, where the insulation is placed between the rafters in the roof, rather then at the base of the attic; and 
           [0029]      FIG. 14  is a schematic end elevation view of a building containing various alternative embodiments of elements including controls, storage tanks, pre-heaters, water heaters, and connecting lines, illustrating various alternative embodiments for maintaining free and forced convection of heating fluids such as water passing through heat exchangers implemented in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    It will be readily understood that the components of the present invention, as generally described and illustrated in the drawings herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in the drawings, is not intended to limit the scope of the invention, as claimed, but is merely representative of various embodiments of the invention. The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
         [0031]    Referring to  FIG. 1 , a system  10 , apparatus  10 , or method  10  in accordance with the invention may include ducting forming a wall defining a channel. It may be appropriate to talk about ducts  12  or ducting  12 , meaning the channel, and the walls, respectively, formed as ducts  12 . In general, the ducting  12  is responsible to conduct air, and need not be closed completely imperviously. For example, conventional heating, ventilating, and air-conditioning (HVAC) technology includes plastic, metal, and sometimes wooden chambers and conduits that effectively act as ducting  12 . Similarly, the entire structure of a room or attic may act as a plenum as part of a circuit (closed or open) of air flow. 
         [0032]    Typically, controlling the restriction on the flow of air, and thus the flow of air through ducting  12 , through one or more vents  14 ,  16  may obstruct the flow and control temperatures. Since temperature is a direct function of mass flow rate of air, for a particular amount of heat available, temperatures may be maintained at higher values (within temperature and transfer limits of a heat source) by restricting the amount of air available to receive the heat transferred into the air. Thus, a vent  14  may be placed low in the air flow system, high (e.g., as upper vent  16 ) in the air flow system, or both may be employed. 
         [0033]    Typically, the vents  14 ,  16  may be completely open at all times. In other situations, the vents  14 ,  16  may be provided with dampers  18  to selectively open and close flows, such as by closing the vents or opening them. In yet other alternative embodiments, the vents  14 ,  16  may be open at any or all times, while the dampers  18  are placed elsewhere in the flow. Regardless of the location of a damper  18 , so long as it is within a single path of ducting  12 , it may partially or completely obstruct the flow through that ducting and thus through any vents  14 ,  16  in the direct path of the air flow through that ducting  12 . 
         [0034]    Dampers  18  may be placed as lower dampers  18   a  or upper dampers  18   b  in the flow. However, a damper  18  may not be placed at a high or low position, but may be placed anywhere within ducting  12  to provide the required obstruction. It is proposed in several embodiments that actuators  19   a,    19   b  associated with dampers  18   a,    18   b  operate based on temperature. 
         [0035]    For example, in one embodiment, an actuator  19  may be a wax actuator having an operating material of a blend of wax that expands with temperature. The wax operates on a piston to expand or contract the overall length of the actuator  19  by extension of a piston thereof. Thus, in one embodiment, the dampers  18  may be controlled by actuators  19 , which are themselves responsive to temperature. 
         [0036]    For example, in one embodiment, an actuator  19  on one damper  18  may act to seal the system  10  off in cold weather. By the same token, another damper  18  may be controlled by an actuator  19  that will restrict airflow to achieve a particular temperature at a particular location in the system  10 , such as near the exchanger  20  to maintain an air temperature sufficiently high to ensure a proper temperature of liquid in the heat exchanger  20 . 
         [0037]    For example, high thermodynamic availability is a direct function of temperature differential. Accordingly, in order to provide a good supply of sufficiently hot water, from the heat exchanger  20 , it may be advisable to restrict the flow of air through the heat exchanger  20 , assuring a higher temperature. Thus, a particular actuator  19  operating on a corresponding damper  18 , may be engineered, designed, or otherwise set to restrict the flow through a damper  18  or past a damper  18  whenever the temperature of the consequent air flow is not at a sufficiently high air temperature, such as one for which the system is designed or desired to operate. 
         [0038]    In general, a line  21  may pass through a heat exchanger carrying a fluid such as water. In certain embodiments, an exchanger  20  may actually be transferring heat into another heat exchange system in a “double loop” arrangement. In such circumstances, the fluid in the line  21  need not be potable water. For example, a water providing with a certain amount of alcohol, glycol, or other antifreeze agent, and may even include salt, as a mechanism to maintain the line  20  free flowing and without any freezing. In other embodiments, the line  21  may carry a once-through pass of culinary water that is simply pre-heated to whatever extent it can be before being sent off to other systems such as a water heater for final heating. 
         [0039]    In one embodiment of an apparatus in accordance with the invention, the line  21  carries water while an array of fins intimately bonded to the line  21  exchanges heat between the passing air and the line  21 . Thus, a comparatively larger area exposed to air may accommodate the reduced heat transfer coefficient in air, while the comparatively smaller area of the line  21  corresponds to the comparatively higher heat transfer coefficient of water. 
         [0040]    In general, a volume  22  such as the enclosed body of an attic  22  may host the ducting  12  and the exchanger  20 . In typical embodiments, a solid conductor  24  basically comprises the roofing. Typically, the conductor is a composite of various materials. For example, tiles, shingles, membranes, and other roofing materials on top of wooden substrates such as plywood, composition board, concrete, steel, or the like may form the conductor  24 . The conductor  24  is typically solid, although it may contain spaces, gaps, air, or the like. 
         [0041]    The conductor  24  may also include insulated spaces. However, in many embodiments of an apparatus in accordance with the invention, the enclosed volume  22  is not interior to insulated spaces. For example, if rafters pitched along the angle of a roof are themselves insulated, then ventilation will typically be provided along the rafters above the insulation, and below the roofing materials that form the conductor  24 . 
         [0042]    Meanwhile, in an uninsulated attic, the enclosed volume  22  of space or air typically sits above insulated joists or horizontal members of truss work. Here, the angled portions of a roof are called rafters. The horizontal portions of roof trusses are called joists here, although people sometimes refer to them as rafters also. Nevertheless, in certain roof structures, insulation is placed exclusively over the ceilings of lower floors, and the attic remains unused. This is typical when truss work fills the attic such that it is not habitable space not readily accessible space for living storage. 
         [0043]    Typically, radiation  26  from the sun  30  may strike an absorber  28 . The absorber  28  is typically an upper surface of the same material that forms the conductor  24 . For example, the stone surface of an asphalt shingle, the surface of a ceramic, clay, concrete, or other tile, or the like may form or define the absorber surface  28 . Accordingly, radiation  26  from the sun  30  strikes the absorber  28  and is converted from its short wavelength, high frequency wave form to thermal energy that may then be transmitted by conduction through the conductor  24 . Of course, a certain amount of the absorbed energy found in the absorber  28  may be convected away by ambient air. 
         [0044]    Opposite the absorber  28  or absorber surface  28  of the conductor  24  is the convector  32  or convector surface  32 . Typically, the convector  32  engages in two types of heat transfer. That is, convection  34  involves conduction of heat from the convector  32  to adjacent air, followed by a net flow or bulk flow of the air as it moves, e.g., as it heats and rises in free convection. Thus, convection  34  is a combination of conduction and bulk air flow. 
         [0045]    Meanwhile, the same surface that operates as a convector  32  may also serve as a radiator  36 . For example, radiator surface  36  maintains a temperature that may be substantially higher than other portions or surfaces within the space  22 . Accordingly, energy may radiate from the radiator  36  as infrared radiation  38 . Infrared radiation is not as effective as solar radiation. For example, typically, radiation  38  transfers between a comparatively warmer surface to a comparatively cooler surface only at about the same rate that energy convects away from a heat transfer service by convection  34 . 
         [0046]    Heat transfer controls many aspects of a temperature within the space  22 . To the extent that a heater exists completely below a particular point in a freely flowing, inviscid fluid of any state (liquid, gas, vapor) the entire process of convection  34  will usually dominate. The heated space  22  has little temperature gradient above the source of heat. Typically, where convection is unobstructed inside, and only free convection cooling is available outside, any type of a heating element in a free pool of fluid will heat all of the fluid above the heating element to some stable temperature. Only a very slight gradient, typically a few degrees will exist between the top of the pool of fluid and the top of the heating element. 
         [0047]    By contrast, throughout the depth of the heating element, a temperature gradient will exist. Below the heating element, the temperature will have a steep gradient or be an almost uniform temperature, depending on whether any significant heat source is below, often having only the gradient imposed by conduction downward. 
         [0048]    Thus, where a conductor  24  is made of the roof of a building, such as a slanted roof, heat is added all the way along the length of the roof. Thus, heat is added at the lowest levels, heat is added at the highest levels, and heat is added at intermediate levels. Accordingly, the enclosed volume  22 , space  22 , or attic  22  will typically stratify, providing strata  40  at different temperatures. 
         [0049]    The air strata  40  may have some maximum temperature stratum  40   a  at the top, with subsequently cooler strata  40   b,    40   c,  etc. down to the lowest stratum  40   m,  and temperature may vary continuously. 
         [0050]    The strata  40  exist exactly because heat is added along substantially the entire height of the space  22  or the volume  22 . In any system where heat is added in one location and extracted from another, a gradient will exist. Nevertheless, it has been found that wherever water or air is free to move, the entire temperature gradient between a heat source and the upper level of the tank or containment vessel holding the fluid is typically only a matter of a degree or two unless some very significant (e.g., liquid or forced convection) is occurring to take heat away from the containment region. 
         [0051]    Otherwise, so long as free convection in air is the only mechanism for removing heat from a volume, across the wall, the temperature gradient above a heating element is comparatively insignificant. Likewise, where a heating element provides substantially constant heat at all levels over the height of a contained mass of fluid, such as water or air, it has been found that a gradient will be established at a very significant slope. The temperature typically will vary substantially linearly from top to bottom between the maximum temperature at the top and the coolest temperature in the contained reservoir or volume at the bottom. 
         [0052]    Typically, an input line  42  connects to or is a part of the line  21  through the exchanger  20 . In certain embodiments, the ducting  12  conducts hot air, while the line  21  is fed water through the inlet line  42 . Typically, the inlet line  42  is cold, comparatively speaking, while the outlet line  44  also connected to the line  21  and the exchanger  20  is comparatively hot. 
         [0053]    When warm air from the normal environment is cooled, its capacity to hold moisture is reduced. To the extent that humidity may be high, cooling of air in an exchanger  20  with comparatively cooler water passing through a line  21  in the exchanger  20  will result in condensate  46  exiting out a condensate line  46  from the exchanger  20 . The condensate line  46  is not carrying away water from the line  21 , or from the inside of the line  21 . The condensate line  46  carries away humidity that has been chilled out, like the sweat on a cold drink container on a hot, humid day. 
         [0054]    As heat  46  is transferred from the air into the water or other fluid in the line  21  of the exchanger  20 , the cooled air resulting may condense out moisture that was previously carried as vapor in the humidity of the air. Accordingly, a condensate line  46  may feed condensed moisture out of the ducting  12  or from regions thereabout for disposal elsewhere. Otherwise, the moisture may cause other problems. 
         [0055]    In general, a flow  50  of air passing through ducting  12  inside or connected to the volume  22  may pass through an exchanger  20 , giving up heat  48  into the fueled line  21  on the exchanger  20 , while condensing out any excess humidity as liquid through the condensate line  46 . 
         [0056]    Referring to  FIG. 2 , while referring generally to  FIGS. 1-14 , a system  10  is shown in one embodiment. Rafters  56  and joists  58  (sometimes called rafters, but referred to as joists simply to distinguish horizontal members  58  from the angled rafters  56 ) together support a building  60  by tying together the walls  61 , and supporting the roof  62 . 
         [0057]    Typically, an attic  64  may define the contained volume  22  of air below the roof  62  and heated thereby. 
         [0058]    Typically, in an attic  64  as illustrated in  FIGS. 2-3 , the bottom  66  of the attic  64  may have insulation  68  to insulate the occupied space of the house  60  or building  60  therebelow. Meanwhile, in such a configuration, the roof  62  is typically uninsulated and vented. Therefore, the flow  50  of ambient air flows up through soffits or vents under the eaves of the roof  62 , and along the undersurface  32  thereof. Typically, the space  22  is completely filled with air, and thus the flow  50 , rises and stays closest to the convector  32  and radiator  36  surfaces of the roof  62 . Accordingly, the absorber  28  of the roof  62  receives radiation  26  from the sun  30  which then heats the conductor  24  or roofing material  24 . Thereupon, the convector  32  and radiator  36  proceed to convect heat into the flow  50 , further heating it, as well as radiating, respectively, to the bottom  66  of the attic  64 . Accordingly, some warming of the bottom  66  of the attic  64  may occur with consequent heating of air thereabove. 
         [0059]    Nevertheless, the majority of heat in the attic  64  will be received by the convector  32  convecting heat into the flow  50  rising along the underside of the roof  62 . Typically, attics will have some type of a venting system. In certain embodiments of an apparatus and method in accordance with the invention, a damper  18  may limit the ability of air to vent through ducts  12 , vents  16 , or both. 
         [0060]    For example, typical construction may include placing vents  16  in roof structures. Vents may be formed as ridge vents, such as the duct  12  feeding air out of the attic  64  under a ridge cap or cap  70  that forms the upper vent  16 . Meanwhile, soffits of eaves may be provided with lower vents  14 . Typically, in a system in accordance with the invention, cooler air enters a lower vent  14 , and is ducted out through one or more openings higher in the attic  64 . 
         [0061]    Some attics may be vented by cross flow of breezes through vents in the gable of the attic. In other situations, a vent cap  70  or ridge cap  70  covers an opening or openings near the ridge of the roof  62  protecting against entry of weather such as rain and snow, while still providing exit of the flow  50  of air through the attic  64 . Thus, the flow  50  may pick up much of the heat impinging on the roof  62  as radiation  26  absorbed therein and carry it out by convection through the upper vent  16 . 
         [0062]    In an apparatus and method in accordance with the invention, maximum thermodynamic availability is desired for obtaining most efficient heat transfer into the exchanger  20  and maximum temperature. Higher thermodynamic availability results in and from higher temperatures at the highest points in the space  22  of the attic  64 , thus providing maximum temperature in the line  21  of the exchanger  20 . 
         [0063]    In the illustrated embodiment, one or more dampers  18  form, control, or both the ducting  12  directing air from the flow  50  through the exchanger  20 . For example, the damper  18  may be placed below, above, or in both places with respect to the exchanger  20  in order to restrict the flow  50  of air. If the flow  50  is left unrestricted then the temperature in the exchanger is left largely to chance. 
         [0064]    In certain embodiments, the exchanger  20  may operate in the absence of a damper  18 . However, in order to provide flows by free convection, substantially exclusively, it may be preferable to rely on the damper  18  to limit the free convection flows  50  of air. This helps maintain the thermodynamic availability of heat in the flow  50  by limiting the net mass flow rate thereof. 
         [0065]    Referring to  FIG. 3 , in particular, while continuing to refer generally to  FIGS. 1-14 , a system  10  in accordance with the invention may take advantage of the strata  40  formed by virtue of the flows  50  of air entering into and rising through the space  22  below a roof  62  of a building  60 . As illustrated, each of the individuals  50   a  into the lower vent  14  of the attic space  22  of a building  60  may rise along the convector surface  32  and radiation surface  36  of the conductor  24 . Typically, if the upper vent  16  is open, then the flows  50  will progress as directly as permitted toward the upper vent  16  to exit the space  22 . 
         [0066]    However, to the extent that the flows  50   b  are heated faster by the convector  32  then they can rise above all the strata  40  then the air may stratify. For example, if a flow  50   b  is rising along a convector  32  and picking up heat, it may come to the point that it exceeds in temperature the surrounding strata  40   a,    40   b,    40   c  and so forth to stratum  40   n.  However, alternatively, if the ducting  12  is restricted, artificially, by design, or simply as a result of the built in floor resistance from either vent  14 ,  16  then a flow  50   b  may find that a particular stratum  40  is already hotter than the flow  50   b.  Thus, the flow  50   b  may spread out as a flow  50   c  at a particular level of a stratum. 
         [0067]    For example, a flow  50   b  may rise along a convector  32 , picking up heat. However, as soon as the net temperature of the convector  32  can no longer effectively heat the flow  50   b  (which is cooling down by entraining fluid from the space  22 ) hotter then a stratum above it, say, for example, the stratum  40   b,  then the flow  50   c  must flow laterally under the stratum  40   b,  and become the stratum  40   c.    
         [0068]    Each of the flows  50   b  will continue to draw into it from each of the strata  40  as the flow  50   b  passes by. For example, as the flow  50  traverses upward along the convector  32 , it continues to pick up additional mass. One may think of this entrainment as the flow  50   b  growing larger, extending further away from the convector  32  and carrying more net mass flow with it. Thus, a certain degree of mixing occurs between the flow  50   b,  and the strata  40  of air in the space  22 . 
         [0069]    Nevertheless, no particular flow  50   b  can rise above a stratum  40  hotter than flow  50   b  is itself. Thus, air in the stratum  40   a  has received heat from all layers below. However, air in the stratum  40   n  receives heat only from the lowest portion of the roof  62 , because the flow  50   b  entrains air from the strata  40 , it exchanges not only momentum but heat energy. Therefore, the net temperature or net average temperature of the flow  50   b  is increasing as the result of additional heat added by the convector  32 , but decreasing as a result of mixing with the various strata  40  in passing. Thus, one may think of the flow  50   b  as being involved in a balancing act between the net temperature gained from the convector  32  balanced against the net temperature loss caused by mixing with the strata  40 . Thus, one can readily see how the flow  50   b  may top out at a particular stratum  40  short of the top stratum  40   a  in the space  22 . 
         [0070]    Ultimately, heated air in the space  22  will arise through the duct  12  and exit out the vent  16 , driven by the net effective density of the air outside the building  60  compared to the net density of the air in the space  22  within. One may thus think of the flow between the vents  14 ,  16  as a chimney-like effect as the more dense column of air outside the building  60  displaces, through the vent  14 , the lighter density air in the space  22 . Accordingly, air incoming through the vent  14 , may actually flow immediately into the bottom stratum  40   n  as hot air from the uppermost stratum  40   a  is rising through the duct  12  and exiting through the vent  16 . 
         [0071]    Referring to  FIGS. 4-8 , specifically, while continuing to refer generally to  FIGS. 1-14 , a system  10  in accordance with the invention may include some type of a barrier  72  restricting air in the space  22  from rising, except through duct work  12 . In the embodiment of  FIG. 4 , for example, flow from the vent  14  into the space  22  may fill the space surrounding the various trusses  54 , including braces  55 , rafters  56 , and joists  58 . As discussed above, the term joist  58  is used even though a roof structure  62  may not have a floor thereabove. Since typically, the horizontal and angled members of the perimeter of a truss may both be called rafters we will refer to the angled member of a truss  54  as the rafter, and the lower member, extending horizontally as the joist  58 . In the illustrated embodiment, a flow  50  rising from a vent  14  may be heated by the roof  28 . Meanwhile, ducting  12  provides the only escape for the flow  50 . 
         [0072]    Accordingly, with exchangers  20  located in the duct work  12 , the flow  50  must pass through one or more exchangers  20  delivering heat into the lines  20  therethrough. Thus, the incoming line  42  provides a substantially and comparatively cooler stream of liquid, such as water, into the exchangers  20 . In the illustrated embodiment, the cooler water from the inlet line  42  goes first to the top exchanger  20 . This is not necessary. In certain embodiments, the inlet line  42  may go first to the bottom exchanger  20 , and then pass into subsequently higher exchangers  20 . In fact, such an arrangement will flow much more readily and naturally without the need for a check valve  74 . A check valve  74  may be placed in the line  42  in order to assure that fluid (water) is always passing only in one direction through the exchangers  20 . 
         [0073]    Also, the water passing through the exchangers  20  may or may not be the same water as is drawn off for use by an independent draw line  76  or draw  76 . For example, insulation  78  around a tank  80  may provide a reservoir of hot water received from the outlet line  44  of the exchangers  20 . Thus, the maximum temperature in the tank  80  is at the top thereof where the inlet line  44  delivers hot water. Meanwhile, the coldest water in the tank  80  is at the bottom thereof and is fed into the inlet line  42  to the exchangers  20 . 
         [0074]    In certain embodiments, the inlet line  42  may simply be connected to the building water supply. Thus, the line  42  may feed directly into the exchangers, and thus provide pressure, flow, and a water supply to be heated by the exchangers  20 . In such an event, the outlet for the tank  80  may simply be in the draw  76 . Alternatively, the line  42  may be thought of as two lines, one serving the building  60  out of the tank  80 , and the other feeding from the building water supply into the exchangers  20 , to provide the outbound outlet line  44 . 
         [0075]    In the illustrated embodiment of  FIG. 4 , the duct  12  exits the roof  62  through a vent  16 . The illustrated vents, sometimes called turtle vents  16 , at particular locations may permit exit of the flow  50  only after that flow  50  has passed through the exchangers  20 . In other embodiments, a ridge vent, or even an end vent may be ducted to dispose of or to conduct out of the space  22 , the hot air being released to ambient air. 
         [0076]    Check valves  74  may assure that water can only flow in one direction into the exchangers  20 . If the highest exchanger  20  is provided the coldest flow of incoming water from the line  42 , then the density in the line  42  is higher and the water column therein is heavier than the column in the exchangers  20  themselves. With the buoyancy effect of hot water against cool water, cool water will tend to drop, thus drawing water backwards out through the line  42 . To assure that this did not occur, the line  21  of the top exchanger  20  may be placed below the lowest point of the tank  40 . Thus, the weight of the column of water in the exit line  44  may be lighter than the column within the tank  80  and the line  42  feeding into the exchangers  20 . Any transient circumstances otherwise may be cured by the check valve  74  prohibiting flow backwards through the line  42 . 
         [0077]    One reason for placement of the feed line  42  feeding water first into the topmost exchanger  20  is to assure that the hottest air from the space  22  strike the bottom exchanger  20  first, and that the bottom exchanger  20  be the last unit to heat the water thus heating water to its maximum possible temperature before being discharged into the tank  80 . 
         [0078]    Alternatively, the free convection flow through the lines  42 , exchangers  20 , and line  44  may be best served by feeding the cool water from the line  42  into the bottom exchanger  20  first. However, this tends to reduce the driving temperature difference heating water in the upper exchangers  20 , driven by the hot water below, already heated by the comparatively hottest air passing through the bottom exchanger  23 . 
         [0079]    Referring to  FIG. 5 , while continuing to refer generally to  FIGS. 1-14 , certain embodiments in the space  22  may be comparatively closed. For example, in the illustrated embodiment, a flow  50   a  is rising along the convector  32  in the space  22 . The flow  50   a  may stratify, mix, or otherwise fill the space  22 . Ultimately, however, from the space  22 , a flow  50   b  may be ducted upward through the ducting  12 . The flow  50   b  may then turn down through the array of exchangers  20 . 
         [0080]    Again, the chilling effect of water in the line  21  is operating to push the colder flow down through the exchangers  20 . Each exchanger  20  chills the flow  50   c,  with the upper exchanger  20  receiving the hottest air available. As the air cools, it drops down through the ducting  12  to the next lowest exchanger  20 . Ultimately, the coolest air drops down past the bottom exchanger  20  and exits as the flow  50   d  back into the bottom of the enclosed space  22 . 
         [0081]    Meanwhile, the tank  80  with its insulation  78  in the illustrated embodiment may feed a flow  42  into the bottom exchanger, from whence the flow  82   a  feeds to the next exchanger  20 , followed by the flow  82   b  to the next exchanger, and so forth until the flow  82   c  feeds the last exchanger  20  at the top of the duct  12 . Finally, the last exchanger  20  provides a flow  44  into the top of the tank  80 . Thus, the air flow  50   c  drops down through the ducting  12 , while the heated water flow rises through the various exchangers  20  and ducting  12 . 
         [0082]    To ensure that maximum temperatures are reached, a thermally activated damper  18  may restrict the ducting  12  at any appropriate location. The damper  18  may be located within the ducting  12  near the exchangers  20 , at the bottom thereof, at the entrance of the air flow  50   b  into the ducting  12 , or the like. 
         [0083]    As a result of the extensive cooling that a flow of cool liquid such as water may cause in the flow  50   c,  in humid environments, a tray  84  may collect water or condensate from the airflow  50   c  for disposal. In certain embodiments, a condensate line  46  imbedded in the tray structure  84  may even be insulated  86  to limit re-evaporation thereof. Thus, the insulation  86  may keep the tray  84 , condensate line  46 , and the condensed water therein somewhat isolated from the heat of the attic space  22 . The condensate line  46  may drain into an appropriate location, such as a household drain, outdoor drain, or the like, limiting humidification of the space  22  that might tend to inhibit operation of the system  10  or damage the structure of the building  60 . 
         [0084]    In addition to the system  10  of  FIG. 5 , additional roof vents may be provided as free vents. For example, certain small vents  16  may be provided that will provide only minimal venting of the space  22 . Thus, the system  10  operates primarily as a ducted system, as far as the air is concerned. However, in order to flush or bleed off moisture in the air and provide a small amount of makeup air, a lower vent  14  and upper vent  16  of comparatively restrictive smaller size may be provided. For clarity, the illustration of  FIG. 5  does not include vents, but the system may be comprised of vents such as those shown in the other  FIGS. 1-14 . 
         [0085]    Referring to  FIGS. 6-8  some attic space  22  may be living space. Even when the attic  64  is constructed in a way to provide only storage or vacant space, the attic  64  may be cooled. For example, in the illustrated embodiment, the vent  14   a  may feed the flow  50   a  into a vent  14   b  extending up between the rafters  56  of a roof  62 . In the illustrated embodiment, a closure  88 , such as sheet rock or other finished material may enclose a living space  22 . The living space  22  may actually be separated completely from the ducting  12 . Instead, the embodiment of  FIG. 6 , air through the vents  14   a,    14   b  may pass into ducting  12  lying above the insulation  90 . The insulation  90  may be separated by a vapor barrier and other material as part of the closure  88 . In typical construction, sheet rock backed by a vapor-liquid-impervious layer may form the closure  88  sealing the heated space  22  away from the exterior space. The ducting  12  above the insulation  90  may conduct the flow  50  upward toward an exchanger  20 . 
         [0086]    Passing through the exchanger  20 , the flow  50  gives up heat into the line  21  for exiting out through the upper vents  16 . A tank  80  (all parts and systems may be plural) may be fixed high among the rafters  56 . In one presently contemplated embodiment, the exchangers  50  may be at or below the lowest point in the tank  80 . Meanwhile, the tank  80  may be insulated  78  by or between the structures holding it. The tank  80  may be accessed by a simple plumbing system as discussed hereinabove, or as described in other art, such as U.S. Pat. No. 5,014,770 to Palmer, incorporated herein by reference. 
         [0087]    In the embodiment of  FIG. 6 , dampers  18  may be placed over the vents  14   a,    14   b , or ducts  12 , as illustrated. Likewise, the damper  18  may be fixed across or within a vent  16 . In certain embodiments, dampers may be placed in more than one location. For example, in one embodiment, the damper  18  may be placed to trigger on cold weather, thus shutting off the system when temperatures are so low outside that the system  10  will not operate properly. Meanwhile, those or other dampers  18 , such as dampers high in the system  10  may be configured to operate only when temperatures are sufficiently high to provide suitably high water temperatures. In certain embodiments, a damper  18  such as the damper  18  of FIG.  6  may be positioned high in the system  10 , such that only the existence of suitable temperatures to provide pre-determined water temperatures within the lines  21  of the exchangers  20  will open the damper  18  to allow the flow  50  to exit the vent  16 . At all temperatures lower, the vent  16  may have substantially zero flow. 
         [0088]    In certain situations, such as systems  10  that receive culinary water destined for use from a water heater, in the “off season” of winter conditions, one may take the exchangers  20  out of service valves by passing them with the culinary water and draining them. Thus, ventilation may continue past them in winter time without fear of freezing pipes or condensation within the ducting  12 . In substantially all embodiments, condensation should be considered, inasmuch as humidity is increased whenever a flow  50  of air is cooled, reducing its capacity to hold moisture. 
         [0089]    Referring to  FIG. 7 , alternative locations for the tank  80  may be not only above the exchangers  20 , but sometimes below, elsewhere in the attic  64 , or even on floors below the attic  64  or in a cellar or basement. However, to the extent that the tank  80  is positioned below the exchangers  20 , flow will not be naturally (freely) convected from the lines  21  into the tank  80 . Water will have to be pumped against the density gradient. 
         [0090]    Palmer, incorporated hereinabove by reference, discusses many systems for manipulating the flow of water by forced convection. Such may be incorporated into an apparatus  10  in accordance with the invention. Nevertheless, such systems are not the principal point of an apparatus and method in accordance with the invention. In contrast to Palmer, an apparatus and method in accordance with the instant invention may rely primarily or completely on passive, convective flows of both air and water. To the extent that a water line from a building may be connected through the tank  80  in a secondary heat exchange loop or path, or directly as a primary fluid in the line  21  of the exchanger  20 , water flow may occur by forced convection. 
         [0091]    Referring to  FIG. 8 , the tank  80  may be formed to be of any arbitrary shape for which space exists. For some roofs, the pitch angle is sufficiently large with respect to horizontal that a large (e.g., narrow triangular or otherwise) space is available, and substantially unusable for living space. Thus, a rectangular or circular tank  80  may be suspended, supported by braces, or otherwise fixed within the structure of the rafters  56 . 
         [0092]    Accordingly, it is contemplated that structural changes to the internal structure of an attic  64 , including selecting, moving, designing, attaching to, or passing around braces  55 , rafters  56 , joists  58 , and other truss members  54  may be done in order to install initially during construction, or as an after-construction modification to a building  60 , such a system  10 . 
         [0093]    Referring to  FIG. 9 , an apparatus and method  10  in accordance with the invention may also be implemented in spaces other than an attic  64 . For example, in the embodiment of  FIG. 9  a wall  61  may support a window  94  having one or more panes through which solar radiation  26  may pass. Glass, for example, provides a window  94  that will pass solar radiation, but will not effectively transmit infrared radiation back from a heated object inside the building  60  of the window  94 . 
         [0094]    In the illustrated embodiment, an exchanger  20  may be located above, behind, or near a driver  96  and may be hidden by a valance  98  in front of a curtain  100 , or by the curtain  100  alone. The curtain  100  may be of any suitable type. When solar radiation  26  is available, and particularly when undesirable to allow it into the space  22 , due to excessive lighting or heating effects, the curtain  100  may itself become the convector  32  and radiator  36 . The radiator  36  may radiate heat back to the window  94 , which heat cannot readily escape. 
         [0095]    Thus, the flow  50  may be heated by the window  94  and convector  32  (outer surface of the curtain  100 ) as the flow  50  rises. The curtain  100  and window  94  form ducting  12  or a duct  12  through which the flow  50  may rise. The curtain  100  may be of any suitable material and may be colored or treated to readily absorb solar radiation. 
         [0096]    For example, the insets show various embodiments including a series of tubes such as reeds, bamboo, plastic, or the like forming a substantially continuous curtain  100 . Likewise, certain expandable (e.g., folded) panels made of paper, polymer fabric, or the like may be extended or drawn up. 
         [0097]    Likewise, Venetian-type curtains  100  having a series of louvers may also operate successfully whether in a substantially open or closed visual configuration. For example, even if the louvers are not in their most nearly vertical orientation, then they still tend to absorb heat, and convect heat back into the flow  50 . 
         [0098]    An exchanger  20  located in the duct  12  formed by the curtain  100 , window  94 , and valance  98  may receive the flow  50  and extract heat therefrom into the contained fluid (e.g., water, etc.) heated and delivered into the tank  80 . In the illustrated embodiment, the line  42  may be the water inlet line to the exchanger  20 . 
         [0099]    In an alternative embodiment, the inlet line  42  may be provided directly to the exchanger  20  from the household water supply. In such an event, the line  99  may actually feed out to some other appliance, such as a water heater, space heater, other energy storage or manipulation system, or the like. For example, in some embodiments, the heat in the water of the tank  80  may actually be used to run a refrigeration cycle. In one embodiment, the energy provided by the heat in the tank  80  may be run through a cycle, such as, for example, the Servel cycle used in natural gas or propane refrigerators. 
         [0100]    Thus, homes  60  in hotter climates where solar energy  26  or solar radiation  26  is most abundant may benefit by a source of energy for driving refrigeration systems. In the Servel cycle, for example heat from the tank  80  may heat a boiler boiling a vapor out of a solution (an absorbed or condensed gas out of the solution). The gas or vapor and liquid may then be cooled at a location cooler than the tank  80 , such as the outdoor ambient. Though the outdoor ambient itself may be uncomfortably warm it is still cooler than the tank  80 . Subsequently, the chemical reaction of absorption of the vapor back into the solution is a net endothermic reaction, cooling the fluids now a solution. The combined, cooled solution may pass through a heat exchanger to cool an enclosed space such as a room or refrigerated space to be cooled. The fluid, now warmed by the exchanged heat, may be run through the entire cycle again. 
         [0101]    In alternative embodiments, the tank  80  may be filled at some location other than its highest point, operating as a pre-heater. As a pre-heater, the tank  80  may provide an initial quantity of energy to a flow, which flow will then be heated further by another auxiliary or principal means. By any of these methods, the net energy gained from the tank  80  resulting in the solar radiation  26  may be used to provide useful work or heating energy. 
         [0102]    Referring to  FIG. 10 , various solar collectors  102  or solar units  102  may also benefit from an apparatus and method  10  in accordance with the invention. For example, a solar unit  102  may have a cover  104  such as a flat plate, various tubes, or the like forming a convective barrier resisting loss of collected heat while providing transmission of light therethrough. Thus, the cover  104  may transmit light to a layer or surface  32  below, acting as a convector  32 . The convector layer  32  may heat up as a result of absorbing solar radiation, thus acting as both a surface absorber  28  and a convector  32 . 
         [0103]    A damper  18  may operate anywhere in a duct  12  to limit the flow of air through the apparatus  102 . Accordingly, the damper  18  controls flow through the duct  12  between the convector  32  and the cover  104 . As air enters the lower vent  14 , a flow  50  passes along between the convector  32  (e.g., absorber  28  acting also as a convector  32 ) and the cover  104 . 
         [0104]    The damper  18  controls the flow to assure that the temperature of the air flow  50  at the exchanger  20  is suitably hot to provide the desired thermodynamic availability of energy. For this reason, one of the simplest configurations may involve placing the damper  14  above or below the exchanger  20  and very close thereto. In this way, when the temperature at the exchanger  20  is at the proper temperature, then a simple wax actuator, for example, may operate to open and close the damper  18  exactly at the temperature required by the design. 
         [0105]    The upper vent  16  may be covered by a shield  106  or flashing  106 . The flashing  106  may assure a suitable degree of weather protection to prevent damage or cooling by precipitation such as rain, snow, ice, or the like. 
         [0106]    As the convector  32  heats air in the duct system  12 , below the cover layer  104  and above the convector surface  32 , the heated air rises along the angle of the roof toward the exchanger  20 . Notwithstanding the flow  50  will be cooled substantially by one or more exchangers  20  in the path of the flow  50 , the long column of hot air in the duct  12  provides sufficient buoyancy to drive the flow  50  out of the upper vent  16 . 
         [0107]    Referring to  FIG. 11 , while continuing to refer generally to  FIGS. 1-14 , a damper system  18  may include more than simply the necessary means to substantially close or obstruct a passage in a duct  12 . For example, in certain ventilation systems, a cover  108 , often having louvered slats therein, and therefore often referred to as a louver  108 , may be secured by fasteners  109  to a wall  61  or other portion of a building  60 . 
         [0108]    The cover  108  may be replaced in the illustrated embodiment by any suitable ventilation port or vent  14 ,  16 . For example, in certain embodiments, the wall  61  illustrated may actually be a soffit under the eave of a house. In other embodiments, the walls  61  may be at the end wall of a gable. In still other embodiments, the wall  61  may actually be a rafter  56  and the cover  108  may be replaced by any suitable type of cover such as a wind turbine, a turtle vent, a ridge vent, or the like. 
         [0109]    In general, a damper system  18  may operate to selectively open and close any a particular vent  14 ,  16  desired. In many embodiments, a damper system  18  may include a closure  110 , such as a vane  110  or panel  110  effectively closing a duct  12  or an opening of a vent  14 ,  16 . 
         [0110]    In one embodiment, a panel  110  or closure  110  may pivot on a hinge  111  or other pivot mechanism  111  between an open and closed position. An actuator  112  may be of any suitable type, including electrical, hydraulic, thermal, or any other type. In response to temperature, the actuator  12  may provide an infinitely variable, continuous range of positions for the panel  110  between fully open and fully closed. 
         [0111]    In one presently contemplated embodiment, the actuator  112  may be a wax actuator (e.g., wax motor), relying on the volumetric change in the wax with temperature. Contained within the apparatus  112 , expansion and contraction of the wax drive a piston  113 . Upon heating, the wax expands pushing the piston  113  in one direction. Upon cooling, the wax drives the piston in the other. 
         [0112]    The actuator  112  may be designed so the piston  113  retracts into the housing upon a rise in temperature or extends out of the housing upon a rise in temperature. This may be done by placing the thermally expanding material either in front of the piston face or behind it with respect to the direction of motion. In the illustrated embodiment, the piston  113  draws into the actuator assembly  112  as temperature increases, thus drawing the piston  113  to reduce in length, drawing the bracket  114  toward the actuator  112 , and opening the panel  110  or closure  110  of the damper system  18 . As the damper system  18  is opened, the flow  50  may enter into the enclosed space  22 . As temperature changes the volume of the wax adjusts to a new equilibrium value, acting as a thermostat. 
         [0113]    Referring to  FIG. 12 , the actuators  112  are not shown, in some instances for purposes of clarity. However, a panel  110   a  may cover a duct  12  by pivoting about a pivot  111   a  in response to operation of an actuator  112  against a bracket  114 . Likewise, as an alternative, or in addition, panels  110   b  may operate between open and closed positions by direct operation of actuators  112 . Whenever the duct  12  is closed off by any amount by any panel  110  of a damper system  18 , the flow of air is restricted. From the contained space  22  or volume  22  of an attic  64  or other heated space, the flow  50  is continuously regulated between maximum flow and substantially stopped. All heat is concentrated into the amount of the airflow  50  permitted to pass, heated from the sun and cooled by the exchangers  20 . Likewise, the comparatively cooler liquid (e.g., water) passing through the lines  21  may be restricted to obtain the maximum temperature rise available in it. 
         [0114]    Referring to  FIG. 13 , in certain embodiments of an apparatus and method in accordance with the invention, a damper system  18  may be positioned within a soffit, or over some upper vent  16 . For example, in the illustrated embodiment, a panel  110  of a damper system  18  may pivot about a hinge  111  or other pivot  111  between an open and closed position ventilating a rafter duct  12  above the insulation  90  thereof. 
         [0115]    In the illustrated embodiment, the exchanger  20  may be idled until such time as the panel  110  is opened. Meanwhile, by a proportional actuator  112 , the panel  110  may be positioned to limit flow through the duct  12  and out the vent  16 , maintaining a proper temperature in the air flow in the duct  12 . Maximum temperature differences provide maximum thermodynamic availability of heat to the air flow  50 , the exchanger  20 , and ultimately to the liquid flowing in the line  21  therein. The damper system  18  may be installed near the upper vent  16 , lower vent  14 , or elsewhere in a duct  12 . 
         [0116]    Referring to  FIG. 14 , various arrangements of storage and use of liquid, such as water, from the exchangers  20  and tank  80  may be implemented. In certain embodiments, a tank  80  may be located high in the attic space  24  of a building  60 . In the schematic diagram of  FIG. 14 , the tank  80  may be disposed in any suitable manner. Typically, the tank  80  will be configured to be above the exchangers  20 . Absent some forced convection imposed on the input line  42 , the exchanger  20  is best located below the top of the tank  80 . To heat the entire tank to the maximum uniform temperature the exchanger  20  is best located entirely below the bottom of the tank  80 . 
         [0117]    Meanwhile, the exchanger  20  may feed the heat transfer fluid flow  82  from the exchanger  20  through the outlet line  44  into the tank  80 , and out the exit line  45  of the tank. As illustrated, various configurations of tanks  80  may be implemented in accordance with the invention. If the tank  80  is in the direct flow of incoming water for the building  60  such as a house  60 , the flow  82  may actually be driven from the city water supply. Accordingly, such an exchanger  20  and tank  80  must accommodate the line pressure of the building  60 . 
         [0118]    In other embodiments, the line  44  incoming into the tank  80  may simply pass heat on to another heat exchanger  118  in the tank  80 , thus heating the water in the tank  80 , before exiting out a line  45 . In such a configuration the flow  119  into and out of the tank  80  is effectively operating in a secondary loop. Thus, the flow  119  may come from the building water supply, while the flow  82  in such a configuration may actually operate in a closed loop with the exchanger  20 . In such a configuration, the flow  82  may be treated with antifreeze, of any chemical variety from salt to glycol to alcohol, and need not even include water. For example, oil, alcohol, and the like may serve well year round. The exchanger  118  provides heat exchange between the heated flow  82  from the exchanger  20  and another fluid such as culinary water or other energy destination. 
         [0119]    In certain embodiments, an auxiliary container such as an extra tank  120  may be used to accumulate heated or preheated water. The tank  120  may be a water heater  122  or may be a reservoir  120  located upstream of the water heater  122  itself. Thus, any amount of heat already captured in the tank  120  may be passed to the water heater  122 , thus reducing the amount of heat the water heater  122  must add. 
         [0120]    In one embodiment, a unit  124  may control the flow  132 . Typically, a unit  124  may include a valve  126  if the flow  132  is coming from the water supply for the house  60  or building  60 . Accordingly, the unit  124  may simply be a valve providing access to water flow  132 . 
         [0121]    In other embodiments, the exchanger  20  may be located other than above the tank  80 . For example, where the exchanger  20  may be high in the space  22 , while the tank  80  is simply replaced by the tank  120  therebelow, forced convection would require a pump  128  in a closed system, or a valve  126  in a system open to regularly receive water from the water supply of the building  60 . 
         [0122]    In the illustrated embodiment, the flow  132  flows up through the input line  42  to the exchanger  20  to be heated. The exchanger  20  then provides the flow  132  out through the exit line  44  into the tank  80 , if present, or directly to a tank  120 . With or without a secondary heat exchanger  118  in the tank  80 , flow through the exit line  45  from the tank  80  may provide a source of hot water, or water as a preheated liquid into a storage tank  120 . Thereafter, the flow  132  may transfer directly to the water heater  122  as the flow  134  therethrough. The flow  134  may be the regular supply of water for the use of the building  60  such as a house  60 . 
         [0123]    In one simple embodiment, the household of water supply may pass through an inlet line  42  to an exchanger  20 . The exchanger may be located in ducting  12  receiving heat collected through and from the conductor  24 . Heat may be passed therethrough by the convector  32  into the air flow  50 . 
         [0124]    In one of the simplest systems to maintain, the exchanger  20  may operate in a closed loop through the tank  80 , and thus rely on a closed loop flow protected against freezing, and therefore never needing to be emptied. In such an embodiment, the flow  82  through an exchanger  118  may exchange water with the tank  80 . The tank  80  may be the tank  120  or feed it. So long as all the lines are properly insulated, the flows  119  from the water supply of the house may run year round. The flow  82  from the closed loop heat exchange cycle need not operate except when effective to augment or supply the heat needed by the tank  80 . 
         [0125]    It may be seen that an apparatus  10  in accordance with the invention and a method  10  in accordance with the invention may operate to constrict flows operating by free convection involving air in a volume  22  receiving heat through a roof  62  of a building  60 . Accordingly, the roof  62  itself may become a passive solar collector and heat exchanger transferring heat from an absorber surface  28  through a conductor  24  constituted by the roofing materials. The convector  32  surface of the roof eventually convects heat out therebelow. 
         [0126]    A flow  50  of air may be substantially enclosed, or may be open to the outdoor environment. By proper use of a damper system  18 , the flow  50  may be constricted to optimize the thermodynamic availability of heat, increasing the temperature in the highest strata of the stratified air in the space  22 . 
         [0127]    By recognizing the appropriate heat transfer mechanisms within the space  22  heated by the conductor  24  and convector  32  (and to a lesser extent by the radiator  36 ), provides substantially improved heat transfer, higher temperatures, and higher thermodynamic availability. 
         [0128]    In certain embodiments, the flows through the system  10  may actually operate in parallel to feed hot water when available and to cease operation when neither sufficient nor economical. Nevertheless, in most embodiments, an apparatus  10  and method  10  in accordance with the invention may typically provide substantial additional energy even if only preheating domestic hot water prior to entry into a water heater  122 . So long as the energy input from the convector  32  into the flow  50  is sufficient to heat a water flow  132  coming into a building  60 , economical preheat may be received into the flow  132 . 
         [0129]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.