Abstract:
A thick film, large area resistance heater including a substrate having an electrically non-conductive surface on which is deposited a film electrical resistor such as a thermally sprayed, photo resist etched foil or sol-gel graphite based material. A combination of an electrically conductive film coated backer board substrate composed of portland cement, sand, cellulose fibers and other selected additives. A mica substrate heater can be cemented to a cement backer board or a vinyl with adhesive backing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to heaters and in particular to a method of heating rooms and spaces through radiant heat from floors, walls and ceilings. There are various known systems and methods used for heating floors. Some include circulating heated water or air through a piping system installed beneath the surface of the floor ( FIG. 1 ). Others include electrically heated insulated wires that are enmeshed in a material placed in between the floor surface and the sub floor, or in a concrete layer that also serves as the finished floor. In other instances a cement board is attached to the sub floor and a finish material such as tile, linoleum or wood are adhered to the cement board. In those systems efficiency is compromised as there is no intimate contact between the source of heat and the surface to be heated. This invention overcomes these shortcomings of the prior art, and provides an improved system for heating surfaces such as floors and walls. 
       SUMMARY OF THE INVENTION 
       [0002]    This invention meets the need for a more efficient, space saving, cost efficient and energy saving method to heat floors, walls, ceilings and surface areas such as countertops. In one preferred embodiment, a resistive film such is applied directly to a backer board by means of spraying, painting or silk screening where the tile or outer surface material is to be applied. 
         [0003]    The width and thickness of the resistive film is selected to provide the desired power as expressed in watts per square inch or watts per square foot. In embodiments utilizing a resistive material, the material requires a firing process for curing. During the curing process it is necessary to control the process so that the cement backer board is not heated sufficiently to degrade the materials within the backer board. In one preferred embodiment the resistive film is cured by the use of infra-red heat processing equipment. 
         [0004]    In another preferred embodiment the resistive film can be patterned onto an insulative material substrate such as mica, and the mica interposed between the subfloor and the finish surface material such as tile or linoleum. In this embodiment the mica is installed by the use of an adhesive applied to the backer board and to the finish surface material. The mica can be in the form of pre-cut tiles with the resistive material patterned onto each tile. 
         [0005]    After the resistive material is applied and cured, spaced apart electrodes or bus-bars are applied to apply a voltage across the patterned resistive material. A protective coating such as Teflon® or silicon is then applied over the resistive material to protect it from moisture and to provide an electrically insulative layer. 
         [0006]    In another aspect of the invention, overheating can be prevented by the use of a temperature sensor embedded in or placed atop the floor assembly. The sensor sends a signal to a controller that reduces or cuts power if a maximum temperature is reached or exceeded. These and other features of the invention will be described below and in reference to the drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional view of a prior art radiant heating assembly. 
           [0008]      FIG. 2  is a top plan view of a preferred embodiment of the invention. 
           [0009]      FIG. 3  is a cross-sectional view of the embodiment shown in  FIG. 1  shown as part of a floor assembly. 
           [0010]      FIG. 4  is a cross-sectional view of the embodiment shown in  FIG. 4  and which also includes a decorative top layer over the heating unit. 
           [0011]      FIG. 5  is a top plan view of a second embodiment of the invention which uses a different interconnect. 
           [0012]      FIG. 6  is a top plan view of the embodiment of the invention shown in  FIG. 2  in which multiple individual units are shown interconnected. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring now to  FIGS. 2-4 , a preferred embodiment of the invention is shown generally at  10 , and includes a backer board substrate  12 , a patterned resistive material  13  disposed on the substrate  12 , and interconnects  14  and  16 . Backer board may be any of a number of materials, but in the preferred embodiment it is formed of a cementous material, and which is designed to underlay tile or other floor finish materials. One such product is known as Hardy Board®. 
         [0014]    In one embodiment the patterned resistive film is a graphite impregnated sol-gel material such as that manufactured by either ThermoCeramix, Inc. of Shirley Mass. or Datec Coating Corporation of Milton, Ontario, Canada. The resistive material is applied directly to the backer board through a means of spraying, painting or silk screening onto the surface of the substrate that will support the tile or other outer finish material. Other methods of applying a resistor include the thermal spraying of the resistive material. The resistive material could be a continuous layer covering the entire surface, but is preferably applied in a pattern to reduce the amount of material required to provide the necessary power. 
         [0015]    In thermal spray, a material in powder or wire form is fed to a heat source where it is melted into fine droplets. The heat source can be created by combustion of fuel gases, an electric arc, or ionized plasma. The droplets are accelerated with a carrier gas and directed towards a prepared surface. The droplets impact the surface and freeze instantaneously. By traversing the spray apparatus repeatedly over the surface, a coating is built up. 
         [0016]    The coatings are deposited using resistive metals or electro conductive ceramics. When metals are used, which is the case when the substrate is mica, the metal is melted in a conventional thermal spray system and subjected to a reactive gas such as oxygen when the metal is in the molten state. The metal forms reaction products such as metal oxides that are incorporated into the deposited coating. The coating will then comprise the free metal starting material together with some proportion of metal oxides that will tend to boost the coating resistivity. In this way, a heater with substantially increase resistivity is formed into a coating. 
         [0017]    Heater starting materials are typically nickel-chrome alloys, iron-chrome alloys, titanium, titanium oxide or zirconium diboride. The heater coating is typically designed to form a pattern that determines the electrical resistance by balancing a combination of the geometric factors of element path length, element thickness, and element width with material factor of element resistivity. 
         [0018]    The resistive film is selected to embody a resistance value that provides the necessary power as expressed in watts per square inch or watts per square foot. This sheet resistance value is affected by a combination of material formulation and thickness of the sol-gel as is well-known to those of skill in the art, including the manufacturers of the material. 
         [0019]    In most instances after the sol-gel resistive material is applied, it must be cured to a finished state by heating. The parameters of the curing process vary according to the resistive material selected, and the invention is not limited to any particular curing process. Those of skill in the art will appreciate the heating/curing process must be controlled so to adequately heat the uncured resistive material on the surface of the cement backer board and also so as not to affect the composite materials within the underlying backer board. One preferred curing method employs the use of an infra red heater, which is particularly well-suited as it can be readily controlled to heat primarily the resistive film and surface of the backer board without overheating the body of the backer board. 
         [0020]    After the resistor material is applied and cured, spaced apart electrodes or bus-bars are applied to apply a voltage across the resistor. A protective coating  15  such as Teflon® or silicon is then applied over the resistor to protect from moisture and to provide an electrical insulation. 
         [0021]    An important feature of the invention with the use of the cement backer board is that the backer board provides an excellent means of connectivity. Affixing fasteners such as threaded screws can be utilized to make safe and secure electrical connections. The backer board serves as an excellent heat and electrical insulator in the invention. 
         [0022]    The use of mica coated heaters can also be utilized by placing the mica heater between the cement backer board and the outer material  18  such as tile, linoleum or laminate as shown in  FIG. 4 , all of which are typically supported on a subfloor  17 . This method requires an additional adhesive layer; one layer  19  adhering the mica to the backer board, and one adhesive layer  20  adhering the top layer  18  to the mica surface. This method is still preferable to the existing products using heated water or wire woven fabrics that take up space and waste energy. 
         [0023]    Referring to  FIG. 7  individual tiles are assembled into a heating assembly by placing two tiles adjacent each other with their respective electrical contacts  15  and  16  overlapping. In the embodiment shown the connectors are connected by screw  23  which is driven through the contacts and into the underlying backer board. In alternative embodiments the contacts could also be connected by adhesives or in any other suitable manner. 
         [0024]    Referring to  FIGS. 5 and 6  another embodiment is shown at  50 . In this embodiment the resistive layer  52  is a rectilinear layer rather than a pattern as in the first embodiment. In this embodiment the contacts  54  and  56  are in the form of long conductive strips that are placed in contact with the resistive material and held in place by a conductive adhesive. The individual tiles are assembled into a floor by placing the edges adjacent one another and interconnecting the conductive strips  54  and  56 . 
         [0025]    In the embodiments described above the individual heating units have been connected in series. Heating units receive current from one adjacent heating unit and provide current to another adjacent heating unit by connection of contacts as described above. In other embodiments the heating units are connected to the power source in a parallel arrangement. The advantage of connecting the heating units in parallel is apparent—the failure of a single heating unit will not adversely affect the remaining heating units. The parallel connection of heating units can be achieved in any suitable manner, and various arrangements for doing so are well-understood by those of skill in the art. The key distinction is that the electrical contacts on each heating unit are connected directly to an electrical supply rather than through an adjacent heating unit. This method of connection can also provide additional advantage in that a lower voltage is required to power the heating units. 
         [0026]    In one such embodiment individual heating units are adhered to a backer board as described above. Rather than each unit being electrically connected in series to the adjacent units, each unit is electrically connected in parallel to a pair of transverse buses. Referring to FIG. ______, in one embodiment the heating unit includes a number of individual heating units adhered to the surface of an underlying support. A series of alternating power and ground electrical buses ______ extend across the support and connect to supply and ground conductors ______ and ______. Each of the individual heating units is connected in parallel to the electrical buses. In this embodiment the transverse electrical buses are connected to a pair of conductors - - - and - - - , one on each side of the panel. In this embodiment each lateral row of heating units is connected in parallel, preventing a significant loss of heating capacity in the event of a failure of one of the heating units. 
         [0027]    While the invention has been described in terms of the preferred embodiments, those of skill in the arts will appreciate that those embodiments can be varied in detail and arrangement without departing from the scope of the invention.