Abstract:
An apparatus and method for radon mitigation using natural convection to remove radon-contaminated air from beneath the slab foundation of a structure, building, or dwelling, comprising a section of vertically mounted convection duct including an internal heat source, an inlet duct extending from said convection duct through the slab into the gas permeable sub-slab layer, an outlet duct extending from said convection duct through the structure and out the roof thereof, the internal heat source comprising a thermally conductive tube disposed concentrically within said convection duct and heated by one or more electrical heater elements to assist the upward airflow of the radon-contaminated air from the structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/773,076 filed on Feb. 5, 2004, which claims priority from U.S. Provisional Patent Application Ser. No. 60/445,135, filed on Feb. 5, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to devices for radon mitigation in homes and other structures. More specifically, the present invention relates to an improved system for ventilating the foundation of a structure to remove radon gas so that it does not accumulate inside the structure.  
         [0003]     Radon is a radioactive gas generated by the natural (radioactive) decay of the uranium that is found in nearly all soils, and can be found all over the United States. The breakdown of uranium in soil, rock and water releases radon into the air we breathe. Radon can get into any type of building—homes, offices, and schools—and cause an elevated indoor radon level. It typically moves up through the ground to the air above and into a dwelling or other structure through cracks, fissures and other holes in the foundation. The structure traps radon inside, where it can accumulate if the structure is not well ventilated. A radon problem can exist in any home, regardless whether the home is new, old, well sealed, or drafty, and may even occur in a home without a basement.  
         [0004]     Like other environmental pollutants, there is some uncertainty about the magnitude of radon health risks. However, we know more about radon risks than risks from most other cancer-causing substances because of studies that have been done of cancer in underground miners.  
         [0005]     One method of radon mitigation in new home construction includes placing a gas permeable layer beneath the concrete slab of the foundation. This layer typically includes 4″ to 6″ of gravel. A polyethylene or equivalent flexible sheeting material is placed on top of the gas permeable layer. At one end of the slab, a hole is cut in the flexible sheeting material. A section of PVC pipe, typically of 3″ or  4 ″ diameter, extends vertically upward from the gas permeable layer, through the hole in the flexible sheeting material and slab, and through the internal space of the structure and out through the roof. At its bottom end, the PVC pipe is supported by a pipe tee located within the gas permeable layer. An inline fan may be installed in the PVC piping, usually in the attic or sometimes outside the structure.  
         [0006]     If a fan is installed, it draws the air up through the bottom inlet (located within the gas permeable layer) and creates a vacuum in the gas permeable layer. The air is exhausted from the outlet of the PVC pipe above the roof. Because this system usually does not have a fresh air input into the gas permeable layer below the slab, airflow is restricted. The effectiveness of this system relies largely on reduced air pressure within the column, i.e., the vacuum created by the fan within the vertical pipe, to pull the radon gas out. The reliance of this method on a fan causes problems with noise and condensation building up on the inside of the pipe. The condensation then drips down on the fan, increasing the noise. Because of the mechanical nature of the fan, its life span is limited.  
         [0007]     Various examples of existing radon mitigation systems are known. U.S. Pat. No. 4,988,237 [Crawshaw] discloses a fan-driven sub-slab ventilation system having an attic mounted fan unit, a sub-slab inlet, and an above-roofline outlet. U.S. Pat. No. 6,524,182 [Kilburn, et al.] discloses a fan-driven system having a sump well inlet and a band-board outlet that is suboptimal, since it does not get the radon-contaminated air sufficiently away from the dwelling. U.S. Pat. No. 4,885,984 [Franceus] discloses a fan-driven system with a sub-slab inlet and an externally mounted fan with an external vertical vent pipe. U.S. Pat. No. 4,922,808 [Smith] discloses a fan-driven system with an inlet drawing air directly from the basement of a house and a band-board outlet. However, all of these fan-inclusive devices suffer from the drawback of having the fan, a mechanical component that is prone to failure. Also, these devices do not heat the cool air being drawn from the sub-slab region (or the basement) and are therefore prone to develop condensation in the outlet piping.  
         [0008]     The United States Environmental Protection Agency Publication EPA/402-K-01-002 (April 2001) entitled “Building Radon Out: A Step-by-Step Guide on How to Build Radon-Resistant Homes” discusses the preparation of building foundations for use with sub-slab ventilation systems and discloses both passive (no driving force for flow other than the free convection stack effect of having the exhaust pipe pass through the heated interior structure space) and active (fan-driven) sub-slab depressurization systems. However, neither these patents nor the EPA Publication suggests the use of an internally heated duct to drive a natural convective flow in a sub-slab depressurization system comparable to that utilized in the present invention.  
         [0009]     Various examples of devices for creating free convection in pipes, all distinguishable from the application of the present invention, are known. U.S. Pat. No. 1,389,252 [Lucas] discloses a heat driven forced draft ventilator which includes a high temperature radiative heating element mounted inside an enlarged section duct which heats the walls of the duct in order to drive the upward flow of heated air. In contrast, the present invention uses a low temperature heating element that will not significantly heat the walls of the duct in which it is mounted. In fact, it is critical that the device of the present invention not heat the duct walls significantly since they are typically made from PVC and not metal. Similarly, the electric heater disclosed in U.S. Pat. No. 1,401,500 [Scott] is intended to be heated to near the point of incandescence (to a point of dull red heat) and therefore will heat not only the air in the duct but also the walls of the duct. For that reason, this device also would not be compatible with PVC piping. U.S. Pat. No. 1,759,830 [Blanchard] discloses a resistance heater that is in contact with and intended to directly heat the metal walls of a chimney cap, again an application that is incompatible with creating an updraft of airflow in PVC piping.  
         [0010]     Somewhat less relevant is U.S. Pat. No. 6,141,495 [Roth], a fan-powered device with an electrical heating element designed to pre-heat a flue. Because the flow in this device is generated by a fan, it does not rely on natural convection. Also distinguishable is U.S. Pat. No. 1,699,739 [Kercher, et al.] that describes a natural convection heater located below the room to be heated. This device relies on the natural convective flow of higher density cool air from the room down ducting to the heater element as well as the natural convective flow of lower density warmed air back up to the room. Therefore, it does not anticipate that an internally heated duct may be used to create a negative pressure at its inlet to actually draw higher density cooler air up into the heated section as is done in the radon mitigation device of the present invention. The pipe heaters of U.S. Pat. No. 4,524,262 [Meyer] and U.S. Pat. No. 1,273,666 [Powers] are also inapposite to the present invention since each applies heat directly to the ducting and neither with the purpose of creating upward flow in the ducting.  
         [0011]     Accordingly, it is an object of the present invention to provide a radon removal device that eliminates the noise and limited lifespan that results from the use of a fan. It is another object of the present invention to provide a radon removal device that increases the temperature of the air being exhausted in order to reduce or eliminate condensation from forming on the inside of the exhaust pipe, but does not excessively increase the air temperature or the temperature of the ducting itself so as to require a temperature control device. It is a further object of the present invention to provide a radon removal device that is more effective at removing the gas from beneath the slab of a building foundation for the reason that it is not susceptible to mechanical failures of the fan-based, vacuum-creating devices currently in use.  
         [0012]     Other objects will appear hereinafter.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a simple, low-maintenance device for actively drawing radon-contaminated air from beneath the foundation of a structure, thus mitigating the risk that radon will enter into and accumulate inside of the structure. By using an internally heated section of duct to create a convective flow of air upward from the gas permeable sub-slab layer, expelling the radon-contaminated air above the roof of the structure, the device of the present invention eliminates the need for a fan. Therefore, the device of the present invention is quieter and less prone to failure than a similarly installed fan-powered radon mitigation device. Additionally, because the device of the present invention heats the air that is being withdrawn from beneath the slab, condensation within the ducting is reduced. The heat source utilized in the present invention is of low wattage so that the temperature of the withdrawn air is maintained within the capabilities of the PVC outlet piping and no temperature control mechanism is required. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     For the purpose of illustrating the invention, there is shown in the drawings forms which are presently preferred; it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
         [0015]      FIG. 1  is an isometric view of the radon mitigation heater pipe of the present invention.  
         [0016]      FIG. 2  is a bottom view of the radon mitigation heater pipe of the present invention, looking upward.  
         [0017]      FIG. 3  is a sectional view taken along Line  3 - 3  of  FIG. 2  showing the interior components of the radon mitigation heater pipe of the present invention.  
         [0018]      FIG. 4  is a diagrammatic view of the use of the radon mitigation heated exhaust pipe and associated air inlet pipe of the present invention installed through and beneath the concrete slab foundation of a building. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The following detailed description is of the best presently contemplated mode of carrying out the invention. The description is not intended in a limiting sense, and is made solely for the purpose of illustrating the general principles of the invention. The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings.  
         [0020]     Referring now to the drawings in detail, where like numerals refer to like parts or elements, there is shown in  FIG. 1  an exterior view of the radon mitigation device  10 . The device is comprised of an external shell portion  12  and an internal heating portion  31 .  
         [0021]     The external shell portion  12  is fabricated by welding several components together; however a one-piece metal casting would be a preferable construction technique for a mass produced device. The base  14  is made from a round flat plate or flange having a centrally disposed hole matching the outer diameter of the lower section  16  of the external shell portion  12 . The base  14  has a plurality of mounting holes  18  for securing the device  10  to the foundation or flooring of a building. In one embodiment, the base  14  is formed from ⅛″ thick steel and is 9″ in diameter with a centered  6 ″ diameter hole. The lower section  16  of the external shell portion  12  is a length of 6″ diameter metal pipe approximately two feet long that is received into the 6″ diameter hole in the base  14 .  
         [0022]     Since the typical exhaust pipe of a radon mitigation system is either 3″ or  4 ″ in diameter, a reducing section  20  and an exhaust pipe section  22  provide a transition from the lower section  16  of the external shell  12  to that smaller diameter. The reducing section  20  may be made from ⅛″ thick steel having an outer diameter of 6″, with a centered hole nominally  4 ″ in diameter for receiving the 4″ diameter exhaust pipe  22 . The exhaust pipe  22  may be manufactured from a short (approximately 6″ long) piece of schedule  40  steel pipe. Typically, the exhaust pipe  22  is coupled to a 4″ diameter PVC (polyvinylchloride) outlet pipe  24  via a flexible rubber coupler  26 , as shown in  FIG. 4 . The PVC outlet pipe  24  extends vertically from the exhaust pipe  22  and is preferably routed up through the heated interior space of the dwelling or structure and out through the roof. The base  14 , the lower section  16  of external shell  12 , the reducing section  20 , and the exhaust pipe  22  may be welded together or may be cast as a unitary piece.  
         [0023]     As is illustrated in  FIGS. 2 and 3 , an aluminum draft pipe  30  is disposed inside the lower section  16  of external shell  12 . The draft pipe  30  is suspended concentrically and co-axially within the lower section  16  of external shell  12  by a mounting bracket  32 , a support bar  34 , and one or more mounting arms  36 . The bracket  32  is welded to the inner wall of the lower section  16  of external shell  12 , near the lower end thereof. The support bar  34  is fastened to the bracket  32  by two or more screw-like fasteners  33  that pass through the support bar  34  and thread into tapped holes in the bracket  32 . The support bar  34  extends vertically upward inside of and parallel to the external shell  12 . The draft pipe  30  is suspended from the support bar  34  by the mounting arms  36  that extend through the pipe  30  and are threadedly secured to the support bar  34  by nuts  37 . Spacing nuts  35  are used to appropriately position the draft pipe  30  at the approximate center of the external shell  12 . Other equivalent mechanical means may be used to secure the draft pipe  30  to the support bar  34  and properly position the pipe  30  within the external shell  12 .  
         [0024]     In one preferred embodiment, the bracket  32  has the dimensions of about 1-½″ in width, 3″ in height and ¼″ in thickness, having two tapped holes. The vertical support  34  has the dimensions of about 1″ in width, 16″ in length (vertical dimension) and ¼″ in thickness. The aluminum draft pipe  30  is preferred to have a 2″ diameter and be about 16″ long, with a wall thickness of approximately 1/4″.  
         [0025]     The draft pipe  30  is electrically heated by at least one band heater  40 . The band heater  40  clamps around the draft pipe  30  and is powered via an electrical cord  42  plugged into a standard grounded wall outlet. A bracket  44  is welded to the inside wall of the external shell  12  to provide a grounding lug  46  for the ground lead of the electrical power cord  42 . The power and neutral leads of the electrical cord  42  are connected through a connector  48  mounted in the external shell  12  to opposite ends of the band heaters  40 .  
         [0026]     In a preferred embodiment, the bracket  44  has the dimensions of about 1-½″ in width, 3″ in height and ¼″ in thickness and includes a tapped hole for receiving the grounding lug  46 . The band heaters  40  have dimensions of about 1-½″ in length and an inner diameter allowing it to be clamped securely around the 2″ diameter draft pipe  30 . The band heaters  40  are powered by 120 VAC and produce approximately 75 watts of power that is converted to heat. The electrical cord  42  is a high temperature three-conductor cable with a standard three-pronged grounded plug at one end to fit a standard grounded wall outlet. The electrical cord  42  passes through the wall of the lower section  16  of external shell  12  via a sealed connector  48  which provides a reinforced mount and strain relief for the electrical cord  42  extending outwardly from the wall of the lower section  16 .  
         [0027]      FIG. 4  is a simplified diagrammatic view of a typical installation of the radon mitigation device  10  in a concrete basement slab foundation. Unless otherwise noted, all PVC pipe and connections are typically 4″ in diameter. On top of the ground  52 , a gas permeable layer  54  is normally laid to a depth of 4″ to 6″, comprising crushed stone between about ½″ and about 2″ in size. A polyethylene or equivalent flexible sheeting material  56  is placed on top of the gas permeable layer  54 , extending to the walls at the perimeter of the foundation of the building. On top of the flexible sheeting material  56 , the concrete slab  58  is poured to complete the foundation of the building.  
         [0028]     At the lower end of the radon mitigation device  10 , a PVC tee  60  sits in the gas permeable layer  54  with two of its openings oriented horizontally and one opening oriented vertically upward. The horizontal openings of the PVC tee  60  may have perforated PVC piping extending within the gas permeable layer  54  up to five feet in either direction to improve gas flow into the radon mitigation device  10 . A short piece of PVC pipe extends from the upward-facing opening of the PVC tee  60  to be flush with the top of the slab  58 , passing through a hole in the flexible sheeting material  56 . Optionally, a second PVC tee  60 A may be similarly placed in the gas permeable layer  54 , the upward-facing opening thereof connecting to an air inlet pipe  62  which extends through the hole in the flexible sheeting material  54  and to the outside of the structure to provide fresh air to the sub-slab gas permeable layer  52  as air is drawn out from below the slab  58  by the radon mitigation device  10  in order to maintain a positive airflow pressure. Above the radon mitigation device  10 , outlet PVC pipe  24  is attached to exhaust pipe  22  by a flexible coupling  26  and extends vertically to direct the radon-contaminated air out above the roof of the structure.  
         [0029]     In operation, the band heaters  40  provide electrical power to heat the draft pipe  30 . Since the draft pipe  30  is aluminum, or another material with a high heat conductivity, the draft pipe  30  reaches a relatively uniform and stable temperature along its length. The band heaters  40  are sized appropriately to keep the temperature of the draft pipe  30  low enough that the resultant exhaust gas is not too warm to be handled by the downstream PVC pipe  24 . In a preferred embodiment, one band heater  40  provides 75 watts. The advantage of such a low wattage band heater  40  is that it can remain on 100% of the time and need not be cycled on and off, thus improving the lifespan of the band heater  40  and saving cost that would be incurred by the requirement for a temperature measuring and switching unit. More than one band heater  40  may be used to provide the required heating of the draft pipe  30 .  
         [0030]     The heated draft pipe  30  creates a natural convective flow of air in the upward direction, along both the inner and outer walls thereof, at an approximate flow rate of 37 cfm. To the extent that the inside structure temperature may be warmer than the temperature of the draft pipe  30 , supplemental natural convection may be gained as the air passes through the exhaust pipe  24  on its way out of the structure.  
         [0031]     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, the described embodiments are to be considered in all respects as being illustrative and not restrictive, with the scope of the invention being indicated by the appended claims, rather than the foregoing detailed description, as indicating the scope of the invention as well as all modifications which may fall within a range of equivalency which are also intended to be embraced therein.