Patent Publication Number: US-2018051903-A1

Title: Apparatus for inhibiting airflow through a conduit

Description:
FIELD 
     This disclosure relates generally to apparatus for inhibiting airflow through a conduit, and more specifically to apparatus for inhibiting airflow through a supply duct when a supply fan upstream of the apparatus is turned off or otherwise disabled. 
     BACKGROUND 
     Ventilation or airflow systems are used to convey air into, out of, and/or within residential, commercial, and/or industrial buildings. For example, most residential buildings have a ventilation system for drawing in, circulating, and exhausting air at one or more locations within the building. In multi-unit residential dwellings, each unit may have its own independent dedicated fresh air intake and air exhaust system. Such an intake air system is usually connected to a hydronic air conditioning appliance. 
     Typically, ventilation systems include at least one flow control mechanism, for example an airflow damper for inhibiting outside air from freely entering the building and/or the ventilation system. Such entry of air could be particularly undesirable in colder climates if the incoming air is not tempered, and may occur if the ventilation system is not forcefully drawing in or exhausting air, for example, while a supply fan is turned off, cycling at a low airflow setting, or malfunctioning. 
     Once a ventilation system has been installed and covered by interior wall and/or ceiling finishes, accessing the ventilation system for service or repair can be time consuming and expensive. 
     SUMMARY 
     The following summary is provided to introduce the reader to the more detailed discussion to follow. The summary is not intended to limit or define any claimed or as yet unclaimed invention. One or more inventions may reside in any combination or sub-combination of the elements or process steps disclosed in any part of this document including its claims and figures. 
     A ventilation system for a residential building and/or an individual residential unit within a multi-unit building may include one or more devices or apparatus for treating or conditioning the air to provide an environment with desired air characteristics, such as temperature, humidity, cleanliness, etc. For example, a heat recovery ventilation system for tempering air in colder climates may include a cross-flow or counter-flow heat exchanger between a fresh air intake and an exhaust air outlet. 
     Residential ventilation systems may include a motorized supply fan to induce an airflow of fresh air into an air conditioning unit for the building or unit in which the ventilation system is installed. In heat recovery ventilation systems, such a supply fan (alternatively referred to as a supply blower) is often located downstream of a heat exchanger. 
     It is often desirable to inhibit fresh air from freely entering the building and/or the ventilation system. For example, particularly in colder climates, it may be desirable to selectively inhibit very cold air from entering the building or unit. If cold air is allowed to flow unimpeded through an air intake or outlet conduit, this may lead to uncomfortably low temperatures in the building or unit. Alternatively, or additionally, devices or appliances within the ventilation system may be damaged by undesired and/or prolonged exposure to cold air. For example, a hydronic coil of an air conditioning unit may freeze as a result of undesired exposure to very cold airflow. 
     In some residential ventilation systems, a motorized damper (e.g. a motorized spring return control damper) may be provided to selectively inhibit outside air from flowing into the building or unit. For example, a motorized damper may be configured to close in response to low temperature air being detected (e.g. as determined by an air temperature sensor). Alternatively, or additionally, a motorized damper may be configured to close when a supply fan or blower is not operating in order to control the rate of airflow into the building or unit. Alternatively, or additionally, a motorized damper may be configured to close if a failure of the supply fan is detected. Typically, such a motorized control damper will be located adjacent the air intake conduit, and in heat recovery ventilation systems, upstream of the heat exchanger. 
     Using a motorized control damper to inhibit outside air from freely entering the building and/or the ventilation system may have one or more disadvantages. For example, such a damper may require separate and/or additional control electronics, which may increase the cost, complexity, and/or failure rate of the control damper. For example, a separate temperature sensor to detect very cold air may be required. Also, if the motorized control damper is a normally closed damper (i.e. supplying power to the motor opens/holds open the damper, while removing power from the motor results in the damper closing), this may require additional power to be supplied to the ventilation system, lowering its overall efficiency and/or its reliability. 
     Also, if the supply fan or blower is periodically cycled into a low or off condition, and the motorized control damper is not closed during periods in which the fan is cycled to low or off, there may still be a relatively uninhibited flow path for exterior air to enter the building or unit, particularly where there is a significant pressure difference between the exterior or fresh air and the air within the building or unit. For example, if the outside air is colder than the inside air, there will be a tendency for the cooler (relatively higher pressure) air to flow into the warmer (relatively lower pressure) building or unit—such a pressure-induced airflow will be greater if the fresh exterior air is colder, potentially exacerbating an undesirable situation. 
     As disclosed herein, an apparatus for inhibiting airflow through a conduit has a frame and a damper blade. The damper blade is moveable between a closed position and an open position, and mechanically biased towards a substantially closed position. A magnet secured to the damper blade is configured to pull the damper blade from the substantially closed position to the closed position, and to releasably retain the damper blade in the closed position. The apparatus is configured so that when a supply fan positioned upstream of the apparatus is not operating (or otherwise fails to supply a predetermined airflow pressure against the damper blade), the damper blade moves to and is maintained in the closed position. The apparatus is also configured so that while a supply fan positioned upstream of the apparatus is turned on (or otherwise increases the airflow pressure it provides above another predetermined threshold) while the damper blade is in the closed position, the magnetic retention is overcome and the damper blade moves to and is maintained in the open position. 
     Preferably, the frame is made of a ferromagnetic material, and the damper blade is made of a paramagnetic material. It has been determined that using a ferromagnetic frame and a paramagnetic damper blade facilitates the provision of a lower magnetic ‘break force’ required to release the damper blade from the closed position. This facilitates the configuration of the apparatus so that the damper blade closes (and remains closed) when the supply fan is not operating, while allowing the damper blade to be reliably opened using airflow pressure generated by the supply fan. 
     The apparatus and methods disclosed herein may have one or more advantages over the use of a motorized spring return control damper. For example, the apparatus does not require a power source or control electronics, instead opening and closing in response to airflow pressures within the ventilation system, such as airflow pressures generated by an upstream supply fan. Also, the apparatus may be factory calibrated based on its expected operating environment, such as the size and power output of an adjacent supply fan. Also, the apparatus may require little or no servicing or maintenance once installed. Also, the apparatus may offer improved reliability over a motorized control damper. Also, the apparatus may be more economical to manufacture, install, and/or maintain. 
     In accordance with a broad aspect, there is provided an apparatus for inhibiting airflow through a conduit, the apparatus comprising: a frame configured for mounting in fixed relation to the conduit in a position a first distance downstream of an airflow fan, the frame having an upstream face and a downstream face, the frame defining a central aperture from the upstream face to the downstream face; a damper blade pivotally secured to the frame and moveable between a closed position in which a perimeter of the damper blade abuts the downstream face of the frame and airflow through the aperture is inhibited and an open position in which the damper blade is spaced apart from the downstream face of the frame and airflow through the aperture is permitted; a biasing member coupled to the frame and configured to be in contact with a downstream face of the damper blade when the damper blade is in the open position to exert a biasing force against the damper blade to urge the damper blade towards a substantially closed position, wherein the biasing member is positioned downstream of the downstream face of the frame; and a magnet coupled to the damper blade and configured to exert a magnetic force between the frame and the magnet to pull the damper blade from the substantially closed position to the closed position; wherein when the damper blade is in the open position and an airflow pressure provided by the airflow fan falls below a first predetermined threshold, the biasing force exerted by the biasing member moves the damper blade to the substantially closed position, and the magnetic force then pulls the damper blade to the closed position and releasably retains the damper blade in the closed position, thereby inhibiting airflow through the conduit, and wherein when the damper blade is in the closed position and an airflow pressure provided by the airflow fan is increased to a second predetermined threshold, the magnetic force is overcome and the damper blade moves towards the open position. 
     In some embodiments, the damper blade is made from a paramagnetic material and the frame is made from a ferromagnetic material. 
     In some embodiments, the damper blade is made from at least one of aluminum, tin, and titanium. 
     In some embodiments, the frame is made from at least one of galvanized steel and stainless steel. 
     In some embodiments, the frame further comprises a flange extending outwardly from the downstream face of the frame, and the biasing member comprises a resiliently flexible member having a first end and a second end, the first end being secured to the flange, and the second end configured to abut the downstream face of the damper blade when the damper blade is in the open position. 
     In some embodiments, the first end of the resiliently flexible member has a width that is greater than a width of the second end of the resiliently flexible member. 
     In some embodiments, the resiliently flexible member is made from at least one of stainless steel, spring steel, and rubber. 
     In some embodiments, the magnet has a strength of at least 32 Gauss. 
     In some embodiments, the magnet has a strength of between 32 and 105 Gauss. 
     In some embodiments, the perimeter of the damper blade is generally rectangular. 
     In some embodiments, the perimeter of the damper blade is generally circular. 
     In some embodiments, the first predetermined threshold is about 0.085 kPa, and the second predetermined threshold is about 0.09 kPa. 
     In some embodiments, the conduit is a supply air inlet of a thermal recovery unit. 
     In some embodiments, the first distance is less than 76 mm. 
     In accordance with another broad aspect, there is provided a thermal recovery unit for installation in a residential dwelling, the thermal recovery unit comprising: a housing having an interior air inlet, an exterior air inlet, an interior air outlet, and an exterior air outlet; a first conduit extending between the interior air inlet and the exterior exhaust outlet; a second conduit extending between the exterior air inlet and the interior air outlet; a supply fan positioned adjacent to the interior air outlet; and a damper assembly positioned downstream of the supply fan by a first distance, the damper assembly comprising: a frame having an upstream face and a downstream face; a damper blade pivotally secured to the frame and moveable between a closed position in which airflow through the second conduit is inhibited and an open position in which airflow through the second conduit is permitted; a biasing member coupled to the frame and configured to be in contact with a downstream face of the damper blade when the damper blade is in the open position to exert a biasing force against the damper blade to urge the damper blade towards a substantially closed position; and a magnet coupled to the damper blade and configured to exert a magnetic force between the frame and the magnet to pull the damper blade from the substantially closed position to the closed position; wherein when the damper blade is in the closed position, the supply fan is operable to provide an airflow pressure sufficient to overcome the magnetic force and move the damper blade to the open position, and wherein when the damper blade is in the open position and the supply fan is turned off, the biasing force exerted by the biasing member moves the damper blade to the substantially closed position, and the magnetic force then pulls the damper blade to the closed position, thereby automatically closing the damper assembly. 
     In some embodiments, the first distance is less than 76 mm. 
     It will be appreciated by a person skilled in the art that a method or apparatus disclosed herein may embody any one or more of the features contained herein and that the features may be used in any particular combination or sub-combination. 
     These and other aspects and features of various embodiments will be described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the described embodiments and to show more clearly how they may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: 
         FIG. 1  is a perspective view of an apparatus for inhibiting airflow through a conduit according to one embodiment, with the damper blade in an open position; 
         FIG. 2  is a perspective view of the apparatus of  FIG. 1 , with the damper blade in the closed position; 
         FIG. 3  is a perspective view of an apparatus for inhibiting airflow through a conduit according to another embodiment, with the damper blade in an open position; 
         FIG. 4  is another perspective view of the apparatus of  FIG. 3 ; 
         FIG. 5  is a perspective view of a thermal recovery unit for use in a multi-unit residential building; 
         FIG. 6  is a top view of the thermal recovery unit of  FIG. 5 ; 
         FIG. 7  is a schematic view of the airflow through the thermal recovery unit of  FIG. 5 . 
     
    
    
     The drawings included herewith are for illustrating various examples of articles, methods, and apparatuses of the teaching of the present specification and are not intended to limit the scope of what is taught in any way. 
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Various apparatuses, methods and compositions are described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover apparatuses and methods that differ from those described below. The claimed inventions are not limited to apparatuses, methods and compositions having all of the features of any one apparatus, method or composition described below or to features common to multiple or all of the apparatuses, methods or compositions described below. It is possible that an apparatus, method or composition described below is not an embodiment of any claimed invention. Any invention disclosed in an apparatus, method or composition described below that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicant(s), inventor(s) and/or owner(s) do not intend to abandon, disclaim, or dedicate to the public any such invention by its disclosure in this document. 
     Furthermore, it will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the example embodiments described herein. However, it will be understood by those of ordinary skill in the art that the example embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the example embodiments described herein. Also, the description is not to be considered as limiting the scope of the example embodiments described herein. 
     While the apparatus and methods disclosed herein are described specifically in relation to the use of the apparatus to inhibit ducted supply air from entering a building or ventilation system due to naturally occurring pressure differentials, for example to inhibit below freezing air from entering a warmer building, or to inhibit air from entering due to wind or stack effects (external and/or internal), etc., it will be appreciated that the same apparatus may alternatively be used in other ventilation systems. For example, the apparatus may be used on systems in which a thermal recovery unit is connected to an air conditioning appliance. 
     Reference is now made to  FIGS. 1 and 2 , which illustrate an apparatus  100  for inhibiting airflow through a conduit. Apparatus  100  may also be referred to as a damper assembly  100 . Damper assembly  100  includes a frame  110  that has an inner perimeter  113  that defines an opening or aperture in the frame  110  through which air can flow. Damper assembly  100  also has a damper blade or flap  120  that is moveable between an open position in which the opening in frame  110  is relatively unobstructed by the damper flap  120  (e.g. as shown in  FIG. 1 ), and a closed position in which the damper flap inhibits (and preferably prevents) airflow through the opening in frame  110  (e.g. as shown in  FIG. 2 ). Damper assembly  100  also includes a biasing member  130  for urging the damper flap from the open position towards a partially closed position, and a magnet  140  for urging the damper flap from the substantially closed position to the closed position, and for releasably retaining the damper blade in the closed position. 
     As will be discussed further below, apparatus or damper assembly  100  is preferably configured so that the damper flap  120  remains in an open position as long as an airflow pressure on the upstream face of the damper flap remains above a first predetermined threshold, such as the airflow pressure expected to be developed by a supply fan positioned upstream of the apparatus during normal operation. Apparatus  100  is also preferably configured so that the damper flap  120  may be released from the closed position and be moved to the open position when an upstream airflow pressure is increased to a second predetermined threshold, such an airflow pressure also capable of being supplied by the supply fan or blower positioned upstream of the apparatus. 
     Accordingly, when installed in (or against the end of) an airflow conduit and downstream of a supply fan, if the supply fan is not operating the damper blade automatically moves to and is maintained in the closed position, thereby inhibiting outside air from freely entering the building and/or the ventilation system. For example, if the supply fan fails, is turned off, or otherwise fails to supply a predetermined airflow pressure against the damper blade, the damper assembly automatically closes. Further, if the upstream supply fan is turned on while the damper blade is in the closed position, the damper blade automatically moves to and is maintained in the open position, thereby allowing outside air to be drawn into the building and/or the ventilation system by the supply fan. Preferably, the apparatus is installed within about 50 to 76 mm of the upstream supply fan wheel. 
     Frame  110  has a first (or upstream) face  112 , a second (or downstream) face  114 , and an inner perimeter  113  that defines an opening from the upstream face  112  to the downstream face  114 . Frame  110  may be made from any suitable material. As will be discussed further below, frame  110  is preferably made from a ferromagnetic material, such as galvanized steel, stainless steel, and the like. 
     Frame  110  is preferably dimensioned so that it can be secured within an airflow conduit, or with the downstream face  114  positioned against an end of an airflow conduit, preferably downstream of a powered airflow fan. 
     Preferably, frame  110  and its opening have the same general shape as the airflow conduit in or against which it is to be mounted. For example, apparatus  100  has a generally circular shape, suitable for use with a generally circular conduit. It will be appreciated that frame  110  may have any suitable shape. 
     Optionally, one or more mounting features, such as mounting projection  115 , may be provided to facilitate the securement of apparatus  100  into the ventilation system. Alternatively, more or fewer (i.e. no) mounting features may be provided. 
     Damper flap  120  has a first (or upstream) face  122  and a second (or downstream) face  124 . Damper flap  120  may be made from any suitable material. As will be discussed further below, damper flap  120  is preferably made from a paramagnetic material, such as aluminum, tin, titanium, and the like. 
     Damper flap  120  is pivotally coupled to frame  110 . In the illustrated embodiment, a pair of projections or flanges  116  extend outwardly from the downstream face  114  of frame  110 . Flanges  116  are each configured to retain an end of a pivot member or shaft  117 , about which the damper flap  120  pivots. Preferably, shaft  117  is generally parallel to the downstream face  114  of frame  110 . In the illustrated embodiment, an upper portion of damper flap  120  is rolled or otherwise formed to create a hollow section or knuckle  126  through which shaft  117  can be inserted or otherwise positioned. When shaft  117  is positioned interior of hollow section or knuckle  126  and the ends of shaft  117  are retained by flanges  116 , damper flap  120  is thereby pivotally coupled to frame  110 . It will be appreciated that any suitable pivotal coupling method may alternatively be used. 
     In  FIG. 1 , damper blade  120  is shown in an open position. In this position, the opening in frame  110  is relatively unobstructed by the damper flap  120 , and airflow though the frame  110  is relatively unimpeded. 
     In  FIG. 2 , damper blade  120  is shown in a closed position. In this position, the upstream face  122  of damper flap  120  is in contact with the downstream face  114  of frame  110  around all (or substantially all) of the inner perimeter  113 , thereby inhibiting airflow through the opening in frame  110 . Preferably, the abutting portions of damper flap  120  and frame  110  are each generally planar. Alternatively, the abutting portions may have complementary, non-planar surfaces. Optionally, one or more seal or gasket members (not shown) may be provided on the downstream face  114  of frame  110  about some or all of perimeter  113  and/or on the upstream face  122  of damper blade  120  to further inhibit airflow through the frame  110  when the damper blade  120  is in the closed positon. 
     Damper assembly  100  also includes a biasing member for urging the damper blade  120  from the open position towards a substantially closed position. In the example illustrated in  FIGS. 1 and 2 , the biasing member comprises a resilient elongate member  130 . A first end  132  of member  130  is secured to a projection or flange  118  that extends outwardly from upstream face  112 . In the illustrated embodiment, end  132  is secured to projection  118  using a pair of rivets  135 . The first end  132  may be secured to the frame  110  using any suitable coupling method (e.g. using a screw and a threaded insert, using a set screw or other mechanical fastener, etc.). Optionally, a suitable adhesive may be used to secure first end  132  to frame  110 . 
     A second end  134  of resilient member  130  is configured to contact the downstream face  124  of damper blade  120  when the damper blade is in an open position (e.g. as shown in  FIG. 1 ), and to exert a biasing force against the damper blade towards a substantially closed position intermediate the open position and the closed position. Optionally, the second end  134  may be configured to remain in contact with the downstream face  124  of damper blade  120  when the damper blade is in the closed position. 
     In the illustrated embodiment, the first end  132  of resilient member  130  is wider than the second end  134 , and resilient member  130  has a generally linear or continuous taper from the first end  132  to the second end  134 . It will be appreciated that resilient member  130  may alternatively have a curved taper, a parabolic taper, a series of tapers, no taper, or any other suitable profile. In some embodiments, the first end  132  of resilient member  130  may be narrower than the second end  134 . 
     Resilient member  130  may be made from any suitable material, such as stainless steel, spring steel, rubber, and the like. 
     When the damper blade  120  is in the open position, the biasing member is configured to exert a biasing force against the damper blade  120  to urge the damper blade towards the partially closed position. In the illustrated embodiment, when the damper blade  120  is in the open position, the second end  134  of resilient member  130  exerts a force against the downstream face  124  of damper blade  120  to urge the damper blade towards the frame  110 . 
     Apparatus  100  is preferably configured so that the force exerted by the biasing member is less than the expected force exerted on the upstream face  122  of the damper blade  120  by the airflow pressure expected to be supplied by a supply fan or blower positioned upstream of the apparatus  100  in an airflow system. 
     For example, if the apparatus is positioned 76 mm downstream of a centrifugal supply fan expected to develop a downstream airflow pressure of 0.09 kPa during normal (e.g. steady-state) operation, the apparatus  100  may be configured so that the damper flap  120  remains in the open position as long as an airflow pressure of 0.085 kPa is applied to the upstream face  122  of damper flap  120 . 
     Also, while in the open position, apparatus  100  preferably introduces a relatively small static pressure increase in the ventilation system in to which it is installed. For example, if the frame  110  has an opening of about 39 cm 2 , for an airflow velocity of 1.54 m/s, the static pressure increase introduced by the damper in the open position is preferably below 0.017 kPa. 
     It will be appreciated that configuration of apparatus  100  so that the damper blade  120  remains in the open position as long as an airflow pressure provided by the airflow fan remains above a predetermined threshold is dependent on a number of parameters. These parameters include, for example: the cross-sectional area of the opening through the frame  110 ; the cross-sectional area of the damper flap  120 ; the size and elasticity of resilient member  130 , friction in the pivotal coupling between frame  110  and damper blade  120 , etc. 
     For example, testing indicated that for a frame  110  with a 35 cm 2  opening and a 38 cm 2  damper blade  120 , a 9 cm×1.8 cm×0.004 inch resilient member  130  made of stainless steel, the damper flap remained in the open position as long as a 0.085 kPa sustained airflow pressure was applied to the upstream face of the damper flap following the application of a 0.09 kPa airflow pressure to open the damper flap. 
     As shown in  FIGS. 1 and 2 , the entire biasing member or resilient member  130  is preferably positioned downstream of the downstream face  114  of the frame  110 . This positioning may result in one or more advantages. For example, this may allow the overall apparatus  100  to have a generally planar upstream face, which may facilitate its installation against an end of an airflow conduit. Also, in this positioning both the pivotal coupling or hinge assembly about which the damper flap pivots and the biasing member are located outside of the expected airstream through the frame  110 , which may reduce the static pressure introduced by the apparatus  100  when the damper flap  120  is in the open position. 
     Also, positioning the entire biasing member  130  downstream of the downstream face  114  of the frame  110  may facilitate the installation of apparatus  100  in close proximity to an upstream supply fan wheel. The relative positioning of the damper blade  120  to the supply fan wheel may impact the ability to develop sufficient velocity pressure to release the damper flap from a closed position. For example, as noted above, apparatus  100  is preferably installed within about 50 to 76 mm of the upstream supply fan wheel. 
     Damper assembly  100  also includes a magnet  140  coupled to the damper blade for urging or pulling the damper blade  120  from a substantially closed position to the closed position. As illustrated in  FIGS. 1 and 2 , magnet  140  is preferably positioned on the damper blade  120  in a location on the opposite side of the frame opening from the pivotal coupling between the frame  110  and the damper flap  120 . When the damper blade  120  is in the substantially closed position, the attractive magnetic force between magnet  140  and frame  110  is sufficient to pull the damper blade  120  into the closed position. While one magnet  140  is shown in the example embodiment, it will be appreciated that two or more magnets may be provided in alternative embodiments. 
     Magnet  140  is also configured to releasably retain the damper blade in the closed position. That is, when the damper blade  120  is in the closed position, magnet  140  provides sufficient holding force against frame  110  to retain the damper blade in the closed position until a predetermined threshold airflow pressure is applied to the upstream face  122  of damper flap  120 . Preferably, apparatus  100  is configured so that this threshold ‘break’ pressure is capable of being supplied by a supply fan or blower positioned upstream of the apparatus  100  in an airflow system. 
     For example, if the apparatus is positioned 76 mm downstream of a centrifugal supply fan capable of developing an airflow pressure of 0.09 kPa, the apparatus  100  may be configured so that the damper flap  120  may be released from the closed position and be moved by the airflow pressure to the open position when an airflow pressure of 0.09 kPa is applied to the upstream face  122  of damper flap  120 . 
     It will be appreciated that configuration of apparatus  100  to provide a suitable threshold ‘break’ pressure to release the damper blade  120  from the closed position is dependent on a number of parameters. These parameters include, for example: the cross-sectional area of the opening through the frame  110 ; the size and strength of magnet  140  (preferably between about 32 and 105 Gauss, although any suitable strength magnet may be used); the size and elasticity of resilient member  130 , friction in the pivotal coupling between frame  110  and damper blade  120 , etc. 
     It has also been found that the material of frame  110  and the material of damper blade  120  have a significant influence on the pressure required to ‘break’ or overcome the holding force provided by magnet  140  when the damper blade  120  is in the closed position. In this respect, experimentation and testing suggest that using a ferromagnetic frame and a paramagnetic damper blade facilitates the provision of a lower or ‘soft break’ force required to release the damper blade from the closed position, as compared to, e.g., the use of a ferromagnetic frame and a ferromagnetic damper blade, which tends to provide a higher or ‘hard break’ force, all else being equal. Thus, use of a ferromagnetic frame and a paramagnetic damper blade may facilitates the configuration of the apparatus so that the damper blade closes (and remains closed) when an upstream supply fan is not operating, while allowing the damper blade to be reliably opened using airflow pressure generated by the supply fan when the fan is operating. 
     For example, testing indicated that for a steel (i.e. ferromagnetic) frame  110  with a 38 cm 2  opening, an aluminum (i.e. paramagnetic) damper blade  120 , and a 32 gauss magnet, the damper flap remained in the closed position until a 0.09 kPa airflow pressure was applied to the upstream face of the damper flap, at which point the magnetic holding force was overcome and the damper blade moved to the open position. 
     As illustrated in  FIGS. 1 and 2 , frame  110  is generally circular, and would be suitable for use with a circular airflow conduit. Reference is now made to  FIGS. 3 and 4 , which illustrate another example embodiment of an apparatus or damper  200  for inhibiting airflow through a conduit. Apparatus  200  has a generally square shape, suitable for use with a generally square conduit. Similar to damper assembly  100 , damper assembly  200  includes a frame  210 , a damper blade or flap  220  that is moveable between an open position (e.g. as shown in  FIGS. 3 and 4 ) and a closed position (not shown), a biasing member  230 , and a magnet  240 . In  FIGS. 3 and 4 , like features as in  FIGS. 1 and 2  are referred to with like reference numerals, with the first digit incremented by 1. 
     As with frame  110  of apparatus  100 , frame  210  of apparatus  200  is preferably dimensioned so that it can be secured in (or against an end of) an airflow conduit, preferably downstream of a powered airflow fan. Preferably, frame  210  and its opening have the same general shape as the airflow conduit against which it is to be mounted. As shown in  FIGS. 3 and 4 , the entire biasing member or resilient member  230  is preferably positioned downstream of the downstream face of the frame  210 . 
     As with apparatus  100 , apparatus or damper  200  is preferably configured so that the damper flap  220  remains in an open position as long as an airflow pressure on the upstream face of the damper flap remains above a first predetermined threshold. That is, apparatus  200  is preferably configured so that the force exerted by the biasing member  230  is less than the expected force exerted on the upstream face  222  of the damper blade  220  by the airflow pressure expected to be supplied by a supply fan or blower positioned upstream of the apparatus  200  during normal operation. 
     It will be appreciated that configuration of apparatus  200  so that the damper blade  220  remains in the open position as long as an airflow pressure provided by the airflow fan remains above a predetermined threshold is dependent on a number of parameters. These parameters include, for example: the cross-sectional area of the opening through the frame  210 ; the cross-sectional area of the damper flap  220 ; the size and elasticity of resilient member  230 , friction in the pivotal coupling between frame  210  and damper blade  220 , etc. 
     For example, testing indicated that for a frame  210  with a 32.2 cm 2  opening and a 36.8 cm 2  damper blade  220 , a 7 cm×1.8 cm×0.005 inch resilient member  230  made of stainless steel, the damper flap remained in the open position as long as a 0.085 kPa sustained airflow pressure was applied to the upstream face of the damper flap following the application of a 0.09 kPa airflow pressure to open the damper flap. 
     Apparatus or damper  200  is also preferably configured so that the damper flap  220  may be released from the closed position and be moved to the open position when an upstream airflow pressure is increased to a second predetermined threshold, such an airflow pressure also capable of being supplied by a supply fan or blower positioned upstream of the apparatus. 
     As with apparatus  100 , apparatus  200  is preferably configured so that the threshold ‘break’ pressure required to overcome the magnetic holding force of magnet  240  is capable of being supplied by the supply fan or blower positioned upstream of the apparatus  200  in an airflow system. The configuration of apparatus  200  to provide a suitable threshold ‘break’ pressure to release the damper blade  220  from the closed position is dependent on a number of parameters. These parameters may, for example: the cross-sectional area of the opening through the frame  210 ; the size and strength of magnet  240 ; the size and elasticity of resilient member  230 , friction in the pivotal coupling between frame  210  and damper blade  220 , etc. 
     As with apparatus  100 , use of a ferromagnetic frame and a paramagnetic damper blade facilitates the provision of a lower or ‘soft break’ force required to release the damper blade from the closed position, as compared to, e.g., the use of a ferromagnetic frame and a ferromagnetic damper blade. 
     For example, testing indicated that for a steel (i.e. ferromagnetic) frame  210  with a 36.8 cm 2  opening, an aluminum (i.e. paramagnetic) damper blade  220 , and a 105 gauss magnet  240 , the damper flap remained in the closed position until a 0.09 kPa airflow pressure was applied to the upstream face of the damper flap, at which point the magnetic holding force was overcome and the damper blade moved to the open position. 
     As discussed above, apparatus  100  or  200  may be used to inhibit ducted supply air from entering a building or ventilation system due to naturally occurring pressure differentials. In a particular application, apparatus  100  or  200  may be used as a non-powered freeze protection system for a supply conduit (i.e. fresh air inflow) of a thermal recovery ventilation system. 
       FIGS. 5, 6, and 7  illustrate a thermal recovery ventilation system or thermal recovery unit  300 . Thermal recovery unit  300  may be designed for use in a residential building and/or a single residential unit within a multi-unit building. For example, thermal recovery unit  300  may be installed so that air from a bathroom, which is typically relatively warm air, may be exhausted through a heat exchanger in order to increase the temperature of outside air as it is drawn into the residential building and/or residential unit. Typically, the outside air conduit is connected to an air conditioning appliance for distribution. 
     In the illustrated example, thermal recovery unit  300  includes a main body  310  and a removable fan and control cassette  320  that houses fans or blowers  360  and  362  (and their controls). Removable cassette  320  is releasably secured to main body  310  using a pair of latches  315 . A lower access panel  312  is also releasably secured to main body  310  using another pair of latches  315 . It will be appreciated that variant designs of thermal recovery units may be used in alternative embodiments. 
     Thermal recovery unit  300  has a first air intake or inlet  332  for drawing in air from outside the building or unit, and a first air outlet or exhaust  334  for supplying air to a device or appliance for tempering or conditioning the air before it enters the residential unit to provide an environment with desired air characteristics. Thermal recovery unit  300  also has a second air intake or inlet  342  for drawing in air from a bathroom or other expected source or warm air, and a second air outlet or exhaust  344  for exhausting the air to the outside of the building. It will be appreciated that one or more of intakes  332 ,  334  and exhausts  334 ,  344  will typically be coupled to airflow conduits or ducting (not shown) to convey air from the intake or exhaust to a remote location in the ventilation system. For example, a conduit may be provided from exhaust  344  to a location proximate the building envelope, such as a shrouded wall exhaust and/or exhaust shaft and the like. 
     As shown in  FIG. 7 , thermal recovery unit  300  also includes a heat exchanger  350 , such as a cross-flow or counter-flow heat exchanger. In use, supply fan  360  induces an airflow to draw air in from inlet  332 , through heat exchanger  350 , and out of outlet  334 . Concurrently, supply fan  362  induces an airflow to draw air in from inlet  342 , through heat exchanger  350 , and out of outlet  344 . In this arrangement, when air from air inlet  342  is warmer than air from inlet  332 , air flowing from inlet  332  to  334  will be passively warmed by air flowing from inlet  342  to  344 . This will often be the case when inlet  342  draws in air from a residential bathroom, and inlet  332  draws in air from the exterior or intake shaft of the residence, particularly in colder climates, 
     To inhibit air (e.g. very cold air) from entering the residential unit or ventilation system appliance or device (e.g. an air conditioning unit) in an uncontrolled manner, an apparatus  100  may be positioned downstream of supply fan  360 . For example, apparatus  100  may be mounted to or over first air outlet or exhaust  334 . As discussed above, apparatus  100  is preferably mounted 76 mm (3 inches) or less from supply fan  360 . 
     In this location, when supply fan  360  is turned on and increases the airflow pressure upstream of the apparatus  100  above a predetermined threshold, damper blade  120  is moved to and is maintained in the open position, thereby allowing outside air to be drawn into the building via inlet  332 . However, if supply fan  360  is turned off, cycled to a lower power setting, or fails, damper blade  120  is moved to and is retained in the closed position, thereby inhibiting outside air from entering the building via inlet  332 . In this manner, apparatus  100  may act as a freeze protection system by inhibiting very cold air from entering the building and/or a downstream air conditioning appliance due to naturally occurring pressure differentials. 
     As used herein, the wording “and/or” is intended to represent an inclusive—or. That is, “X and/or Y” is intended to mean X or Y or both, for example. As a further example, “X, Y, and/or Z” is intended to mean X or Y or Z or any combination thereof. 
     While the above description describes features of example embodiments, it will be appreciated that some features and/or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. For example, the various characteristics which are described by means of the represented embodiments or examples may be selectively combined with each other. Accordingly, what has been described above is intended to be illustrative of the claimed concept and non-limiting. It will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto. The scope of the claims should not be limited by the preferred embodiments and examples, but should be given the broadest interpretation consistent with the description as a whole.