US3791281A

US3791281A - Exhaust fan assembly

Info

Publication number: US3791281A
Application number: US00227031A
Authority: US
Inventors: R Steiner
Original assignee: Emerson Electric Co
Current assignee: Emerson Electric Co
Priority date: 1972-02-17
Filing date: 1972-02-17
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: CA977721A

Abstract

An improved exhaust fan assembly is provided in which a fan and damper are mounted in a frame, the damper being mounted at the fan outlet, within an exhaust fan housing structure. Two springs are utilized to hold the damper closed. The damper opens by the force of air exerted during fan operation. During that operation, the springs exert a force which helps to hold the damper open. At least one spring has a top extension that contacts a pin on the damper, keeping the over-center force from becoming so great to hold the damper open permanently. The fan and damper are selfcontained with respect to the rest of the fan housing and the length of the exhaust fan housing can be varied for differing thicknesses of wall through which the housing is to extend.

Description

United States Patent 91 Steiner [45] Feb. 12,1974

[ EXHAUST FAN ASSEMBLY Primary Examiner-William F. ODea Assistant Examiner-Peter D. Ferguson 75 l t R rt E. S S l 1 men or obe miner 1 Louis M Attorney, Agent, or Firm-Polster and Polster [73] Assignee: Emerson Electric Co., St. Louis, Mo.

[22] Filed: Feb. 17, 1972 I [57] ABSTRACT [21] Appl. No.: 227,031 An improved exhaust fan assembly is provided in which a fan and damper are mounted in a frame, the damper being mounted at the fan outlet, within an exg 98/43 49/ haust fan housing structure. Two springs are utilized "i...- to the ampe ose e dalnper opens the [58] meld of Search 98/43 49/197 386 force of air exerted during fan operation. During that operation, the springs exert a force which helps to [56] References cued hold the damper open. At least one spring has a top UNITED STATES PATENTS extension that contacts a pin on the damper, keeping 1,792,514 2/1931 Spear 98/43 the over-center force from becoming so great to hold 1,528,267 3/1925 Root the damper open permanently. The fan and damper lg; x g are self-contained with respect to the rest of the fan a er h 3,120,798 2/1964 Nash 98/43 X ousmg and the length of the exhaust fan housing can be varied for differing thicknesses of wall through which the housing is to extend.

' 11 Claims, 9 Drawing Figures PAIENTED FEB 1 21974 SHEEI 8 BF 3 FIG. 5.

FIG. 4. j

BACKGROUND OF THE INVENTION This invention relates to back draft dampers for exhaust fans, and in particular, to through-wall exhaust fans. Those skilled in the art, however, will recognize the applicability of the concepts of this invention to other damper applications.

It is common to use exhaust fans in the construction of houses and the like in a number of areas.

For example, commonly a new home has a throughwall exhaust fan near the cooking area of the home to dissipate the heat and odors generated in food preparation. Conventionally, the exhaust units have a fan mounted in a housing. the housing extends through the wall structure of the house and the blower or fan is adapted to blow air through the housing to the external atmosphere. Because these units are vented to the outside atmosphere, it is common to utilize some form of back draft damper in conjunction with the units to prevent rain, cold air, insects and the like from getting in. Conventionally, the damper has been installed on the external side of the through-wall unit.

While these units work well for their intended purposes, they exhibit inherent disadvantages heretofore unresolved in the prior art. Prior art dampers conventionally are constructed from sheet metal and are attached to a sheet metal enclosure. Such construction presents a highly objectionable noise problem as the back draft damper tends to close with some force. It is conventional to line the damper-enclosure contact area with some form of padding, conventionally of the felt type, to lessen the metal chatter. Unfortunately, it is difficult to attach the padding to the damper and have it remain there for any extended period of time. It also generally is difficult to position the padding properly.

While recent improvements in damper constructions, for example, that disclosed in the US Pat. to Steiner No. 3,589,268, issued June 29, 1971, alleviated cerain objectional features of the stamped metal damper by utilizing a non-metallic material for the damper, the use of non-metallic material for damper construction in certain applications, and in particular, in through-wall applications, has been heretofore unfeasible because of the conventional placement of that damper on the outside, along the outlet of the exhaust fan structure, re-

mounting procedure is simplified by utilizing a flanged opening at the unit impeller and by designing the damper to seal about the flange.

One of the objects of this invention is to provide an exhaust unit having a back draft damper mounted adjacent the impeller of the blower.

mote from the impeller of the blower unit. In some through-wall units, this means that the damper 'is exposed to the ravages of the environment, and not all non-metallic dampers will stand up under the conditions of extreme heat and cold to which they are liable to be exposed. In addition, the through-wall unit, to be economically feasible in a large scale production, must be adaptable to a variety of wall thicknesses. Heretofore, no reliable non-metallic unit was considered commercially feasible because the adjustability requirement for the housing was believed to preclude mounting of the damper at any point except near the outlet of the exhaust unit.

My invention permits use of a non-metallic back draft damper by mounting the damper adjacent the unit impeller. A unique system of back biasing the damper is provided which enables the damper to close regardless of the thickness-of the wall, hence the length of the housing, thereby retaining the necessary adjustability features provided by older systems. The damper Another object of this invention is to provide such a through-wall exhaust unit having a nonmetallic back draft damper.

Yet another object of this invention is to provide novel means for back biasing a back draft damper.

Still another object of this invention is to provide a damper characterized by silent operation.

Yet another object of this invention is to provide increased air flow in an exhaust unit by utilizing louvers in the external side of a through-wall exhaust system.

Other objects will be apparent to those skilled in the art in light of the following description and accompanying drawings.

SUMMARY OF THE INVENTION In accordance with this invention, generally stated, a through-wall exhaust unit is provided having a nonmetallic back draft damper mounted adjacent the impeller of the blower. The damper is back biased by a spring arrangement which prevents the damper from locking itself in an open position.

In the preferred embodiment, the damper is mounted in a housing which is adjustable to correspond to a variety of wall thicknesses. The external portion of the housing is louvered to increase exhaust fan (blower) output without requiring increased blower capacity.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a view in perspective of one illustrative embodiment of exhaust fan unit of this invention;

FIG. 2 is a view in front elevation of a self-contained fan-damper assembly of the unit shown in FIG. 1; I

FIG. 3 is a view in rear elevation taken along the line 3-3 of FIG. 1;

FIG.4 is a view in side elevation taken along the line 4-4 of FIG. 3;

FIG. 5 is a top plan view of the assembly shown in FIGS. 2-4;

FIG. 6 is an enlarged plan view of a bumper unit utilized in conjunction with the damper shown in its mounted position in FIG. 2; a

FIG. 7 isa view in side elevation taken along the line 7-7 of FIG. 6;

FIG. 8 is an enlarged view in side elevation of a spring utilized to back bias the damper of FIG. 2; and

FIG. 9 is a view in side elevation taken along the line 9-9 of FIG. 8.

DESCRIPTION OF THE PREFERRED I EMBODIMENT and includes a stator assembly 5 having a plurality of windings 6 associated with it in accordance with conventional motor design theory. In the embodiment il- Blower 2 includes a motor 8. Motor 8 is conventional lustrated, the windings 6 are enclosed by a shroud l4. Stator assembly has an axial opening through it in which a rotor assembly, not shown, is rotatably mounted. A rotor assembly includes a shaft operatively connected to it, likewise not shown, on one end of which is mounted an impeller 7. Motor 8 is attached to a bracket 9. Bracket 9 is made of sheet metal, rectangular in plan, U-shaped in side elevation, with integral mounting tabs at each end, and a central mounting portion 12. The bracket is blanked out between the tabs 15 and the central mounting portion 12 and turned up at the edges to form flanged rails 11. Portion .12 has a square opening 16 in it, through which a cover attaching bolt 13 is placed. Bolt 13, in the embodiment illustrated, is nothing more than a conventional, flat headed bolt with a square shank. The motor 8, when mounted to the bracket 9 locks the bolt 13 within the opening 16.

Motor 8 is attached to the central portion 12 of bracket 9 by a conventional nut-bolt combination l0.

Mounting tabs 15 have an opening 17 in them, through which fasteners are run to attach the motor bracket to the enclosure 3. In the particular embodiment illustrated, an annular vortex ring 18 is connected to the bracket by spotwelding or the like along the tabs 15. Ring 18 has a transverse lower wall 19 and a vortex wall 20, tending axially with respect to the axis of rotation of the fan. The purpose and function of ring 18 is described in detail hereinafter.

Motor 8 is electrically connected to a cord 21 with two conductors in it and a male plug connected to them.

In the embodiment illustrated, enclosure 3 comprises a motor mounting frame 22, telescoping shell 23 and an external portion 24.

Motor mounting frame 22, in the embodiment illustrated, is substantially square and has a top 25, a bottom 26, and

side walls

27 and 28. Motor mounting frame 22 has a open back 29 and a front wall 30. Front wall 30 has a circular opening 31 through it defined by an annular flange 32 which projects outwardly with respect to the motor 8 and is perpendicular to the front wall 30. When the motor 8 is mounted within motor mounting frame 22, the wall 19 of the ring 18 abuts the back of the front wall 30 while the wall is adjacent but spaced from the annular flange 32. This arrangement permits effective sealing of the unit to back drafts, as later explained.

The front wall 30 has mounting means associated with it for attaching the bracket 9 to it. Common screws through the opening 17 work well. A connection box 33, with a female socket mounted in it to receive the male plug on the motor cord may be utilized to connect the electrical connections 21 for the motor 8 to the external source of power not shown. In FIG. 1, the connection box 33 is shown as mounted in the lower right hand corner, to show the female socket. In FIG. 3, it is shown as mounted in the upper left hand corner. The construction of the frame 22 permits mounting either place. The top and the bottom 26 have a pair of parallel slits 34 in them. The slits 34 provide a portion of the adjustable feature for the exhaust unit 1 as later explained in detail.

Referring now to FIGS. 2, 4 and 5, it is observed that a pair of legs 35 extend perpendicularly to the front wall from opposite ends of the top 25. The legs are parallel to one another and aligned, and have laterally aligned openings defined by integrally formed journals 48. In the embodiment of FIG. 4, a second pair of legs extend from the wall 30 near the bottom 26. The legs near the bottom 26 serve no function, but are shown because the top and bottom are identical and are obtained in a single punching operation. For the functioning of my invention, however, only one pair of legs 35, or their equivalent, need be provided. While the particular embodiment illustrated has the legs 35 formed integrally with the top-bottom members of motor mounting section 22, those skilled in the art will recognize that the legs 35 may be formed from or with a front wall 30 or individually formed and later attached to the section 22. The front wall 30 has a pair of T-shaped openings 36 in it, along each side of the annular flange 32.

Damper assembly 4 comprises a damper 37 and a pair of mounting springs 38. Damper 37 is constructed of a vibration absorbing, synthetic material and includes a front 38, a side wall 39 and a back 40. The side wall 39 extends about the perimeter of the damper 37 and is perpendicular to the front 37 and back 40, extending rearwardly, toward the motor 8, from the back 40. Side wall 39 has a pair of oppositely disposed, integral bosses 42 from which pivot pins 41 project, on opposite sides of the damper. The pivot pins 41 are aligned on an axis along a top edge 43 of the side wall 39.

Spaced below the bosses 42, on both sides of the damper, spring anchors 44 project from the side wall 39. They are aligned transversely of the damper and are offset rearwardly with respect to the axis of the pivot pins 41 when the damper is closed, as shown in FIG. 4. Spring anchors 44 are formed integrally with the wall 39. In the embodiment shown, they are cylindrical and have an annular channel in them as may best be seen in FIG. 2.

Side wall 39 also has a spring stop 45 extending outwardly from it on one side of the damper. Spring stop 45 is a cylindrical section formed integrally with side wall 39.

Side wall 39 has a plurality of fastening bosses 46 formed integrally with and extending outwardly from it. Fastening bosses 46, in the embodiment illustrated, are made up of facing angles defining an inverted T-shaped channel. Fastening bosses 46 are intended to receive a resilient stop 47, described in detail hereinafter.

The pivot pins 41 are rotatably mounted in the pair of fixed journals 48 formed in each leg 35. Placement of pivot pins 41 within journals 48 pivotally mounts damper 37 at the opening 31. The side 39 is adapted closely but freely to surround annular flange 32 while resilient stops 47 are sized to abut front wall 30 in the closed position of the damper 37 illustrated in FIG. 4.

Spring 38 is a broad band, serpentine design, best illustrated in FIGS. 8, 2 and 4. As can be seen by comparing those Figures, the spring is inverted in FIG. 8, with respect to its orientation in use. Spring 38 has a free outer end 49, an attaching crook 51, a compression loop 50, and an inner mounted end 52. The end 52, in the embodiment illustrated, has notches 53 in it to define a T-shaped end. As is best illustrated in FIGS. 2 and 4, one spring 38 is frictionally mounted to each spring anchor 44 by snapping the attaching crook 51 over it. The spring 38 on the side of the damper from which-the spring stop 45 projects is so positioned that the end 49 abuts spring stop 45 when the damper is opened. This abutment is important in the operation of the damper, as is explained in detail hereinafter. Although only one spring stop 45 is illustrated in the drawings, those skilled in the art will recognize that two can be provided and both of the springs will be so positioned.

The inner end 52 of each spring is inserted in the opening 36 against the bias of the spring, and the notches 53 permit the end to go into the stem of the T, with the cross of the T of the end 52 behind the front wall 30. The downward bias of the spring keeps it in place.

Resilient stop 47 is thin and flat, and dart shaped in plan, as best illustrated in FlGS. 6 and 7. As there shown, stop 47 has a wedgeshaped blunt end 54 and a base 55 joined by a reduced part 56. The part 56 is caged within the channel of the fastening bosses 46. End 54 strikes the front wall 30 when the damper is closed.

Telescoping shell 23 is square in front elevation and includes a top 57, a

side wall

58 and 59, and a bottom 60. Top 57 and bottom 60 have a pair of openings 61 for receiving conventional metal screws 62. The openings 61 are positioned so as to align with the slits 34 of the motor mounting frame 22. The opening defined by the walls of the telescoping shell 23 is large enough to permit insertion of motor mounting frame 22.

External portion 24 includes a rectangular frame 63, a flange part 64 and a pyramidal section 65.

Rectangular frame 63 has four walls, a top 66, sides 67 and 68 and a bottom 69. Top 66 and bottom 67 have a pair of slits 70 in them. The slits 70 are parallel and designed to align with at least a pair of openings 61 in top 57 and bottom 60 of telescoping shell 23.

Flange 64 extends outwardly about the perimeter of the frame 63 and may be formed integrally with either frame 63 or section 65, or may be constructed separately and attached to either part. In the embodiment illustrated, the flange 64 is formed integrally with the,

section 65 and the frame 63 is flared along an end 70 for attachment therewith. Section 65 includes a front wall 71 and a first and a

second side

72 and 73 respectively. The

sides

72 and 73 have a plurality of louvers 74 lanced in them. The pyramidal section 65 is open bottomed.

As thus described, enclosure 3 comprises three sheet metal units which may be varied in length to correspond to a plurality of wall thicknesses. That is, motor mounting frame 22, telescoping shell 23, and external portion 24 are telescopically mounted to one another and may be adjusted for a variety of wall thicknesses ranging, in the embodiment illustrated, between four and ten inches. The various parts are adjusted along the slots 34 and slots 70 by loosening the middle screws 61 and 75 along both the upper and lower portions of the enclosure 3. Once installed, the flange 64 abuts the external wall covering and the pyramidal section 65 extends outwardly therefrom. Motor mounting frame 22 is positioned so that the back 29 opens within the building. Back 29 may be closed by a decorative grill which also serves as an inlet to the blower 2. The outlet of the blower 2 is the opening 31 in the front wall 30 of motor mounting section 22. As previously indicated, it is at ln operation, air is forced against damper 37 by the impeller 7 which causes the damper 37 to swing out, around the axis of the pins 41. As the damper 37 opens, the spring anchor 44 swings outwardly with respect to the plane of the pivot pins 41 and the springs 38 would tend to exert a force to hold the damper open were it not for the abutment of the spring stop 45 and the end 49 of the spring. The damper is prevented from being biased open permanently under the influence of the springs by the restraining action of end 49 of the spring. The end 49 and stop 45 not only prevent the damper and spring from over-centering completely but resiliently bias the damper toward closing when the fan is turned off. Thus, the spring keeps the damper firmly closed when the fan is off, and tends to counter balance the damper to permit it to be and remain opened wide by the fan, the amount of opening being regulated by the end 49. The resilient stops 47 cushion the closing material, thereby permitting extremely quiet damper that location that the damper 37 is mounted. The

damper 37 will always be so mounted regardless of the position of the frame and housing elements of the enclosure 3.

operation. Removal of the damper from attachment with external unit 24, permits use of the louvers 74 in the sides 72. Dampers in the prior art generally were arranged to close the open bottom of the pyramidal section 65. Louvers in the sides of the section 65 were then impractical because their use defeated the purpose of the damper. With the placement of the damper as described above, louvers can in fact be used. Such use greatly increases the efficiency of the blower 2 by relieving pressure on the impeller 7.

Numerous variations, within the scope of the appended claims, will be apparent to those skilled in the art in light of the foregoing description and accompanying drawings. Thus, various parts described as integrally constructed may be constructed separately and later attached. The shape of the enclosure 3 may be varied. Thus, while I find it convenient to utilize three elements for the enclosure 3, other element combinations may be used. External portion 24 may be altered. While I find the'use of louvers 74 extremely effective, other embodiments of my invention may eliminate them, alter their size or shape or change their positioning. Other forms of resilient stops 47 may be used in place of stops shown or in conjunction with them. For example, conventional felt padding may, be utilized along the interior portion of side wall 39 and back 40 of damper 37 in place of or in conjunction with the resilient stops 47 illustrated in the drawings and described above. I prefer the use of the stops 47 because of the difficulties in placing and maintaining the position of the felt and the hazards inherent in the use of felt. The design and configuration of the springs 38 may be varied. These variations are merely illustrative.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. An exhaust fan unit comprising:

a housing including an external section and a blower mounting frame adjustably mounted with respect to said external section, said mounting frame having an inlet side and an outlet side;

a blower mounted to said mounting frame so as to direct a volume of air from said inlet side to said outlet side;

a back draft damper mounted to the outlet side of said mounting frame, said damper being mounted for pivotal movement with respect to said frame, about a pivot axis, between closed and opened positions;

a spring stop connected to said damper;

a spring anchor mounted to said damper, said spring anchor being offset, toward the inlet side, from said pivot axis when said damper is closed; and

at least one spring element attached to said spring anchor having a serpentine contour between a first end and a second end, one of said first and said second ends of said spring element being attached to said housing and the other of said first and said second ends being free and projecting towards the outlet side, the said free end and said spring stop being positioned to abut when the damper is opened, thereby limiting the opening of the damper.

2. The fan unit of claim 1 wherein the opening of the damper moves the spring anchor to a point just short of over-centering, whereby the effect of the bias of the spring is reduced in the damper-open position.

3. A through wall exhaust fan unit comprising:

a housing, said housing including a blower mounting frame having an inlet side and an oulet side;

a blower mounted to said blower mounting frame so as to direct a volume of air from said inlet side to said outlet side;

a spring anchor mounted to said damper, said spring anchor being offset, toward the inlet side, from said pivot axis when said damper is closed;

at least one spring element having a first end and a second end, one of said first and said second ends being attached to said blower mounting frame and the other of said first and said second ends being free and projecting toward the outlet side, said spring element being attached to said spring anchor between said first and said second ends; and

a spring stop mounted to said damper, said free end of said spring and said spring stop being arranged to abut in at least one open position of said damper to limit opening of said damper.

4. The fan unit of claim 3 wherein said housing is further characterized by an external portion and a shell portion, said blower mounting frame, said shell portion and said external portion being adjustably mounted to one another, said external portion having a pyramidal silhouette having three walls, at least one of said walls having a plurality of louvered openings in them.

5. in an exhaust fan unit having a housing, said housing including a blower mounting frame having an inlet side and an outlet side, the improvement which comprises a back draft damper pivotally mounted to said blower mounting frame at said outlet side and adapted for rotation with respect to said housing about a pivot axis so as to define at least a closed position, a plurality of opened positions, and an outermost open position, said damper comprising a structure having a pair of pivots and a pair of spring anchors attached to it, one each of said pivots and said spring anchors projecting outwardly on opposite sides of said damper along parallel but laterally displaced axis, and at least one spring element having a first end and a second end, said first end being operatively connected between said housing and said spring anchor so as to exert a force tending to open said damper in said plurality of open positions of said damper, said second end of said spring coacting with said spring stop to enable said spring to exert a force tending to close said damper in said outermost open position of said damper.

6. The improvement of claim 5 wherein said pivot pair, said spring anchor pair, and said spring stop are integrally formed with said damper.

7. The improvement of claim 6 wherein said outlet comprises a wall having an opening in it for defining said outlet side, said opening having a flange extending about its perimeter, said damper comprising a flange engaging portion adapted to seal said outlet opening in a closed position of said damper.

8. In an exhaust fan unit housing having an inlet side and an outlet side, a blower mounted in said housing, the improvement which comprises a back draft damper pivotally mounted to said housing adjacent said blower, said damper being adapted for rotation with respect to said housing between at least closed and opened positions, said damper comprising a resilient material structure having a pair of pivots projecting oppositely outwardly from said damper along a pivot axis, at least one anchor means projecting outwardly from said damper along a second axis, said second axis being parallel to but laterally spaced from said pivot axis, at least one spring operatively connected between said anchor means and said housing, and a spring stop mounted to said damper, said spring stop and said spring coacting to exert a closing force on said damper in said opened position.

9. The improvement of claim 8 wherein said outlet side comprises a wall having an opening in it for defining said outlet side, said opening having a flange extending about its perimeter, said damper comprising a flange engaging portion adapted to seal said outlet opening in a closed position of said damper.

10. In an exhaust fan unit having a housing and a back draft damper rotatably mounted between a closed position, a plurality of open positions and an outermost open position, the improvement which comprises means for exerting a force tending to open said damper in said plurality of open positions of said damper, and means for exerting a force tending to close said damper in an outermost open position of said damper, said open force exerting means including a spring element operatively connected between said housing and said damper, said closed force exerting means including a spring stop attached to said damper which coacts with said spring in at least the outermost open position of said damper to enable said spring element to exert a force on said damper in a direction of the closed position of said damper.

11. An exhaust fan unit comprising:

a housing selectively adjustable between a minimum housing length and a maximum housing length including an external section and a blower mounting frame adjustably mounted with respect to said external section, said mounting frame having an inlet side, a wall having an opening in it attached to said mounting frame, said opening defining an outlet side for said mounting frame;

a blower mounted to said mounting frame so as to dipositions of said damper;

rect the volume of air from said inlet side to said at least one spring element operatively connected beoutlet side; tween said housing and said damper, said spring ela back draft damper mounted to the outlet side of ement havingafree end extending outboard of said said mounting frame, said damper being mounted wall in the direction of air volume flow for all posifor pivotal movement with respect to said mounttions of said damper; and ing frame between a closed position blocking said means for engaging the free end of said spring eleopening in said wall and an open position, said ment to exert a closing force on said damper in its damper being mounted so as to be surrounded by closed position. said housing at all lengths of said housing and at all UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,791 ,281 Dated February 12, 1974 Inventor(s) Robert E te1ner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 16, after "housing.", "the" should-read the line 40, "cerain" should read certain Column 4, line 20, after "a front" the reference numeral "38" should read 90 line 22, "the front 37" should read the front 90 Column 9, line 2, "the" should read I a- Column 10, line 9, "closed" should read open I DRAWINGS V Figure 2, the reference numeral "38", shown in the center of the drawing figure, should read 90 Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) USCOMM-DC 60376-P69 I v.5. GOVERNMENT PRINTING OFFICE: 930

Claims

1. An exhaust fan unit comprising: a housing including an external section and a blower mounting frame adjustably mounted with respect to said external section, said mounting frame having an inlet side and an outlet side; a blower mounted to said mounting frame so as to direct a volume of air from said inlet side to said outlet side; a back draft damper mounted to the outlet side of said mounting frame, said damper being mounted for pivotal movement with respect to said frame, about a pivot axis, between closed and opened positions; a spring stop connected to said damper; a spring anchor mounted to said damper, said spring anchor being offset, toward the inlet side, from said pivot axis when said damper is closed; and at least one spring element attached to said spring anchor having a serpentine contour between a first end and a second end, one of said first and said second ends of said spring element being attached to said housing and the other of said first and said second ends being free and projecting towards the outlet side, the said free end and said spring stop being positioned to abut when the damper is opened, thereby limiting the opening of the damper.

3. A through wall exhaust fan unit comprising: a housing, said housing including a blower mounting frame having an inlet side and an oulet side; a blower mounted to said blower mounting frame so as to direct a volume of air from said inlet side to said outlet side; a back draft damper mounted to the outlet side of said mounting frame, said damper being mounted for pivotal movement with respect to said frame, about a pivot axis, between closed and opened positions; a spring anchor mounted to said damper, said spring anchor being offset, toward the inlet side, from said pivot axis when said damper is closed; at least one spring element having a first end and a second end, one of said first and said second ends being attached to said blower mounting frame and the other of said first and said second ends being free and projecting toward the outlet side, said spring element being attached to said spring anchor between said first and said second ends; and a spring stop mounted to said damper, said free end of said spring and said spring stop being arranged to abut in at least one open position of said damper to limit opening of said damper.

11. An exhaust fan unit comprising: a housing selectively adjustable between a minimum housing length and a maximum housing length including an external section and a blower mounting frame adjustably mounted with respect to said external section, said mounting frame having an inlet side, a wall having an opening in it attached to said mounting frame, said opening defining an outlet side for said mounting frame; a blower mounted to said mounting frame so as to direct the volume of air from said inlet side to said outlet side; a back draft damper mounted to the outlet side of said mounting frame, said damper being mounted for pivotal movement with respect to said mounting frame between a closed position blocking said opening in said wall and an open position, said damper being mounted so as to be surrounded by said housing at all lengths of said housing and at all positions of said damper; at least one spring element operatively connected between said housing and said damper, said spring element having a free end extending outboard of said wall in the direction of air volume flow for all positions of said damper; and means for engaging the free end of said spring element to exert a closing force on said damper in its closed position.