referring to fig1 a and 1b , these show , in a general schematic manner , the disposition of the flow of air through a heat exchange element or heat recovery core in accordance with the present invention . in these two figures the heat recovery core is generally designated with the reference number 1 ; the same reference numerals are used to designate the air paths in both of these figures as well . thus in fig1 a and 1b there are four main air path elements , namely the air path 2 for delivering exhaust or stale air from the building to the heat recovery core , air path 3 for exhausting the stale air which has passed through the heat recover core 1 to the exterior of the building , air path 4 for delivering fresh air to the heat recovery core and air path 5 for delivering fresh air which has passed through the heat recover core 1 to the interior of the building . there are also two defrost air paths , namely defrost air paths 2a and 3a . fig1 a , represents a ventilation configuration for a ventilation system in accordance with the present invention ; thus the defrost air paths 2a and 3a are closed off as is represented symbolically by the break in these paths . in accordance with fig1 a exhaust air will pass through air path 2 , through the heat recovery core 1 and through the air path 3 for delivery to the exterior of the building . on the other hand , fresh air will pass through air path 4 , through the heat recovery core 1 and through the air path 5 for delivery to the interior of the building . fig1 b , represents a defrost configuration for a ventilation system in accordance with the present invention ; thus the defrost air paths 2a and 3a are open while the air paths 3 and 4 are blocked as is represented symbolically by the break in these latter paths . in accordance with fig1 b , fresh air will not pass into the core 1 because air path 4 is blocked off ; exhaust or stale air will also not be exhausted to the exterior of the building since air path 3 is also blocked off . on the other hand , exhaust or stale air will now pass through air paths 2 and 2a , through the heat recovery core 1 and through the air paths 3a and 5 for delivery back to the interior of the building ; in this way the core 1 is defrosted without inducing any substantial negative air pressure in the building . fig2 shows a preferred construction of the defrostable ventilation apparatus of the present invention . the apparatus shown in fig2 includes a cabinet indicated generally by the reference number 6 . in fig2 the door ( not shown ) of the cabinet has been removed ; the door when present in hingedly attached to the hinge members 7 and 8 and may be kept shut for example by snap locking members 9 and 10 which cooperate with members on the door . the cabinet 6 is preferably provided with insulated walls and partition walls ( at least for the fresh air intake side , the fresh air discharge side and the exhaust air discharge side ). a heat exchange element or ( heat recovery ) core 11 is mounted in the cabinet 6 . although one such core 11 is shown it will nevertheless be appreciated that two or more such cores may , if desired , be used provided that appropriate modifications are made to the partitioning members so as to accommodate the required air flow patterns . referring to fig8 the entire inside face of the door is provided with a layer of sponge like insulating material 12 or the like which when the door is shut facilitates the sealing of the cabinet and also sealingly engages the front face 13 of the core 11 . the rear face 14 of the core 11 sealingly abuts the partition wall indicated generally by the reference number 15 . returning to fig2 the core 11 has a rectangular parallelepiped shape and is constructed generally as described in the u . s . pat . no . 5 , 002 , 118 mentioned above . thus , the core 11 has first and second air paths . the first air path is defined by a plurality of channels one of which is shown designated by the reference number 16 ; the second air path is also defined by a plurality of channels one of which is shown designated by the reference number 17 . as can be seen the air paths of the core 11 are disposed at right angles to each other ; the air paths are also separated by heat exchanging walls . the core 11 is mounted in the cabinet 6 such that the air paths are diagonally disposed ( e . g . about 45 degrees from the vertical ) so that they are self draining ; any moisture from the exhaust air including moisture caused by defrosting will drain downwardly for collection in a drain pan ( not shown ) from which it may be removed in any convenient manner . the corners 18 , 19 , 20 and 21 of the core 11 are provided with slidable sealing engagement members 22 , 23 , 24 and 25 so that the core 11 may be removed for cleaning , replacement , repair , etc . ; fig2 a and 2b show in more detail the sliding engagement members 22 and 25 . the sealing engagement members have l - shaped elements the inner angle part of which embraces a corresponding corner of the core 11 . each of the sealing engagement members is also provided with a sealing groove element which extends outwardly from the l - shaped element from a respective corner ( see for example sealing groove element 22a in fig2 a ). each sealing groove element extends from the front face 13 to the rear face ( 14 in fig8 ) of the core 11 such that when the door is closed one end of each sealing engagement member will sealingly abut the partition wall 15 and the opposite end will so abut the door insulating material 12 . the sliding engagement members are connected to a respective top wall 26 , side wall 27 , bottom wall 28 and / or side wall 29 ( as the case may be ) by a sliding and sealing engagement of each sealing groove with a sliding rail element of a respective wall extension member . the wall extension members may comprise only a rail fixed to a wall ( s ) or may , as desired or necessary , extend from other extension elements which are fixed to wall ( s ); for example , the wall extension member which engages with the sliding engagement member 22 also includes the partition wall 30 . the partition wall 30 is shown in fig2 on edge ; in fig2 a a portion of the rail 22b is shown which is integral with the partition wall 30 . the wall extension members including the rail elements extend forwardly from partition wall 15 up to the front door . the sealing engagement members and the wall extension members are so constructed that they removeably and slidingly engage with a sufficient sealing character such that when the door of the cabinet is closed four corner air pockets are defined in the interior of the cabinet 6 about the core 11 . the core 11 may be removed by sliding it out of position and vis versa to install it . as shown in fig2 partition wall 30 extends perpendicularly to the plain of the sheet of paper ; it also has a support bracket 31 . the partition wall 30 extends from the partition wall 15 to a point at the front of the cabinet sufficient to allow it to sealingly engage the material 12 fixed to the cabinet door when the door is closed . a part of the bracket 31 is shown as being cut away so as to expose the defrost opening 32 ( which traverses the partition wall 30 ) and the damper member 33 . the damper member 33 is so sized and shaped such that as shown in fig2 a side 33a of the damper member can block off the defrost opening 32 so that air cannot go through this opening ( i . e . in the ventilation configuration ). the damper member 33 is hinged at pivot point 34 by a rod which passes through the upper part of the body of the damper member 33 . the rod is rotatably engaged by a correspondingly shaped opening in the body of the damper member , the rod being fixed at one end thereof to the partition wall 15 and at the other end to the bracket 31 . the rod and its related opening in the damper body are sized and configured so that the damper member 33 can be rotatably displaced in the direction of the arrow 35 between the ventilation configuration and the defrost configuration ; in the latter configuration the exhaust air opening in the top wall 26 ( around which is disposed the duct collar 36 ) will be blocked off by the appropriately sized side 33b of the damper member 33 and the defrost opening 32 will at the same time be unblocked . the damper member 33 is displaced between the ventilation configuration and the defrost configuration by means of a motor 37 and rod 38 arrangement which includes a spring member 39 for biasing the damper member 33 in one configuration , the motor 37 being used to displace the damper member 33 to the other configuration . referring to fig2 c the motor - rod assembly is shown in more detail . thus the motor 37 has a shaft 37a which is fixed to an actuating arm 37b ; the motor is fixed to a support platform 37c which is in turn fixed to the partition wall 15 . one end of the rod 38 is rotatably disposed in a hole at the distal end of the actuating arm 37b while the other end of rod 38 is rotatably fixed in a similar manner ( not shown ) to the damper member 33 ; i . e . via a small plate / hole member fixed to the damper member . the spring member 39 is a ( helical ) coil spring disposed around the shaft 37a ; one end 37d of the coil is fixed with respect to the platform ( see dotted line ) and the other end 37e is fixed to the actuating arm 37b so as to bias the rod 38 ( and thus the damper member ) with respect to one configuration . activation and deactivation of the motor will thus cause the damper to be displaced between the ventilation and defrost configurations . any suitable motor ( such as for example a synchronous motor as made by hansen manufacturing company , inc .) may for example be used for this purpose . any other suitable damper mechanism may of course be used , keeping in mind that the purpose of the damper is to block off the appropriate air path for the ventilation cycle and the defrost cycle while leaving the other necessary air paths unobstructed . referring back to fig2 fresh air opening 40 and exhaust air opening 41 are provided which pass through the partition wall 15 at the foot thereof . the fresh air blower wheel 42 can be seen through the opening 40 ; the exhaust air blower wheel 43 can be seen through the opening 41 . these blower wheels form part of the blower assembly which shall be described below with reference to fig7 and 8 . the apparatus also includes air filters 44 and 45 ; only parts of which are shown . in fig2 the apparatus is shown as being in a ventilation configuration . in this configuration : exhaust air will proceed from the opening 41 into the lower right air pocket in the direction of the arrow 46a ; the exhaust air will then pass through the core 11 into the upper left air pocket in the direction of the arrow 46b ; the exhaust air will finally pass through to the exterior atmosphere in the direction of the arrow 46c . fresh air will proceed from the opening 40 into the lower left air pocket in the direction of the arrow 47a ; the fresh air will then pass through the core 11 into the upper right air pocket in the direction of the arrow 47b ; the fresh air will finally pass through the opening in the top wall 26 ( surrounded by duct collar 48 ) in the direction of the arrow 47c for delivery to the interior of the building . fig3 shows the apparatus of fig2 with the rear wall removed . the view shown is of the rear side of the partition wall 15 . the part of the cabinet on this side of the portion wall 15 has a portion wall 49 ( perpendicular to partition wall 15 ) and a blower housing 50 in which is disposed fan means for moving air through the apparatus . the partition wall 49 and the housing 50 partition this side of the cabinet interior into an exhaust air inlet pocket and a fresh air inlet pocket . the housing 50 has side openings ( not shown here , but see fig7 ) for communicating with each air pocket . the duct collars 51 and 52 respectively encircle openings in the top wall 26 for exhaust air and fresh air . the opening in the top wall leading to the fresh air pocket is encircled on the interior with a collar 53 ; collar 53 has a lower mouth which is cut at an inclined angle . the partition wall 49 is provided with a defrost opening 54 . a damper member 55 is also provided . the damper member 55 is sized and shaped such that as shown in fig3 a side 55a of the damper member can block off the defrost opening 54 so that air cannot go through this opening ( i . e . in the ventilation configuration ). the damper member 55 is hinged at pivot point 56 in a manner similar to that of the damper member 33 i . e . so that damper member 55 can be rotatably displaced in the direction of the arrow 57 between the ventilation configuration and the defrost configuration ; in the latter configuration ( dotted image of the damper member 55 ) the fresh air opening in the top wall 26 ( around which is disposed the duct collar 53 ) will be blocked off by the appropriately sized side 55b of the damper member 55 and the defrost opening 54 will at the same time be unblocked . the damper member 55 is displaced between the ventilation configuration and the defrost configuration by means of a motor 58 and rod 59 arrangement which includes a spring member 60 for biasing the damper member 55 in one configuration , the motor being used to displace the damper member 55 to the other configuration ; the motor - rod assembly takes the same form as the assembly shown in fig2 c . in fig3 the apparatus is shown as being in a ventilation configuration . in this configuration : exhaust air will proceed from the opening encircled by the collar 51 into the left exhaust air pocket in the direction of the arrow 61a ; the exhaust air will then pass through a side opening and a top opening ( not shown ) in the housing 50 in the direction of the arrows 61b ; the exhaust air will finally pass through to the blower means and on to opening 41 ( see fig2 ). fresh air will proceed from the opening encircled by the collars 52 and 53 into the right fresh air pocket in the direction of the arrow 62a ; the fresh air will then pass through a side opening ( not shown ) in the housing 50 in the direction of the arrow 62b ; the fresh air will then pass through to the blower means and on to opening 40 ( see fig2 ). the sides 33a , 33b , 55a and 55b of the damper members 33 and 55 may , for example , be provided with a layer of sponge like resilient material or the like in order to facilitate the sealing of the various openings to be blocked by these members ; such layers are shown in fig2 and 3 by the narrow strips on the blocking sides of the members . referring to fig4 a , 4b , 5a , 5b , 6a and and 6b , these figures illustrate in a schematic manner the disposition of the partitioned air pockets of the apparatus shown in fig2 and 3 , the general structure of the partitioning members and the consequent flow of air through the apparatus during the ventilation cycle and the defrost cycle ; ( the same reference numbers as used in fig2 and 3 are as far as possible used with respect to these latter figures ; in fig5 a and 6a , the blower housing is , for illustration purposes only , not shown , partition wall 49 being shown instead as extending right up to the bottom wall 28 ). fig4 a shows a view of the top of the apparatus , in a ventilation configuration , with the attendant air flows numbered as in fig2 and 3 . as can be seen the interior of the cabinet 6 is partitioned into a number of air pockets by partition members such as partition walls 15 , 30 and 49 . the dampers ( not shown in fig4 a ) block off the openings 32 and 54 in the partition walls 30 and 49 such that during a ventilation cycle fresh air flow is confined to the fresh air path defined in the cabinet and the exhaust air flow is confined to the exhaust air path also defined in the cabinet . thus , as may be seen in fig5 a and 5b , exhaust air is confined to the air path outlined by the arrows 61a , 61b , 46a , 46b and 46c ; on the other hand the fresh air is confined to the air path outlined by the arrows 62a , 62b , 47a , 47b and 47c . fig4 b shows the apparatus in a defrost configuration with corresponding attendant air flows . in this configuration the fresh air inlet side of the fresh air path is blocked off by the damper 55 ; the exhaust air outlet side of the exhaust air path is also blocked off by the damper 33 . at the same time , the defrost openings 32 and 54 are no longer obstructed by the damper members . the input exhaust air shown by arrow 61a is split into two air streams shown by arrows 61b and 61c . on the other hand the air delivered back to the building denoted by the 47 &# 39 ; c is composed of two merged air streams denoted by the arrows 46d and 47b . thus , as may be seen in fig6 a and 6b , fresh air is excluded from the apparatus and the exhaust air path to the exterior of the building is also blocked off . at the same time , exhaust or stale warm air from the interior of the building circulates through the apparatus as outlined by the arrows 61a , 61b , 61c , 61d , 46a , 47a , 47b , 46b , 46d and 47 &# 39 ; c . turning to fig7 and 8 , the same reference numerals will be used with respect of those elements which are common with the elements referred to in fig2 and 3 . fig7 and 8 provide a more detailed view of the blower assembly of the apparatus . fig7 shows the apparatus as illustrated in fig2 but with a portion of the interior structure removed so as to expose the interior of the blower housing 50 . a motor 63 is disposed within the housing 50 . the blower housing 50 has two blower housing portions 64 and 65 which are disposed on either side of the motor 63 ; the two blower wheels 42 and 43 are disposed in respective blower housing portions ; the two blower wheels 42 and 43 are also disposed on opposite sides of the motor 63 and are mounted directly on corresponding ends 66a and 66b of the shaft of the motor 63 . the blower housing 50 ( including portions 64 and 65 ) is made of an expanded polystyrene material and is spaced apart from motor 63 . the motor 63 is disposed on a stiff bracket 67 . the motor 63 is rigidly joined by a suitable fixation means ( such as nut / bolt ) to the shelf member 68 of the bracket 67 . preferably the motor is fixed to a stiff bracket in the manner described in the co - pending u . s . pat . application ser . no . 785 , 058 ; i . e . apart from stiff corner edges of the cabinet , the stiff bracket 67 does not contact other elements of the apparatus ; the other elements include elements such as insolation , dividing walls , etc .. the blower housing portion 64 is provided with exhaust air ( intake ) openings 69 and 70a ; the end 66a of the ( motor ) shaft extends through the opening 70a . the blower housing 50 has an additional opening 70b in the top part thereof which also communicates with the exhaust air inlet pocket ; i . e . exhaust air may thus enter the blower housing portion 64 not only via opening 69 but also via openings 70a and 70b . for the openings 70a and 70b , exhaust air passes through the opening 70b , around the motor ( providing a cooling effect therefore ) and then passes through the opening 70a . the blower housing portion 65 on the other hand is provided with a fresh air ( intake ) opening 71 but does not have an opening corresponding to the opening 70a . the blower housing portion 65 has in place of such opening a wall 71a which is provided with an opening which is large enough to allow the end 66b of the ( motor ) shaft to rotatably pass therethrough but small enough to stop or impede the passage of exhaust air into the interior of the housing portion 65 ; if desired or necessary , however , any suitable sealing gasket means may be disposed around the shaft in the hole to provide an airtight seal . fig8 shows a side view of the apparatus illustrated in fig2 and 3 with the side wall removed to expose the internal elements . these elements include the heat recovery core 11 , air filter 44 ( these are all of known construction -- see for example u . s . pat . no . 5 , 002 , 118 ). for fig8 the blower housing 65 and blower wheel 42 have been removed to expose the motor 63 and the blower housing 64 . the exhaust air inlet opening 70 is hidden from view on the opposite side of the housing portion 64 . in fig8 the door of the enclosure is present as indicated generally by the reference numeral 72 . in fig8 the outline of the border of the blower channel in housing portion 64 is illustrated generally by the dashed line 73 . the cabinet can also , if desired , includes appropriate temperature sensor ( s ), electric wiring , control mechanisms for controlling the various motors for the ventilation and defrost cycles , etc .. ( none of which is shown in the figures but which can be provided in any suitable or desired conventional manner ). these mechanism may example include programmable computer type controls . the defrost cycle for example would normally be triggered by a thermistor or thermostat connected to a timer . it is to be understood that the apparatus of the present invention may take many other forms without departing from the spirit and scope thereof as described in the present specification ; the specific embodiment illustrated above being provided by way of illustrative example only .