Concentric snap-together direct vent structure and associated fabrication methods

Each longitudinal section of a concentric direct vent duct structure for a fuel-fired appliance, such as a furnace, is formed using a tubular outer duct having an axially spaced pair of triangular stand-off frame members anchored therein, and a smaller diameter inner duct having an axially spaced pair of ramped external annular locking projections thereon. To assemble the concentric duct section, the inner duct is simply axially inserted into the outer duct. As the inner duct is inserted into the outer duct, a ramped annular side surface on the leading inner duct locking projection resiliently and radially outwardly deflects side wall portions of the stand-off frame members and then permits them to snap back to their original undeflected positions. When the inner duct reaches its operative position within the outer duct, facing radially extending side surfaces on the locking projections straddle the stand-off frame members and act as abutments therewith to prevent removal of the inserted inner duct from the outer duct in either axial direction. The cooperation between the inner duct locking projections and the resiliently deflectable side wall portions of the stand-off frame members thus provides for a convenient snap-locking installation of the inner duct within the outer duct.

BACKGROUND OF THE INVENTION 
The present invention generally relates to vent apparatus for fuel-fired 
direct vent heating appliances and, in a preferred embodiment thereof, 
more particularly relates to a specially designed snap-together concentric 
direct vent structure section for such appliances, and methods fabricating 
the vent structure section. 
Gas fired heating appliances, such as furnaces, located in interior paces 
of buildings require during their firing a continuous supply of combustion 
air. At the same time, such appliances generate hot combustion gases which 
must be appropriately discharged therefrom to the exterior of the 
building. One technique for supplying combustion air to a fuel-fired 
heating appliance, while at the same time creating a path through which 
its generated combustion gases may be discharged to the exterior of the 
building, is to operatively connect a concentric direct vent structure to 
the appliance. 
As conventionally constructed, a direct vent structure of this general type 
is defined by lengths of concentric inner and outer ducts extended between 
the furnace and a combination intake/discharge assembly mounted on the 
outer side of an exterior wall of the building. The concentric inner and 
outer ducts define therebetween an annular space, with combustion gases 
from the furnace being discharged through the interior of the inner duct 
and out the intake/discharge assembly, while outside combustion air is 
drawn inwardly through the intake/discharge assembly and through the inner 
duct/outer duct annulus to the furnace. 
To maintain the inner and outer ducts in a concentric alignment, stand-off 
structures are secured within the outer duct, extend through the inner 
duct/outer duct annulus, and provide lateral centering support for the 
inner duct. The overall concentric direct vent structure is typically made 
up from axial sections of concentric inner duct/outer duct assemblies 
which are appropriately connected in end-to-end relationships to form the 
desired length of the finished direct vent structure. From a materials 
standpoint, each of these sections conventionally includes an aluminum 
inner duct, a galvanized steel outer duct, and one or more non-galvanized 
steel stand-off members secured within the outer duct and supporting the 
inner duct therein. 
Various well-known problems, limitations and disadvantages have heretofore 
been associated with the conventional manufacture and assembly of the 
individual longitudinal sections of this general type of concentric direct 
vent structure. For example, the installation of the stand-off structures 
within the outer duct tends to be difficult and relatively expensive. 
Additionally, the connection of the stand-off structures to the inner duct 
also tends to be difficult and relatively expensive. Typically, this 
connection of the stand-off structures to the inner duct undesirably 
required penetration of the inner duct (for example, with rivets), and 
thus resulting gas leaks in the inner duct, since spot welding techniques 
could not be used to join the aluminum inner duct to the steel stand-off 
structures. Moreover, the proper insertion and positioning of the inner 
duct section within its associated outer duct section tends to be a 
tedious and time-consuming task, which undesirably adds to the overall 
fabrication cost of conventionally constructed concentric direct vent 
assemblies of this general type. 
From the foregoing it can be seen that a need exists for improvements in 
the manner in which a longitudinal section of a concentric duct direct 
vent structure is constructed. It is to this need that the present 
invention is directed. 
SUMMARY OF THE INVENTION 
In carrying out principles of the present invention, in accordance with a 
preferred embodiment thereof, each longitudinal section of a concentric 
direct vent duct structure for a fuel-fired appliance, such as a furnace, 
is provided with a specially designed snap-together construction which 
simplifies and reduces the expense of the construction of the overall 
direct vent structure. 
From a broad perspective, each longitudinal section of the direct vent 
structure comprises an outer duct extending along a first axis, specially 
designed stand-off apparatus anchored within the outer duct and preferably 
including first and second axially spaced first and second standoff 
members anchored within the outer duct and circumscribing the first axis; 
an inner duct extending along a second axis and being laterally smaller 
than the outer duct; and first and second axially spaced exterior 
projections disposed on and extending laterally around the inner duct. 
To assemble the concentric duct section, the inner duct is simply inserted 
into the outer duct to a predetermined operative position therein. The 
stand-off members in the outer duct and the exterior projections on the 
inner duct are configured to be cooperable, in response to axial insertion 
of the inner duct into the outer duct to the predetermined operative inner 
duct position therein, to provide a snap-fitted relationship between the 
inner and outer ducts, in which the centered inner duct is locked against 
removal in either axial direction from the outer duct, when the inner duct 
reaches its predetermined operative position within the outer duct. 
Preferably, each of the inner and outer ducts has a circular cross-section, 
and each of the stand-off members has a frame-like polygonal 
configuration, representatively triangular, and has plate-shaped side wall 
portions which are resiliently deflectable in radially outward directions 
and have opposite, axially facing side edge portions. The locking portions 
on the inner duct are preferably annular external ribs having axially 
inwardly facing, radially extending first side surfaces, and annular 
second side surfaces that are ramped in axially inward and radially 
outward directions. 
As the inner duct is inserted into the outer duct, the ramped annular side 
surface on the leading inner duct projection resiliently and radially 
outwardly deflects side wall portions of the stand-off frame members and 
then permits them to snap back to their original undeflected positions. 
When the inner duct reaches its operative position within the outer duct, 
the facing radially extending side surfaces on the inner duct projections 
straddle the stand-off frame members and act as abutments therefor to 
prevent removal of the inserted inner duct from the outer duct in either 
axial direction. The cooperation between the inner duct locking 
projections and the resiliently deflectable side wall portions of the 
stand-off frame members thus provides for a convenient snap-locking 
installation of the inner duct within the outer duct. 
To facilitate the end-to-end connection of a series of the assembled 
longitudinal concentric duct sections to form the overall concentric 
direct vent structure, end portions of the inner and outer ducts in each 
assembled longitudinal concentric duct sections are provided with annular 
interior resilient seal members carried in annular exterior seal pocket 
areas and radially projecting into the duct. In this manner, a series of 
assembled concentric duct sections may be simply telescoped in end-to-end 
relationships to sealingly join the facing ends of each opposing pair of 
inner ducts and outer ducts and thereby form the overall concentric direct 
vent duct structure. 
While the overall stand-off apparatus anchored within the outer duct is 
preferably defined by axially spaced first and second individual stand-off 
members, the stand-off apparatus, particularly in the case of relatively 
short concentric duct sections, could alternatively be defined by a single 
stand-off member. In this case, axially spaced first and second portions 
of the single stand-off member would perform the same inner duct locking 
function as the separate axially spaced stand-off members.

DETAILED DESCRIPTION 
Schematically shown in FIG. 1 is a fuel-fired heating appliance, such as a 
gas-fired furnace 10, to which a specially designed concentric direct vent 
structure 12 is attached. Direct vent structure 12 is connected between 
the furnace 10 and a conventional combination intake/exhaust assembly 13 
on an outside wall 14, and is used to flow outside combustion air 16 into 
the furnace 10, while at the same time exhausting combustion gases 18 from 
the furnace 10. 
Referring now to FIGS. 1-5, the concentric direct vent structure 12 is 
representatively formed from three joined-together tubular sections S1-S3, 
each having a tubular outer galvanized steel combustion air intake duct 
20, and a smaller diameter tubular inner aluminum combustion discharge 
duct 22 held concentrically in place within the outer duct 20 by means of 
a unique stand-off apparatus, representatively in the form of an axially 
spaced pair of specially designed stand-off members 24a,24b anchored 
within the outer duct 20 and cooperating with structures on the inner duct 
22 to provide a unique snap-together construction for each of the sections 
S1-S3 as later described herein. In each assembled concentric inner/outer 
duct section S, the inner duct 22 is axially locked within its associated 
outer duct 20 by the axially spaced stand-off members 24a,24b as later 
described herein. 
As can be seen in FIG. 5, the ducts 20,22 in each section S form 
therebetween an annular space 26. During operation of the furnace 10, 
outside combustion air 16 is drawn into the furnace 10 through the annular 
spaces 26, while hot combustion gases 18 discharged from the furnace flow 
outwardly through the interiors of the inner ducts 20. 
In each section S, the outer duct 20 has a continuous butt weld seam 28 
along its length (see FIG. 7), an outwardly projecting annular seal pocket 
30 adjacent one of its ends 32 (see FIGS. 2 and 7), and a longitudinally 
spaced pair of annular external stiffening portions 34 positioned between 
the seal pocket 30 and the opposite end 35 of the outer duct 20. Each seal 
pocket 30 has interiorly disposed therein an annular elastomeric sealing 
member 36 (see FIG. 3) having the indicated webbed and ridged 
cross-sectional configuration, the sealing member being of a conventional 
type for this application and being commercially available from Selkirk, 
Inc., 14801 Quorum Drive, Dallas, Tex. 75240-7584. 
The inner duct 22 in each section S has a continuous butt-weld seam 38 
along its length, an outwardly projecting annular seal pocket 40 adjacent 
one of its ends 42, and an opposite end 44. Additionally, the inner duct 
22 in each section S has an axially spaced pair of specially configured 
ramped annular stiffening portions 46a,46b (see FIGS. 4-6). In addition to 
stiffening the inner duct 22, these annular external portions 46a,46b also 
function, as later described herein, to provide a unique snap-fit 
connection of the inner duct 22 within its associated outer duct 20. 
Representatively, in each section S the end 42 of the inner duct 22 is 
adjacent the end 35 of the outer duct 20. Seal pocket 40 receives an 
annular, inwardly projecting elastomeric seal 36 (see FIG. 2) similar to 
the seal 36 received in the seal pocket 30 of the outer duct 20 and 
illustrated in FIG. 3. 
As schematically illustrated in FIG. 7, adjacent sections S (for example, 
the illustrated sections S1 and S2) may be very rapidly joined by simply 
telescoping the two sections, as indicated by the arrow 48 in FIG. 7, to 
(1) cause the end 35 of the outer duct 20 in section S1 to enter the end 
32 of the outer duct 20 in section S2 and be sealingly engaged by the 
annular seal 36 within the seal pocket 30 of the outer duct 20 of section 
S2, and (2) cause the end 44 of the inner duct 22 of section S2 to enter 
the end 42 of the inner duct 22 of section S1 and be sealingly engaged by 
the annular seal 36 within the seal pocket 40 of the inner duct 22 of 
section S1. 
According to a key aspect of the present invention, the specially 
configured, axially spaced pairs of stand-off frame members 24a,24b and 
annular exterior stiffening portions 46a,46b on each inner duct 22 permit 
a unique snap-together assembly of each associated outer and inner duct 
pair 20,22 which will now be described with reference to FIGS. 4-6. 
Each of the two axially spaced apart stand-off members 24a,24b is 
representatively formed from a rectangular strip of non-galvanized steel, 
and is bent to a generally triangular shape having three plate-shaped side 
walls 50,52,54 with truncated apex portions 56 positioned between the side 
walls. Each apex portion 56 is anchored to the inner side of the outer is 
duct 20, as by spot welds 58. The side walls 50,52,54 have, relative to 
the outer duct 20, axially inwardly facing side edges 50a,52a,54a and 
axially outwardly facing side edges 50b,52b,54b. 
As may be best seen in FIG. 6, each of the annular external ribs 46a,46b on 
the inner duct 22 has a generally radially extending, axially inwardly 
facing annular side surface 60, and an opposite annular ramped annular 
side surface 62 which is sloped axially inwardly and radially outwardly. 
To assemble the illustrated representative longitudinal section of the 
concentric direct vent structure 12 shown in FIG. 4, the inner duct 22 is 
representatively inserted leftwardly into the outer duct 20 as indicated 
by the arrow 48 in FIGS. 4 and 7. AS the left annular ramped rib 46a on 
the inner duct 22 sequentially passes through the right and left stand-off 
members 24b and 24a, the ramped annular side surface 62 of the rib 46a 
resiliently cams the side walls 50,52,54 of the stand-off members 24b,24a 
radially outwardly, as indicated by the arrows 64 in FIG. 6, to permit 
passage of the rib 46a sequentially through the stand-off members 46b,46a. 
After the rib 46a passes through the stand-off members 46b,46a the 
outwardly deflected side wall portions 50,52,54 of each stand-off member 
snap back to their original undeflected positions shown in FIG. 5. Just as 
the rib 46a leftwardly passes through the left stand-off member 24a, the 
right, unramped annular side surface 60 of the rib 46a is leftwardly 
adjacent the left side edges 50b,52b,54b of the side walls 50,52,54 of the 
left stand-off member 24a, and the left, unramped annular side surface 60 
of the right rib 46b is rightwardly adjacent the right side edges 
50b,52b,54b of the side walls 50,52,54 of the right stand-off member 24b. 
Thus, the cooperation between the axially spaced apart ribs 46a,46b on the 
inner duct 22 and the axially spaced apart stand-off members 24a,24b 
anchored to the outer duct 20 permit the inner duct 22 to simply be 
axially snapped into place within the outer duct 20. While the 
installation was illustrated and described with the inner duct 22 being 
leftwardly inserted into the outer duct 20 (see FIG. 4), the oppositely 
sloped annular side surfaces 62 on the inner duct ribs 46a,46b permit the 
operative axial snap-fit insertion of the inner duct 22 in either axial 
direction into the outer duct 20. 
With the inner duct 22 in its snapped-into-place installed FIG. 4 
orientation within the outer duct 20, circumferential portions X of the 
unramped, axially inwardly facing annular side edges 60 (see FIG. 5) 
radially outwardly overlap the facing side edges of the stand-off member 
side walls 50,52,54 (see FIG. 4 also) in a manner locking the inserted 
inner duct 22 within the outer duct 20 against removal in either axial 
direction therefrom. The stand-off members 24a,24b thus laterally position 
the inner duct 22 in a concentric relationship with the outer duct 20 and 
axially lock it in place therein by means of the cooperation between the 
radially resilient stand-off members 24a,24b and the inner duct ribs 
46a,46b. 
While the side walls 50,52,54 of the stand-off members 24a,24b are radially 
resilient, they are quite stiff in the axial direction, thereby firmly 
preventing appreciable relative axial movement between the outer and inner 
duct portions 20,22 of each section S of the overall direct vent structure 
12. Thus, when the sections S are joined end-to-end as previously 
discussed in conjunction with FIG. 7, the inner duct 22 is firmly braced 
in opposite axial directions within the outer duct 20. The stand-off 
mebers 24a,24b have been representatively illustrated as having generally 
triangular configurations. However, they alternatively could have other 
polygonal configurations if desired. 
As can be seen from the foregoing, the present invention provides a 
uniquely simple and rapid snap-together construction for an axial section 
of a concentric direct vent structure which, in turn, simplifies and 
reduces the cost of constructing the overall concentric direct vent 
structure compared to conventional fabrication techniques. In addition to 
these advantages, this construction technique eliminates completely the 
necessity of penetrating the inner duct to operatively associate it with 
the stand-off members carried within the outer duct. In fact, no fasteners 
of any kind are required to operatively associate the inner duct with the 
stand-off members--once the inner duct is axially snapped into place 
within the outer duct, this operative association between the inner duct 
and the stand-of members within the outer duct is automatically achieved. 
While the overall stand-off apparatus anchored within the outer duct 20 is 
preferably defined by axially spaced first and second individual stand-off 
members 24a and 24b, it will be appreciated by those of skill in this 
particular art that such stand-off apparatus, particularly in the case of 
s shorter concentric duct sections, could alternatively be defined by a 
single stand-off member. In this case, axially spaced first and second 
portions of the single stand-off member would perform the same inner duct 
locking function as the separate axially spaced stand-off members 24a,24b 
representatively illustrated and described herein. 
The foregoing detailed description is to be clearly understood as being 
given by way of illustration and example, the spirit and scope of the 
present invention being limited solely by the appended claims.