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NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
LENNOX HEARTH PRODUCTS • MERIT
®
PLUS DIRECT VENT GAS FIREPLACES (MPD33/35/40/45) • INSTALLATION INSTRUCTIONS
Figure 31: Rear Vent - ONE 90-DEGREE ELBOW
Figure 30: Top Vent - STRAIGHT
Figure 32: Top Vent - TWO 90-DEGREE ELBOWS
VERTICAL VENT FIGURES/TABLES
Note: It is very important that the horizontal/
inclined run be maintained in a straight (no dips)
and recommended to be in a slightly elevated
plane, in a direction away from the fireplace of
1/4" rise per foot (20 mm per meter) which is
ideal, though rise per foot run ratios that are
smaller are acceptable all the way down to at
or near level.
Note: SV4.5VF (Secure Vent), SF4.5VF (Secure
Flex) firestop/spacer must be used anytime vent
pipe passes through a combustible floor or ceil-
ing. SV4.5HF (Secure Vent), SF4.5HF (Secure
Flex)firestop/spacer must be used anytime vent
pipe passes through a combustible wall.
Note: Two 45 degree elbows may be used in
place of one 90 degree elbow. The same rise
to run ratios, as shown in the venting figures
for 90 elbows, must be followed if 45 degree
elbows are used.
Note: An elbow is acceptable as 1 foot of
vertical rise, except where an elbow is the
only vertical component in the system. (See
Figure 39 on page 21 ).
Note: Secure Vent (rigid vent pipe) is shown
in the figures; Secure Flex (flexible vent pipe)
may also be used.
A Vent Restrictor, as shown in Figure
18 on Page 13, must be used in this
application.
This table shows a 1 (V) to 2 (H) ratio.
For every 1 foot of (V) vertical, you are
allowed 2 feet of (H) horizontal run, up
to a maximum horizontal run of 8 feet.
Example: If 8 feet of (H) horizontal vent
run is needed, then 4 feet minimum (V)
vertical vent will be required.
TABLE A
H Maximum V Minimum
feet feet
2 (0.610) 1 (0.305)
4 (1.219) 2 (0.610)
6 (1.829) 3 (0.914)
8 (2.438) 4 (1.219)
V + H = 40 feet (12.2 meters) Max.
H = 8 feet (2.4 meters) Max.
Ratio V to H ratio is 1:2
(meters)
(meters)
Example: If 20 feet of (H) horizon-
tal vent run is needed, then 4 feet
minimum of (V) vertical vent will
be required.
This table shows a 1 (V) to 5 (H) ratio.
For every 1 foot of (V) vertical, you
are allowed 5 feet of (H) horizontal
run, up to a maximum horizontal
run of 20 feet.
An elbow is acceptable as 1 foot of
vertical rise except where an elbow
is the only vertical component in the
system. See Figure 39.
TABLE B
H Maximum
V Minimum
feet
(meters)
feet
5
(1.524)
Elbow Only
5
(1.524)
1
(0.305)
10
(3.048)
2
(0.610)
15
(4.572)
3
(0.914)
20
(6.096)
4
(1.219)
V + V
1
+ H = 40 feet (12.2 m) Max.
H = 20 feet (6.096 meters) Max.
(meters)
WARNING
Separate sections of concentric flexible vent pipe may NOT be joined
together under any circumstances.
Max. 40 ft
(12.2 m)
*When using
Secure Flex, use
Ceiling Firestop/
Spacer SF4.5VF.
Ceiling
Firestop/
Spacer
SV4.5VF*
V
H
Ceiling
Firestop/Spacer
SV4.5VF*
*When using Secure Flex, use
Ceiling Firestop/Spacer
SF4.5VF.
H
V
V1
Ceiling
Firestop/Spacer
SV4.5VF*
Wall
Firestop/Spacer
SV4.5HF**
*When using Secure Flex, use
Ceiling Firestop/Spacer SF4.5VF.
**When using Secure Flex, use
Wall Firestop/Spacer SF4.5HF.