Lennox Hearth MERIT PLUS MP-36OD - Manual (Page 22)

NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE. 22
Calculating Minimum Thickness if
Multiple Materials are Used
At times it is important to know what
combination of materials are acceptable for use
as fl oor protection. The “R values” are used to
determine acceptable combinations of materials
because “R values” are additive where r and k
values are not.
“R value” = 1/k = "r" x thickness of material
used
Example: Given that the required “R value”
for a suitable fl oor protector used must be
equal to or greater than:
“R” = r
L
x T
L
= 1.19 x 1” = 1.19."
If it is desired to elevate a marble hearth
extension to a level of 5” or more above
the fl oor surface. What combination of
noncombustible materials can be used to
accomplish this?
If common brick is used so that the 3 1/2”
dimension is the height, “R” for the common
brick becomes:
brick
“R”
M
= r
M
x T
S
= 0.20 x 3 1/2" = .70
Using 1/2” of mortar to set the brick, “R” for
the mortar is calculated as follows:
mortar
“R”
M
= r
M
x T
S
= 0.20 x 1/2" = .10
Next, a 3/4" marble slab set in 1/2" mortar cov-
ers the brick, “R” for the marble and mortar
becomes:
marble
“R”
M
= r
M
x T
S
= 0.05 x 3/4" = .038
mortar
“R”
M
= r
M
x T
S
= 0.20 x 1/2" = .10
The sum of all “R values” is: .70 + .10 +. 038
+ .10 = .938
This would NOT be an acceptable combination
of material for the hearth extension since the
total calculated “R value” of the materials used
is under the required “R value” of 1.19. An
additional layer of insulating materials must
be used.
Note: Also s ee NFI Certifi cation Manuals for
expanded explanation on calculating “R val-
ues” when multiple materials are used.
Secure the hearth extension to the fl oor to
prevent possible shifting.
FINISH TO YOUR TASTE
There are a wide variety of “fi nished looks” for
these fi replaces, from formal wall decor with
elaborate mantels to rustic wood paneling or
warm brick facings.
Only noncombustible materials like stone, tile,
brick, etc., may overlap the black front facing.
Do not extend these materials beyond the black
facing and into the fi rebox area. Be sure not to
interfere with the installation and operation of
glass doors or block the upper or lower grilles.
Seal all joints between the black facing and
wall surrounds to prevent air intrusion. Use
noncombustible caulking material only to seal
the black metal facing to the surround material
on the fi nished wall.
Combustible materials may project beyond the
sides of the fi replace opening as long as they
are kept within the shaded areas illustrated in
Figure 55.
Figure 54
7"
Max. Thickness Of Hearth Extension When
Non-Circulating Models Are On The Floor
Figure 55
45 °
Combustible Ma-
terials Allowed In
Shaded Area “Safe
Zone”
12"
Fireplace
7"
7"
1"
12" Min. Distance To
Unprotected Side Wall
Min. Distance To
Protected Side Wall
WARNING
The crack between the fi replace
and the hearth extension must be
sealed with a noncombustible
material.
AVERTISSEMENT
La fente entre le foyer et
l’extension d’âtre doit être
scellée avec un matériel non-
combustible.
AVERTISSEMENT
En installant la prolongation de foyer
devant un modèle decirculation de
cheminée, la cheminée doit être
augmentée si la taille de la prolon-
gation de foyer excède 7”(pouces)
au-dessus du fond de la cheminée
(Figure 54).
En installant l’extension d’âtre faites
attention à ne pas bloquer la grille
d’entrée d’air sur les modèles à
circulation d’air.
WARNING
When installing hearth extension
in front of a non-circulating fi re-
place model, the fi replace must be
raised if height of hearth extension
exceeds 7" above the bottom of
fi replace (Figure 54).
When installing the hearth exten-
sion be careful not to block the
heat-circulating air inlet grill on
circulating models.
1/2"