Lennox Hearth LA41TCF, LA41CF Hearth Extension Dimensions, Using the k formula Minimum, Thickness

Note: Any noncombustible material whose k value is less than .84 or whose r value is more than 1.19 is acceptable.

If the fireplace is installed on a combustible floor, use the metal safety strips (provided) on the floor extending half under the fire- place and half under the hearth extension.

A wall shield is required where a continuous perpendicular side wall is within 15" of the fireplace opening (see Figure 44 ). Use a 40" W x 40" H wall shield constructed of a durable, noncombustible material having an equal or better (lower k value) insulating value than k =

.84 BTU IN/FT2 HR ° F. At no time may a perpendicular side wall be located closer than 12" from the fireplace opening.

If fireplace is installed diagonally across a 90° corner; no wall shields are required.

Hearth Extension Dimensions

(For Figures 43 and 44 )

A20"

B41"

C12"

D65"

C B C

A

D

Figure 43

Wall Shield Required

Where Less Than 15"

24”

36”

C

B

Methods of Determining Hearth Extension and Wall Shield

Equivalents - To determine the thickness required for the alternate material when either the “k” value or “r” value is known, use either the k formula or r formula.

Example: If Micore 160 is to be used for the hearth extension fireplace, how thick must this material be?

kM = k value per inch of alternate material rM = r value per inch of alternate material

TM = minimum thickness required for alternate material TS = standard thickness of the alternate material

kL = k value per inch of listed material rL = r value per inch of listed material

TL = minimum thickness of listed material

Note: An asterix “*” indicates, it is a value taken from Table 1.

Using the k formula:

Minimum		k-value (per Inch) of		Specified min.
thickness of	=	alternate material (kM) x		thickness
alternate		k-value (per inch)		of listed
material (TM)		of listed material (kM)		material (TL)
TM (inches)	=	kM	x	TL
		*.84
TM (inches)	=	*.35	x	1"
		*.84
.416 (inches) =		.416	x	1"

Answer - The minimum required thickness of the Micore 160 is .417”, therefore round up to nearest standard thickness available which is 1/2”.

Using the r formula:

Minimum		r-value (per Inch)		Specified min.
thickness of	=	of listed material	x	thickness
alternate		r-value (per inch)		of listed
material		of alternate material		material
TM (inches)	=	rL	x	TL
		rM
TM (inches)	=	*1.19	x	1"
		*2.86
.416 (inches) =		.416	x	1"

16

D

Figure 44

C A

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.