White Mountain Hearth DVTL27FP90N-1 Installation Instructions And Owner's Manual page 27

VERTICAL TERMINATION (continued)
The included flue restrictor disc may be used in vertical vent runs
of 10 to 40 feet as measured from the bottom of the fireplace.
Use the restrictor disc when a more yellow flame is desired.
If the flue restrictor is installed on a natural gas fireplace, the rear
burner's air shutter must be adjusted to 1/8-in. open.
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