HearthStone Manchester 8360 Owner's Manual page 21

PRIMARY AIR CONTROL: The primary air control
Lever is located under the center of the ash lip. The
primary air control allows you to regulate the amount
of air entering the firebox. Generally, the more air
allowed into the firebox, the faster the rate of burn;
conversely, less air creates a slower burn. For
maximum air flow, move the lever to the left as far as
possible; move the lever as far right as possible for
minimum air flow (does not close completely).
Figure 13 – Air Control Low Position
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Hearthstone Quality Home Heating Products, Inc ® Manchester Model 8360