Page 16 from my 3100i ACT manual
"... The story of the Three Burning Stages . . .
It helps to know a little about the actual burn process, which entails three discernible burning stages. The first stage
is called the kindling stage. During this stage the fuel reaches the boiling temperature of water, 212°F, evaporating the
moisture found to some degree in all wood.
Because the process takes heat from the insert during this initial drying stage, each new load of wood reduces the chances for a good, clean burn. For this reason it is always best to burn dry, seasoned firewood, and operate the controls properly.
The control on the right side of your insert is called the Start-Up Air Control and is used primarily during this first kindling stage of burning.
During the secondary stage, the wood gives off flammable gases which burn above the fuel with bright flames. These flames above the fuel must be maintained until the third stage to insure proper burning. During this stage you may adjust your insert for a low burn rate. To achieve a low burn rate it is necessary to close down the air while still maintaining some flames. If the flames tend to go out, the setting is too low. The Primary Control, located in the center of the insert beneath
the ashcatcher, will assist you in adjusting the insert for a low burn rate.
The third stage of burning is the charcoal stage. This happens when the flammable gases have been burned and the charcoal remains. The coals burning with hot blue flames is a naturally clean portion of the burn. It is very important to reload your insert while enough lively hot coals remain in order to provide the amount of heat needed to dry and rekindle the next load of wood. Open up both controls for a short while before reloading to liven up the coal bed. You should also
break up any large chunks and distribute the coals so the new wood is laid on hot coals. Leave both controls open until the new wood load is burning well enough to maintain the secondary stage of burning and then set controls to your desired heat output setting..."
Keep in mind you will only get about 10k btus (maybe only 80% of that) of heat at the low setting, so this setting is really good after you have brought the space to desired temp. or if you are supplementing your regular house heat.