When you have a rip-roaring woodstove fire going, and a chimney charged with rising superheated exhaust gases, the air flow through the firebox is considerable. If you cut down the supply of air too abruptly, the fire instantly consumes the available air, creating a powerful vacuum inside the stove. If strong enough, this vacuum will sometimes reverse the flow inside the chimney, pulling a “gulp” of air back down the flue into the firebox.
When this pocket of air hits the fire, a mini-explosion occurs, and the resulting sudden extreme pressurization inside the firebox forces smoke out through the draft control, door gasketing and other tiny openings that exist in even the most “airtight” woodstoves.
This brief period of pressurization is followed immediately by extreme depressurization, as the explosion consumes all the available oxygen, and another gulp of air is pulled down the chimney, causing the process to repeat. We call this “whuffing”, due to the usual accompanying sound of muffled explosions. In extreme cases, these repeated explosions can cause the stove to actually move around on the hearth!
Although whuffing usually only occurs for a short time (until the starved-for-air fire dies down, reducing the vacuum effect), it should be avoided, as the repeated pressurization inside the stove caused by the mini-explosions could fill the house with smoke, blow the door open, disconnect the exhaust pipe, or damage the stove.
Whenever you’ve had a hot fire going and want to “bank” it down for the night, care must be taken not to cut down the air supply too suddenly. Adjust your draft control to, say, half throttle (you’ll have to experiment a bit to find the setting that works best for you), for a few minutes, then continue to turn it down in gradual stages, so the fire can quench down slowly. If the stove starts to whuff, open the draft control a bit to supply more air to the fire for a few minutes, then resume your gradual reduction of combustion air until you reach your all-night burn setting.