So, last night I decided to load up our Century Fireplace exactly as the manual directed: Build up a quick fire under smaller logs, close the damper, and let it burn to coals. Once you have a bed of coals you fill the entire box with unsplit logs (ours are on the small side, about 6-8") and totally close off the air. Yes... you actually close both the damper and the flu!
Even though we'd used it as a fireplace (doors open) and then loaded it up just before bed, I wasn't prepared for just how much "more efficient" the recommended method turned out to be. Instead of waiting a couple hours for enough coals it took about 30 minutes and once the thing was loaded up it felt as though there was a fire under the floor, it was so warm! The upstairs temperature was brought to 76 degrees (F) by bedtime so we all slept warm and cozy.