I've noticed that we don't seem to be able to get an extended burn time much beyond five hours. Our VC Sequoia (see avatar) is fairly tight air-wise with the exception of about a small 1 inch vertical gap at the top of the door where the doors meet. No cracks inside the box that we are aware of. We have seasoned wood: elm, poplar, hack berry, black walnut. The damper appears to fit tightly when closed. The air control device functions. The firebox will accommodate up to 22" logs. We have triple wall pipe. Can you advise me about those elements that may affect the burn time.? Do certain species of wood garner greater burn times than others? Thanks in advance for your help.