Glass doors must be wide open when burning. All CHILLBUSTERs are also certified to the vented decorative standard, so that the damper may be slightly opened for reduced venting applications and can be used in localities where vent-free is not permitted. Although most states allow the use of vent-free heaters, check your local codes and ordinances for permitted uses. The "DFC7" Chillbuster Single SeeThru Burner has detailed logs with heavy bark, natural split, and a charred window to the burning embers. Opposed louver, lanced port burner with Enhanced Ember-Lift technology and a heavy 5/8" grate.Vent-free applications are rated for a maximum elevation of 3500 feet.