If there is a concentrated spot of heat like a ring around the pipe then this is an indication of a possible internal flue fire. This is caused by a build up of creosote within the flue pipe which has then caught fire. If this is the case then a sweep would be recommended. Make sure you are using a sweep that has done a Pyro fire before or has watched our video about flue cleaning. It would also pay to check the moisture content in your wood by splitting one of your logs in half and spiking it with the moisture meter that came with your Pyro Classic (Pyroclassic IV). Wood should be less than 20% to burn clean & efficiently.