I have just installed my first wood burning appliance. An Englander 28-3500 furnace. Chimney is all installed up through the roof. I went to connect the double walled pipe to the top of the furnace last week only to see that the pipe would not just hook up to the collar. I was told I needed an adaptor that would sit around the stove collar and also go inside and down into the collar. So I ordered the Selkirk DSP6SA-1 adaptor (I have all Selkirk pipes). Just got it today and it doesn't fit snuggly as I thought it would. The furnace chimney collar has an inside diameter of 6 1/8". The adaptor inside pipe that is supposed to fit down inside the collar only has an outside diam. of 5 3/4", leaving a 3/8" difference. Lots of play there. This obviously can't be right. What do I do to close up this space? How have you who have these furnaces connected the chimney to this collar? Measuring the male end of my regular DSP pipe also measures 5 3/4" O.D. so that won't match up either. Any advice would be Great. Thanks