I have pulled a cast iron stove insert out of a stone woodburning fireplace. now there is a remaining heat delivery system that has three 4" pipes running directly across the flue, and I think is inhibiting the flow of smoke up the chimney. This is causing smoke to backup into my home. I have cleaned the chimney and can see that there is no major obstruction blocking the flow except the pipes for the heater. I am going to cut two of the three pipes out of the flue area and don't know the best way to
1. Cut through the cast iron pipes(angle grinder, sawzawl?)
2.How to seal the open end of the pipe after it is cut, it will be exposed to some heat and I am not a welder.
Any advice would be greatly appreciated.