You all need to know the title for this thread "Changing furnace filter" contains a basic misunderstanding because it is definitely a boiler shown in the original poster's photographs and not a furnace, and that a boiler is VERY different from a furnace! .....
A boiler will heat up WATER for heat, while a furnace will heat up AIR for heat, making it a "hot water boiler" or a "hot air furnace" ...... you understand! ..
For a very good chimney inspection/cleaning/repair/expertise, suggest you try the Chimney Crickett from Exit 29 in Thornton, Rick Vollmer, who has been doing chimneys for a long time! He goes all over central NH in his big white Chimney Crickett van with the ladders on the roof.