This is my third year in my house and have just recently noticed that there is a sand like substance that runs up the flu from my furnace. My question is should there be a sand like substance in my flu duct. I am sure it has been there since I purchased the house and never really looked that close because everything is working fine. I am just making sure it is suppose to be there but in my thinking it should be empty. Any ideas?

Thank you

Bob

Gas or oil?
Masonry Chimney termination?

Ok I hope I answer this correct. My father in law is also helping me answer this question.

It is Natural Gas

My flu pipe is

Double walled metal pipe they call is metalbestes (sp? )

Fireplace goes through a masonry chimney and both item terminate on the roof

Hope this helps thank you for your time on this issue.