Blocking off the bottom of my chimney on my furnace.

[Brian26]

Wasn't sure what section to post this so please move it if its the wrong section. This has to do with my oil furnace that is vented into its own 8x8 clay flue. I get somewhat low draft readings as the bottom of the oil flue is open to an ash pit. I can't tell but it might be connected to the ash pit next to it that was there for the fireplace. I have an insert in my fireplace so that flue is separate and lined. When I open the cleanout door for the furnace the draft drops off and the bottom of the flue pulls in air like crazy. Even with the furnace off the bottom of this flue is drafting like crazy. Just as a test I taped the bottom of it and my draft for the oil furnace went way up. Is it safe to block it off with a removable piece of metal or something? Here are some pictures of what I am talking about.

 

[BoiledOver]

Does your chimney have two separate clay lined flues? Are you asking if it would be beneficial to divide the ash compartment to isolate the two flues from one another? Would the divider be located between the two access doors?

 

[brenndatomu]

How would this piece of metal be different than just closing the cleanout door?
What makes you think the furnace has low draft? Have a meter?