Chimney liner clearence height above 2nd flue in double chimney

[Ogre]

I have a double chimney.
1 flue is the existing masonry firplace and the other is the flue for an old forced air oil furnace.
The old oil furnace is long gone and that flue is capped in the basement.

I'm installing an insert and lining the masonry chimney. With the flues so close at the top of the double chimeny I'm concerned abt smoke from the woodburner rolling back down the other (oil furnace / capped off) flue.

What is the right thing to do here? Is there a height adjustment I should make in the liner above the chimney / other flue? I'm currently thinking to extend the liner (rigid double wall) just an 1" or 2" above the other flue's rain cap... Just hoping the rain cap would deflect any smoke rolling back down...(I know this wont really work but it makesme feel better)

Is there an established dimension for this type of install, or other advice, suggestions?

Thanks all.

 

[theheatelement]

You won't have any issues there, is the other one capped with a rain cap or just a cover..

 

[Ogre]

Vented rain cap at the top, like a critter cage. this is terra cotta tile...

and is capped at the flue fipe in the basement. Probably not sealed air tight but is capped.