Mark Meier : A basement location is more than problematical! Google
Stack Effect and
Whole House Stack Effect! This is not a Nice to know exercise -but a real world condition
that you are bumping your head up against .
I am glad that you tried your system before you
cob'd everything in, As a working rule you should
Count on no more than 30 horizontal feet for a 6'' RMH System, and 50'
for a 8'' system. Every Horizontal Elbow counts 5' against your system ! This includes horizontal connections in your final vertical chimney.
Your RMHs Final vertical Chimney needs to rise to a point at least 1 stove pipe length above the peak of your roof, ideally it needs to be on the lee or downwind side of your house.
Because your RMHs present location is in the basement make sure all windows and doors are closed, and do not run your clothes drier, or any Fossil fuel fired
Hot water heater
that is vented to the outside OR the exhaust fan over your Cooking Stove or the exhaust Fan for your Bathroom !
Especially if these steps cause your problem to go away- you seriously need to reconsider your RMHs location ! For the good of the Craft ! Big AL