I recently bought a ranch home that has 2 full length 3wall non insulated chimneys installed. Id like to hook up my woodchuck 2900 to the chimney. I am a bit confused because both 8 inch chimenys are identicle and placed 5 inchs apart. One has suit. The other appears to never been used. It appears these chimneys are prob 30 years old. Is there any reason I cant use the new appearing chimney?