That exhaust pipe is really long. Too long. The heat transfer efficiency must be nearly perfect given the length, but that's going to be a problem. After some (probably short) distance the temperatures will be low enough that creosote will condense out and cause major problems with that pipe. It's flanged, so that's nice, but it's still going to be a high maintenance item.

Creosote is a big problem and it's counter-intuitive, but you have to keep your temps up or it will cake out on the worst places possible. You should try to keep every point of your chimney above 250F or creosote will condense out and burn your house down.