Welcome to the Fourm.
From your manual of the 550M:
"Outside Air Supply (optional)
Depending on your location and home construction, outside air may be necessary for optimal performance.
Metal pipe (solid or flexible) must be used for the outside air installation. PVC pipe is NOT approved and should NEVER
be used.
A wind shield over the termination of the outside air pipe or a 90-degree elbow or bend away from the prevailing winds
MUST be used when an outside air pipe is installed through the side of a building. The outside air termination MUST
be at least 1-foot away from the exhaust system termination.
The outside air pipe on your King/Ashley stove is 1 7/8†OD. The outside air connecting pipe must be at least 1 7/8†ID.
The outside air connection used MUST NOT restrict the amount of air available to your stove. The outside air connecting
pipe must be as short and free of bends as possible, and it must fit over, not inside, the outside air connection to the stove."
You could always crack a window open to see if it is an OAK problem before installing one. But I would be more inclined to check for leaks, feed rate settings, how is you venting system set up and type of pellets you are burning. More info is needed. Pics of your set up would really help the pellet guru's on here of which I am not.