It s hard to see them but the chimney needs to be free of any downdrafts.
Chimney leaking smoke into attic.
Rerendering may stop the smoke but my advice would be to fit a twin wall stainless steel liner.
Water leaking into a chimney increases the chance of damage to the chimney flue or chimney structure.
There is tremendous heat from chimney fires i have seen stacks split from the heat so for peace of mind get a liner in preferably by a hetas registered installer.
The other thing to check is downdrafts.
If you see water entry in the attic on the uphill side of the chimney we know a chimney cricket is needed so water is not ponding in this area.
Another event that could happen although i have never heard of it actually happening is that you could get so much moisture in your attic that it could condense and roll down the stringer onto your chimney.
If you have no smoke coming into your attic or other fireplace you ve confirmed.
Smoke a little like water can end up in places where you would not expect.
Start a small test fire and this time open a window in the same room as the fireplace with the restored chimney.
Have a certified chimney sweep inspect the chimney and make recommendations.
Don t let one observation at a chimney distract you or convince you that further up close inspection is not needed.
This blocks the wind from coming down the chimney while at the same time allowing for the smoke to exit the chimney.
Gas fireplaces give off fumes with moisture due to the lower temperatures and of the correct liner isn t installed that moisture can build up and seep through your chimney walls.
Once the outer walls of the flue start to leak then action is imperative if you plan to keep on using the fireplace.
Sometimes a leak can occur without visible problems making the correct fix hard to identify.
If you ever had a chimney fire i wouldnt rely on the render stopping further damage.
If the top of the chimmney is below the roof line downdrafts could be swirling down toward the chimney.
Solving chimney leaks can be difficult as there are many potential spots that could cause the leak.
It has happened and usually isn t discovered until people have spent a terrible sum fixing everything else.
Code requires a chimney have a cricket if it is thirty inches or wider.
Often ponding water will rust a hole in the simple flashing found on most chimneys.