Stove closed but burning hot

Hi all, we have the flu and bottom for on our Hitzer 503 E-Z Flo Fireplace Insert closed but its still burning pretty good. Last year we rarely had to open it any more than that. Any ideas on what would make it burn so well? We think it might be getting air somewhere but not sure how to test. Thanks in advance for any feedback.

Smoke from a lit cigarette or the like will show where it's getting pulled into the stove.

The double nut on the slider on the inside of the 503 door can loosen. It creates a gap between the slider and the air intake holes. So even if it is closed all the way air can still leak in.

Also check your door glass gasket. Mine got eaten up and air was leaking from the bottom of the glass. They sell the new glass with gasket attached... it is the easy way to go.

Loose slider nut if the ash door wiggles at all it lets in lots of air...
Then test the gasket with a dollar bill...

Thanks everyone. We will check it out this weekend!