I have a Hastings that is probably 10+ yrs old. I do a thorough cleaning on it each year including the exhaust, ash traps, etc. This winter I'm having issues with burning on settings 4 & 5. If I burn on 1 or 2 the glass starts to brown a little which is expected. If I turn up to setting 3 the glass will actually clear up for the most part. This seems to be my most efficient setting. If I set it on 4 or 5 the glass completely darkens up within about a half hour. I'm figuring it has to be something where there is not enough air flow to burn on the higher heat, but can't figure out exactly what's going on. As I mentioned I clear the ash traps (hammering the back wall) at least every week or so and give the tubes a nice thorough cleaning. Any ideas on what may be the issue on my higher settings?