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After you dry pellets whether it be microwave or in withe the preheat. Do you let the pellets cool down before you lite them? I have dried them both ways and lite them right away and a whole AMAZNS full of pellets will bun up in a few hours.
It looks like they flamed up and jumped rows, the burn never turned the corners.
How long did you nuke them?
Did you blow out the flame before putting in the mailbox?
How much air is getting in the mailbox? I have seen flame ups when too much air gets to the pellets like on a windy day.
How high did you fill the rows? If the rows are filled over the top then the burn can jump. I only fill to just below the top, never above.
I live in Missouri where humidity is high and pellets keep burning out. I done a test run one pellets I microwaved for 1 minute and they lit right up (15 seconds on the torch) and burnt complete very fast. My second attempt I filled box up and put in smoker during warm up (without water pan) removed it after almost an hour and they too lit right up. So I think I need to not dry them near as long and maybe let them cool down before lighting them?