Merry Christmas fellow AHS/AA boiler explosion troubleshooters. Horizontal run length has zero effect on the problem. It's all about having proper draft, keeping/maintaining proper draft; and, allowing the boiler to operate as designed. I honestly believe you're turning a simple problem into a complicated one. I've had ZERO puff-backs; large or small, since performing the following:
1. Added fresh make-up air with 4" dust collection hose directed at the tombstone intake.
2. Significantly loosened the sight tube cover (per recommendation from AHS); such that, the sight tube cover DOESN"T form a tight seal when the boiler is running. Loosening the spring assembly holds the top of the sight tube cover out; such that, air is always being drawn over the top of the coal bed. Per AHS: "A tight seal over the sight tube isn't necessary. Keeping the sight tube cover loose has no effect on boiler performance."
3. Reduced ashing temp from 140 to 130 degrees; keeping, 10 degrees of hysteresis. I recommend finding an ashing temp which works for your boiler; and, stay with this setting---130 seems to be working great for me.
4. Installed a new Field Controls type M baro; and, resisted the primal temptation to touch/play with/caress/fondle the baro weights. Field Controls ships their dampers with the weights set and calibrated for drawing 0.06" of water---keep the baro that way.
5. Continuously monitor your draft with an accurate manometer. If you can't maintain at least 0.04" of draft with or without the boiler running you'll have problems.
6. Burn pea coal per AHS' recommendation; although, other coal sizes can probably be mixed in and might provide different results---YMMV.
MERRY CHRISTMAS TO ALL!!!