Originally Posted by
Bob Harper
On a water heater or boiler, swing a 6" stick from every contact point on the single walled pipe and draft hood just like rays of the sun.
True, the arc is how the heat travels in distance (like the rays of the sun radiating outward), however, the greatest length of that arc (maximum clearance) will be where the arc is perpendicular to the surface being measured from.
Swing the arc far enough and you will be back at the surface you are measuring from.
For a smoke pipe, use two 18" blocks down where you can easily reach the smoke pipe, use a laser level or other laser pointing device, and align the laser beam with the end of each 18" block, setting the laser beam parallel with, but 18" from, the smoke pipe. The laser beam at the ceiling should not be on wood. If you use a laser level, and if you know the beam is 1/4" up from the bottom of the level (the laser level on the block moves the laser beam an extra 1/4" away from the smoke pipe), the laser beam still should not be on wood at the top, you would still want that 1/4" clearance you are starting out with (not that you could see it that clearly anyway) 'just for comfort factor).