You've got it Jerry, when you swing the arcs from the closest point to a combustible you get a rather large cone.

Here is another way of looking at combustible clearances. Have you all seen woodstoves vented into factory chimney supported in a cathedral ceiling box on a steep roof? You have a 2" clearance off the factory chimney but 18" off the single walled smoke pipe leading to it. Therefore, as you approach the ceiling, the smoke pipe will eventually get too close. The solution is to extend the factory chimney down far enough so the smoke pipe does not hit the ceiling. We teach these trig. calculations in the NFI Wood Cert. course. Based upon the roof pitch, you have to extend the chimney further and further down to meet the clearance. When measuring for clearance, you can measure perpendicular to get close but to be accurate, you would need to swing arcs. Make a stick the length of your clearance (18" in the case with wood and single walled smoke pipe). Try to connect the combustible to the single walled pipe from every point and every angle possible. This is what Jerry and I are discussing.

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. If you don't hit anything, you're ok.

Back when I was with the mfr. we have gauge blocks made up for inspectors that were 1,2, and 3" with a 1/2" rabbet on one corner. These were great when taking post installation pics to verify clearances. You can make one yourself out of wood, though ours were aluminum. You can buy a telescoping pointer from the office supply just like lecturer's use to point at charts. Mark the shaft at 6,9, 12, and 18". After that, use a wide tape rule.

There is no need to insulation to be that close to the water heater. Sorry but they are going to have to live with a little bit of exposed pipe.

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