Thread: insualation clearance from draft hood

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• Draft hoods , combustible material

Originally Posted by dan orourke

Is not the clearances from the water supply line insulation from the draft hood 6 inches?

Also, would you agree that the vent pipe does not have the 12 inche rise before making a turn?

Type B gas vent has a 1" clearance to combustible material, and that is less than 1".

The draft hood, yes, that has a 6" clearance to combustible material.

And, again, yes, that does not look like it would meet the minimum 1 foot rise.

Looks like some melting as already occurred at the insulation on the right.

One inch from B-vent
Six inches from draft hood
Twelve inch rise off hood min.
Draft hood connector
venting secured to draft hood

Those water flex lines look a bit kinked. Should be a broad radius turn. Somebody really like brass nipples.

Originally Posted by Bob Harper

Six inches from draft hood
Twelve inch rise off hood min.

Meaning, if I have it correctly, the 6" clearance from the draft hood extends up 12" from the draft hood.

Where do you measure that 12" vertical from the draft hood at: a) the flange at the bottom (the wide part); b) the top where the B vent goes; c) the 12" follows the shape of the draft hood?

The way I understand it Jerry is according to the vent connector sizing charts, all offsets begin with a 1 ft(12") rise.

As for the clearance, The 1" clearance is afforded by the double walled section of the B-vent. Therefore, any part of the vent including the single walled connector below the insulated portion and including the draft hood proper would carry a 6" clearance. Let's say you have a draft hood connector attached to B-vent and the distance from the insulated portion of the B-vent to the widest part of the draft hood where the air enters is 5". Then this 5" section carries a 6" clearance. However, once you reach that first roll in the Bvent, it drops to 1". So, from the opening inlet of the draft hood up about 5" I then measure 6" from there would mean the lowest combustibles could reach down a pipe towards the draft hood would be about 11-12". You have to remember this clearance is measured by swinging arcs so the single walled portion of the draft hood, which carries a 6" clearance is not measured perpendicular but tangential in all directions. This means the first 6" of so of the B-vent actually must be at a 6" clearanc for the single walled connector under it.
Is this confusing enough? Whew!
Bob

"The way I understand it Jerry is according to the vent connector sizing charts, all offsets begin with a 1 ft(12") rise."

That I am aware of. Don't always see it, but know it should be there.

"As for the clearance, The 1" clearance is afforded by the double walled section of the B-vent."

That part I got too.

"Therefore, any part of the vent including the single walled connector below the insulated portion and including the draft hood proper would carry a 6" clearance. Let's say you have a draft hood connector attached to B-vent and the distance from the insulated portion of the B-vent to the widest part of the draft hood where the air enters is 5"."

Okay, that's getting to what I was asking about. That lower, wide, part of the draft hood, you measure out 6" from it, right"

"Then this 5" section carries a 6" clearance."

As measure out from the widest part and sloping in as the draft hood size reduces?

"However, once you reach that first roll in the Bvent, it drops to 1"."

Yep, that I also have down.

"So, from the opening inlet of the draft hood up about 5" I then measure 6" from there would mean the lowest combustibles could reach down a pipe toward the draft hood would be about 11-12"."

Okay, do you measure 6" out from the wide part of the draft hood, as though you were sliding a cylinder over it? Or, does your measurement taper in to match the profile of the draft hood, only you are measuring 6" wider?

"You have to remember this clearance is measured by swinging arcs so the single walled portion of the draft hood, which carries a 6" clearance is not measured perpendicular but tangential in all directions. This means the first 6" of so of the B-vent actually must be at a 6" clearanc for the single walled connector under it."

Got that too, but what about the questions I have above?

Let's say the draft hood is 6" wide at the bottom and 3" wide at the top, with the transition being 45 degrees.

You would measure 6" out from the bottom, at the bottom. This means an 18" diameter circle.

You would measure 6" out at the top, from the top (including up 6" from the bottom of the B vent). This means a 15" diameter circle.

And the space between those two are measured out to that 45 degree slope. Meaning the 6" clearance circle goes from a 15" diameter to an 18" diameter at a 45 degree connecting slope.

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.

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).

Jeff,

See if this helps.

The 6", 1" and 12" are *not to scale*, they just represent the 'method' of measuring.

The 12" represent the vertical height (minimum) before a bend or elbow is allowed.