3-2-10 rule revisited

[Benjamin Thompson]

Does the 3 feet above the roof line only apply to wood burning fireplaces, not to gas appliances? I can't find anything in the IRC regarding this.

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[Bob Harper]

chimneys

[Benjamin Thompson]

Thanks Bob. Does a prefabricated gas fireplace with type B vent count as a chimney? Is it OK to have type B vents 4-6 inches above the roof line?

[Bob Harper]

Nope and nope.

B-vent is a *vent*--not a *chimney*.

Refer to the gas codes but they start at 12" above a flat roof then go up based upon roof pitch.

[Benjamin Thompson]

Thanks again Bob! Here is a picture of the vent I thought was too low. It's about 3 feet from the parapet and well below it but not within 3 feet of the corner. I am not sure I understand why a gas appliance would not back draft when a fireplace would? Any insight?

Bvent.jpg

[Bob Harper]

You have a vertical obstruction within 8ft horizontally so the flue gas outlet must extend at least 2 feet above the top of that parapet. The 3 ft rule is to allow sparks from wood burning to cool before they land on a possibly combustible roof deck. The 2/10 is to hopefully extend the vent above any pressure zones created by wind effect over a ridge.

The height of a vent above a roofline would have an effect on backdrafting only if it was wind related in which case it may need to extend a lot more than just the code minimum. Codes do not guarantee performance and can be exceeded.

[Jerry Peck]

... code minimum. Codes do not guarantee performance and can be exceeded.

Expanding on what Bob said - codes are the minimum standard, with the emphasis on "MINIMUM" standard. Any 'good' builder/contractor will exceed code as code is the crappiest they are legally allowed to build.