B vent clearance

[William Mize]

Greetings Friends;

I did an inspecton yesterday, it was inspected about a year ago by another inspector, who evidentaly called out clearance issues on the
B vent for the furnace. In the photo showing the rafter there is about 1/2 inch of clearance between the vent pipe and the flashing nailed around the truss. at the roof the extra flashing appears to be just jammed up between the vent pipe and the roof deck. What do you think, I do not believe just jamming a piece fo extra flashing up there is adequate and extra distance needs to be in place.

Thanks Willie

[Brandon Whitmore]

There has to be an air space between the combustible material at flashing/ deflection shield for that to work.

[Bob Harper]

Stated clearances are air spaces. However, NFPA 211 does not recognize clearance reduction less than 12 inches even with a ventilated shield.

The shields shown will act to either:
a) conduct heat if in direct contact
b) provide a target to absorb more radiant heat than had it been left alone
c) impeded what little cooling air flow there was
d) are a violation of the codes and listing.

HTH,

[Billy Stephens]

Stated clearances are air spaces. However, NFPA 211 does not recognize clearance reduction less than 12 inches even with a ventilated shield.

The shields shown will act to either:
a) conduct heat if in direct contact
b) provide a target to absorb more radiant heat than had it been left alone
c) impeded what little cooling air flow there was
d) are a violation of the codes and listing.

HTH,

Mr. Harper,

Would not the B vent clearance be 1 inch to combustibles not 12 inches ?
.

[Bob Harper]

Sure, B-vent carries a 1 inch clearance to combustibles. Those same listed instructions you referenced also warn you against tampering with their product or installing it outside the listed instructions.

Billy, my point is those clearance reduction systems with the ventilated heat shields derived from NFPA 211 12.6.2 were intended for solid fuel appliances. Within solid fuel appliances, the maximum reduction must still leave at least 12" to the combustible. Over the years, the concept of a heat shield with a 1" ventilated air space has been loosely applied and adopted to gas, oil and other heat related applications. Your AHJ can rule on what he accepts or not. However, I know of no application that can reduce B-vent to less than 1", listed or otherwise.

HTH.

[William Mize]

Thanks Guys

I thought the bandaid approach was not proper I will call the local AHJ this morning and get his thoughts. It never fails, it is easier to do it right than to go back and redo.

I will advise my customer as to the consensus of this group. and MY thoughts.

Thanks Again

willie

[Dan Hagman]

This is an improper B-Vent installation at the top and at the sheet metal flashing. A B-Vent must have a 1 inch clearance to combustables, FYI- and a single wall Vent Connector from the appliance to the B-Vent must have a 6 inch clearance to combustables. This is a easy fix! I can see a connection just below the lower metal flashing, an offset elbow would provide the proper clearance and re-cut the roof deck and move over the flashing 1 - 2 inches would take care of this problem. This is a 2 hour job at the most.