B-Vent size

[frankt]

I currently have an 8" B-Vent flue with about 30' of heigth. It is serving 2 nat. venting gas furnaces (160,000 and 108,000 btu) and a 75,000 btu nat venting gas water heater. If I have the 160,000 btu furnace changed out and vented with PVC will the flue have to be lined to a smaller size, or can it remain at 8" while only serving the remaining 183,000 btu max.

Thanks

[Moose]

Go with what your contractor says - he/they will have to deal with the AHJ (authority having jurisdiction)

[rsmith46]

As long as both the furnace & WH are natural draft hoods 8" is still ok.

[frankt]

I looked at a couple B-Vent sizing charts and in both charts there was only a MAX btu for natural drafted, no minimum listed. Fan drafted has both a min. and max. SO am I to assume there is no btu minimum for natural drafted?

[gasguy]

I currently have an 8" B-Vent flue with about 30' of heigth. It is serving 2 nat. venting gas furnaces (160,000 and 108,000 btu) and a 75,000 btu nat venting gas water heater. If I have the 160,000 btu furnace changed out and vented with PVC will the flue have to be lined to a smaller size, or can it remain at 8" while only serving the remaining 183,000 btu max.

Thanks

My tables show 2 spillage succeptable (nat draft) appliances with 183,000 btu input and 30' common vent height and double wall (b-vent) vent connectors needs a minimum of a 6" b-vent.

In the general venting requirments, it states "When the table permits more than one diameter of pipe to be used for a vent or vent connector, the smallest permitted diameter shall be preferred."

I would take that to mean you need a liner. I would assume too large of a vent would cause condensation and backdrafting issues. Your local codes and inspectors may be different though, so check with the authority having jurisdiction in your area.

YMMV