double wall chimney

[coolmen]

Hello,
Im getting ready to replace a 80% furnace with a 94 %. The current chimney is I think is 7" (8" outer diaI measured) bvent feeding feeding the 120,000 old furnace and the 75,000 hot water heater. Do i need to run a smaller chimney liner down the bvent to handle the 75,000 btu HWH . or what?.
I would allways on a clay chimney but had not come across this with the bevent.
Thanks.
CJ

[beenthere]

You should be fine.
7" is ok for a 3" hood.
That 75,000BTU water heater should be a 4", so you should be safe.

[heaterman]

What is the the total height of the existing "B" vent chimney?

[coolmen]

The chimney hight looks to be about 25 ft. out side wall

[heaterman]

The total capacity of the existing B vent at the given height is in the neighborhood of 300,000 btu's. I'm assuming the hot water tank is not draft induced so dilution of the flue gases will be a bigger issue in addition to the extra surface area of the inside diameter of the B vent (alot of surface area to warm after an extended off cycle). You refer to 25' outside wall I'm assuming 25' exposed which is more cooled surface area as technically speaking it's not an "insulated" chimney. Myself, to avoid a possible problem down the road as far as condensation, poor draft or no draft at all I would install a properly sized liner and be done. Happy customer and a more restfull sleep.

[hearthman]

You have to compare the cost of replacing the B-vent with replacing the WH with a power vented unit out the side of the house. Liners are not tested or intended for use down B-vent. Since it is on the outside wall, it should be relatively straightforward to replace the B-vent with brand new 4" B-vent with a new 25 yr warranty. How old is that 75K WH?

[heaterman]

You have to compare the cost of replacing the B-vent with replacing the WH with a power vented unit out the side of the house. Liners are not tested or intended for use down B-vent. Since it is on the outside wall, it should be relatively straightforward to replace the B-vent with brand new 4" B-vent with a new 25 yr warranty. How old is that 75K WH?

Good points although Z-Flex does offer the necessary hardware for lining B vent. If the chimney is in fact located outside of the structure up an outside wall, it really is not an approved method of installation according to most manufacturers.