Exterior \"chimney\" / B-vent requirements

bryan_4

Somewhere Tim must have covered this that i missed hopefully. Is it actually acceptable to run B-vent exposed on the exterior of a home without a chase or not? NFPA doesn't SEEM to cover this. One manufacturer says you can not use B-vent alone and that it must be enclosed (because that is what NFPA state), yet another states that it IS acceptable to just use the B-vent all by itself. NFPA 54 is specific about the tables not covering vents exposed to the outdoors below the roof line, so i'm wondering if there is something covering what they do not cover? The chimney is collapsed and i'm searching for the correct option to vent an existing natural draft gas boiler and water heater.

Boilerpro_3

Personally,

I would enclose it and insulate the enclosure. The water heater venting in helps enormously to help prevent condensation. I have found that even on lined chimneys, heavy condensation is often formed if a newer boiler with stack damper is venting into the chimney without a water heater. Have you looked at installing a power ventor for the boiler, and possibly the water heater together? With energy costs what they are, I suggest replacement with wall vented equipment may be good idea.

Boilerpro

Robert O'Connor_12

bryan

Exterior chimneys require greater heat input to make them work. An alternative to the continued use of an exterior chimney would be relining the chimney or abandoning the chimney and using high-efficiency appliances. It takes aprox. 50,000 btu's to heat up a "properly" sized masonry chimney to 120 degrees whereas venting through a aluminum liner (making a comparison between masonry chimney to type B vent) it would take 135,000 btu's to heat from 0 degrees to 120 degrees. Use of type B vents is prohibited in cold climates when attempting to run it up the outside of the building unprotected. The use of type B vents passing through an unused masonry chimney flue however shall not be concidered to be exposed to the "outdoors". Outdoor vents obviously suffer from poorer draft and increase condensation formation because they are exposed to the elements, low temperatures being the main factor. "Stack effect" in the building interior can cause back drafts (reverse flow) to occur in any vent or chimney, particulary vents exposed to the outdoors. For the same reasons, vents should extend above the roof line no more than required by the code because the more vent pipe that extends above the roof, the more the vent pipe will be exposed to the cold and wind.

Robert O'Connor/NJ

JackFre

You are correct that it is :sketchie\"

in the code book. The requirements for Bvent were written decades ago. Back in the day when a gas furnace was designed to be 60% efficient. We didn't pay anything for fuel and no one cared...well not many anyway. With the new min eff of 78% we have put more into the house and there is less energy to drive the flue gases out of the stack. While b-vent is an insulated pipe you are asking a lot of it to operate in an exposed condition. Poor operation is exacerbated by oversizing and short run times also. By the time you buy the vent, build the enclosure and do the "other" work, wouldn't it be better to buy a condensing appliance and skip the bvent?

Bob Harper

collapsed?

AS in the entire chimney or just the liner? If it is just the liner, you'd probably be better off relining it. Contact your local sweep.

Steve_35

NFPA 54

Says you can't use single wall metal pipe outdoors in cold climates. It doesn't say anything about not using B vent outdoors in cold climates so that would seem to indicate it's okay. They also say it must be installed in accordance with the manufacturer's instructions. That's probably where you need to look.