Newer furnace flue slipped inside larger flue

[Lon Henderson]

I have never seen this before. The furnace was replaced about 20 years ago and the flue looks like it was shoved up into an existing flue that goes through the roof deck. The flue is about 3-4" inside the larger flue. Is this permitted?

The photo is in the attic.31784

[Jim Luttrall]

I would suspect the roof flashing is incorrect. The roof flashing should fit the newer pipe with all the same manufacturers listed components including roof flashing, storm collar, and cap. I'd bet this one is just stuck up inside a "universal" bonnet style which is no Bueno.

[Raymond Wand]

Something is amiss. Water staining on flue, rust staining...

[Scott Patterson]

No that is not correct. I don't think you will find any manufacturer that will allow that design. The flue gases in the new pipe are condensing before they get out of the home and that is why we see the stains.

I'm curious as to how long is the flue pipe from the furnace to the top of the roof?

[Lon Henderson]

Water stains date back. The shingles were replaced last month. The boot jack was installed correctly.

[Jerry Peck]

The flue is about 3-4" inside the larger flue. Is this permitted?

No.

The Type B gas vent needs to extend through the roof and be connected to a proper Type B gas vent cap, the parts connect together in a way which seals the inner vent so that the combustion by-products are vented up and through to the outdoors ... not into the other vent where they may, or may not, vent to the outdoors.

Looks like someone went to extra work to not do the extra work necessary when it was installed, and will be going through even more extra work to correct it, including re-doing the shingles around a proper Type B gas vent and its flashings when it is replaced.

[Lon Henderson]

No.


Thanks. That is what I suspected but I was having trouble finding a reference that addressed this.
It also occurred to me that depending on wind direction and velocity, you might get backdrafting through this arrangement into the attic.

[Jerry Peck]

Thanks. That is what I suspected but I was having trouble finding a reference that addressed this.

The code and the venting type required (Type B gas vents) and the manufacturer's installation instructions.


It also occurred to me that depending on wind direction and velocity, you might get backdrafting through this arrangement into the attic.

Might? :D

I suspect that there likely will be, but I would call is "spilllage" rather than "backdrafting" as the combustion byproducts will spill out the end of the Type B gas vent, some going up due to heat rising, some going back down due to gravity - unless there is another overpowering force ... gravity *always* "wins". :)

[Dave Borowski]

It's difficult to evaluate from your photo. If that B vent simply terminates in the old roof flashing.... that's a serious void and the furnace should be tagged until a remedy is made by a gas or mechanical contractor. It is true as posted - the mfg brand of B vent can not be comingled with other brands. However, to my knowledge there is no probation to employing an oversize roof collar (penetration) as long as an appropriate storm collar and the B vent extends to the correct height and support. You can see detailed drawings in NFPA54 and consult the mfg installation guidelines of the vent pipe.