Thread: Chimney used as B-Vent and HVAC chase

Went back to Mondays’ inspection this morning to meet the HVAC guy as the property is just down the street and I was curious to learn if I had correctly called out a possible heat exchanger breech based on changes in flame pattern when the blower kicked in (I had).

Anyway, we were discussing the option for furnace replacement and adding central AC – difficult at this house because of the limited duct system cobbled onto the previous ductwork for a gravity system – and he mentioned that one option was to sidewall vent a cat-IV furnace, install run of b-vent up the chimney for the WH, and then use the balance of the space in the unlined masonry chimney to accommodate a HVAC supply duct leading to the two small second floor bedrooms. (Essentially, the chimney would be acting as a chase.)

Assuming that you could properly support everything I don’t see any obvious objection to this strategy - there would be no combustible in material in the chase” and there are already openings in the chimney at the second floor - but somehow it does not seem quite “right”, either… though perhaps I’m just not “thinking outside the box"...

Comments?

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Originally Posted by Michael Thomas

Went back to Mondays’ inspection this morning to meet the HVAC guy as the property is just down the street and I was curious to learn if I had correctly called out a possible heat exchanger breech based on changes in flame pattern when the blower kicked in (I had).

Anyway, we were discussing the option for furnace replacement and adding central AC – difficult at this house because of the limited duct system cobbled onto the previous ductwork for a gravity system – and he mentioned that one option was to sidewall vent a cat-IV furnace, install run of b-vent up the chimney for the WH, and then use the balance of the space in the unlined masonry chimney to accommodate a HVAC supply duct leading to the two small second floor bedrooms. (Essentially, the chimney would be acting as a chase.)

Assuming that you could properly support everything I don’t see any obvious objection to this strategy - there would be no combustible in material in the chase” and there are already openings in the chimney at the second floor - but somehow it does not seem quite “right”, either… though perhaps I’m just not “thinking outside the box"...

Comments?

Michael,

I'm not an HVAC guy, but I'll address the chimney issue that I see here. This is an unlined chimney with what appears to be some pretty major degradation of the brick masonry units.

I recommend a CSIA Certified Chimney Sweep be hired to remove all the loose materials to determine just how deep the damage to the chimney structure is.

We've found these so bad in some cases that the chimney had to be torn down due to structural concerns.

Bart Ogden

So let's see .. you and the hvac guy are going to guarantee that
- the b-vent won't leak at some point down the road;
- the supply duct will be completely taped/sealed so air won't leak out or exhaust won't filter in if the b-vent deteriorates
- the top of the chimney will remain sealed so that moisture won't deteriorate either metal system to cause failure?
There's probably a code against it, regardless it's a bad idea.
There isn't a closet somewhere that they can run the duct up in the corner and pop through the wall or ceiling? That seems extremely un-usual. The bedrooms have closets right? Or maybe 6" oval through the 2x4 stud chases.

Originally Posted by Markus Keller

So let's see .. you and the hvac guy are going to guarantee that
- the b-vent won't leak at some point down the road;
- the supply duct will be completely taped/sealed so air won't leak out or exhaust won't filter in if the b-vent deteriorates
- the top of the chimney will remain sealed so that moisture won't deteriorate either metal system to cause failure?
There's probably a code against it, regardless it's a bad idea.
There isn't a closet somewhere that they can run the duct up in the corner and pop through the wall or ceiling? That seems extremely un-usual. The bedrooms have closets right? Or maybe 6" oval through the 2x4 stud chases.

Who is the question directed to, Markus?

Bart Ogden

That would be Michael and the rocket scientist hvac guy.

Markus, I'm not going to "guarantee" anything.

I'm skeptical about this solution, which I way I opened the topic for discussion in the first place.

This seems like the last route, figuratively and literally, that I would take. The costs and PM to make sure that the B vent stays intact as well as the supply issues EVEN if the chimney were capped/crowned seem exorbitant. From an energy standpoint, I never like seeing ducts outside the conditioned space if avoidable.

To play flipside, I know I've seen bladders inserted into chimneys and filled with a type of refractory cement that MAY somehow address this issue and separate the the vent from supply in their own "chase" if you will. Still lean towards no...

The chimney must be inspected by a pro first. If it is unstable, it must be torn down & rebuilt or repaired. Bladder or "pumped" liners require a stable chimney or else you could end up with blowouts dumping wet cement into the home. While there are B-vent mfrs. who allow their vent to be used inside chimneys in lieu of listed liners, you cannot inspect the joints or screw it back together should it separate. However, if you reline the flue, you cannot run ducts in the flue with the liner. If you run B-vent, your AHJ might allow such ducts but I doubt it. The code prohibits using the space around a liner for ventilation. Ducts are not intended for use in a cold, damp chimney so what do you do not if but when it rusts out?
Maybe it would be better to switch out that WH with a power vented one. One possibility might be to get the AHJ to sign off on using a large diameter aluminum liner as a warm air duct.
HTH,
Bob