Gas fireplace… insulated vent chase warranted?
Hi,
My wife and I are building a home in upstate NY (near Albany). I’m the GC (yep, brave, stupid, all of the above). The custom house plans are for: two-story, rectangular footprint, 3200 SF, R-20 foundation, R-40 double-stud walls (two layers of rigid foam + Roxul layer), R-50 attic (cellulose), as air-tight as we can reasonably achieve. The primary heating/cooling is five mini-splits with strategically located heads (and a ducted ERV system design that helps move the conditioned air around the house). As a back-up, alternate-fuel heat source, we’ve planned for a gas fireplace (natural gas, direct-vent, adjustable 18k – 35k BTU, sealed-firebox), but now that it is time to install the appliance, I’m having concerns about the details of the vent routing, vent chase, and chase air-tightness/insulation. The fireplace installation manual is fairly clear about the vent chase needing to be air-tight and insulated (in colder climates), but the sales rep is saying an insulated chase is not needed (because the system — vent and firebox — is “sealed” / air-tight). The firebox will be located on an interior partition wall on the first floor. The horizontal vent run, within the 24″ tall, 24″ on-center, floor trusses above, to an outside wall is 20 feet (yes, the specs state we can run up to 24′ horizontally with the 6′ of vertical pipe coming off the appliance — page 27 @ http://downloads.hearthnhome.com/installManuals/2165_900.pdf). Regardless of the system “air-tightness”, my concern is re. the potential heat loss within the conditioned floor system space to the relatively cold metal vent pipe on a cold winter day when there is no fire in the firebox. I.e. if the 6″ diameter metal vent is not enclosed in an insulated chase, why wouldn’t interior warmth radiate to the presumably colder metal vent and 1) conduct down the length of the vent to the outside termination cap, and 2) radiate into the air space within the vent, presumably creating micro-convection within the vent with the outside cold air. My question is whether my concern is warranted. Perhaps any micro-convection within the pipe is so minimal and the conduction down the pipe material is so minimal that the expense/hassle of a fully insulated, air-tight chase is not justifiable…? I’m hoping someone may have some heat loss stats for this scenario, or at least help validate my concern with like-minded thinking/concern. What I’m ultimately hoping for is grounded justification for going the extra mile and building an insulated, air-tight chase for the 20″ vent run, as well as an insulated framed box around the firebox (maintaining proper clearances to combustibles all-around).
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Replies
Jay. This doesn't address your question, but I noticed your plan includes rigid foam on the exterior of a double-stud wall. Please see Martin's article why this is a bad idea: https://www.greenbuildingadvisor.com/articles/dept/musings/exterior-rigid-foam-double-stud-walls-no-no
I think he meant that the rigid foam was between the studs, with rock wool in the stud bays. (?).
If the unit is truly air tight, there won't be much air moving through the vent, and what you're left with is just the necessary thermal bridging at the wall penetration. If you insulate the chase, you would also be giving up some of the heat coming off the vent piping into the house, at a much higher delta-T for more BTU per hour (but probably fewer hours) than you would have of parasitic loss. Even if it's a net loss, it's not clear that it would be "worth" the effort & expense of insulating 24' of chase fully inside of conditioned space.
Thank you Steve & Dana.
You're correct Dana. The two rigid foam layers are within the wall system (3" XPS ripped to fit in the stud bays of the outer studded wall and 2" XPS between the two studded walls). It's been a labor of love foam sealing around each XPS panel in each layer. Hoping for very low air-leakage.
Due to this gas fireplace being a secondary/backup heat source (and occasional ambiance creator), we don't foresee it being fired very often. So, I'm figuring the heat loss via the uninsulated vent will be a net loss. I agree, though, it's just not clear whether an insulated chase is worth the effort. Furthermore, if we opt to not construct a chase at all, there's an option to route the vent out a side wall of the house which would be a shorter 14' horizontal run, as opposed to the back wall of the house which would be a 20' horizontal run. The shorter run would be a decent cost savings in vent pipe, as well as presumably offer better drafting/venting performance. The reason for the two different vent layouts is entirely for ease of the considered chase construction (parallel to / between floor trusses) as opposed to the shorter horizontal run (perpendicular to trusses / through truss open webs -- not a simple task, in my mind, to construct a continuous, air-tight chase when routing through triangular truss open webs). Anyhow, the shorter vent run option may tip the scale toward the "non-chase" approach. Still, I long for some clarity around the actual heat loss of the uninsulated vent pipe.