Venting an exterior chase

[dskup]

Before I write my question, I will preface by saying I am perfectly aware that
an exterior chase is the worst possible scenario for a fireplace chimney. No need
to school me on the stacking effect. Unfortunately it's a done deal, so I can't really
change it.

That said, I do have a Quad 7100 installed in a 26 ft. exterior chase. The first ten feet is
heavily insulated, including the ceiling. However the remaining 16 ft. is not. When the chase
was being built (and was uninsulated), I noticed lots of condensation inside. This makes
sense, since the warmer air from the house was rising, trapped by the top sheeting, and condensing.
Now it's all boxed in and insulated, and I'm wondering if I should install two vents...
one just above the ten foot mark, and one near the top?

I'm thinking along the lines of a typical attic scenario, whereby we would vent the soffit
and the ridge to keep air moving in the cold space. Are there any downsides to this? Is
it unnecessary?

Thanks!

 

[Corey]

As others on here have quoted, "I don't really know, but that won't stop me from giving an answer" My biggest question would be do you still see the condensation or traces of it? If the bottom is insulated (and hopefully has some vapor barrier) I would suspect the moisture transfer would be fairly minimal. On the surface, it seems like an attic scenario, but with an attic, you may have 2,000 sq ft of house (bathrooms, showers, kitchen, etc) ceiling passing moisture into a relatively small volume - the attic space itself. And the attic is topped with roofing which is specifically designed to be a barrier to moisture.

With your chase, you may have a relatively small 'contact patch' with the heated space (just guessing maybe 5'x5' or 25sf) - (probably in a family room where there is minimal moisture anyway) venting into a relatively large volume (25 x 16 = 400 cf). Plus the walls are probably wood or siding or other material which can 'breathe' anyway.

IMHO, I would just leave it unless you see physical signs of condensation. The vents would probably cause more trouble than they would solve. But others may have differing opinions.

 

[Metal]

I had condensation problems in mine so I added two circular 2-inch vents about a foot above the bottom of the chase on both sides. When painted you barely notice them. These are what I used, you can probably find them at any hardware store (see link below). I have B-Vent running up the chase, so top vents were not needed as the vent is not airtight and allows air movement, but it is sometimes recommended to raise your chase cover about a half inch with some spacers to allow the chase to "breath."

(broken link removed to http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId;=-1&catalogId=10053&productId=100090036)

 

[JohnnyBravo]

that should solve the moisture problem. what is your flashing at the top of the chase like? some of them have vents in them just below the storm colar. keep in mind that when you do this your chase will be much colder and you may end up with draft issues. if that is the case you will have to insulate the chase and treat it as a warm zone.

 

[dskup]

Thanks guys.

Metal... I'm curious why you would vent one foot from the bottom of your chase? Is yours
insulated? Did you vent one foot from the insulated ceiling of the chase?

CozyHeat... You bring up good points. Yes, the chase area is only 12 sq ft. Once I got it
insulated, I could no longer inspect for condensation above the ceiling of the chase (it's capped
off from above.) Below the insulation, I was seeing condiderable condensation on the outside air
intake manifold. This is the sheet metal box that surrounds the chimney pipe at the top of the
fireplace unit, and has a 4" off-take for outside air (air cooled chimney). After running the unit
for awhile, then letting it return to room temp, I don't get as much condensation. I suspec there
was some residual moister in the room from building.

Johnny Bravo.. the chase cover and flashing have no ventilation. However, I have not yet intalled
the chase cover. Just plywood covers it right now. I'm waiting for it to be fabricated. I will have to
remove the cap and storm collar and put in in place soon.

Regarding creating more problems than it solves... do you really think it will be appreciably colder
with a couple of vents? This part of the chase is uninsulated... just 1/2" sheeting and some cedar
siding.

By the way, I've read a lot of bad things about exterior chases. I paid a lot of attention to detail when
insulating and installing vapor barrier. I used R21 on the sides, R38 on the ceiling. This thing has a monsterous draft, even when the unit is not running . I imagine that is in part due to the height- it's 26 ft. tall, the ceiling is 17.5 ft.

 

[James04]

Dskup,

I am not fully understanding about a ceiling in the chase. However if you want to keep th chase warm and you have access via the attic. I am assuming the condensation is moisture from vapor inside the house getting into the chase. I would try to install vapor barrier to stop that from happening.

James

 

[Metal]

The base of my chase is framed with a 2x12 (an extension of my main level floor) so I didn't want to drill through it. I just drilled through the siding and sheathing with a hole saw and then siliconed the vents in and painted them. If you can get under the chase you could put a few there instead. The top of my chase is not air tight (since it is a b-vent installation the pipe naturally pulls some air in from the chase) so I did not put any vents up top.