Air Intakes Location--ZC High Efficiency Fireplaces

[builderbob]

In my study of getting to know a few final candidates of high efficiency woodburning fireplaces for a new home we are building, I stumbled upon a question about location of outside air intakes.

I happened to pickup some literature somewhere which was a few years old on one particular brand and model. Then I happened to see similar more recent literature for the same fireplace.

The old literature showed and talked of the outside air intakes being "on top of" the unit. The newer literature showed the outside air intakes at the "bottom side" of the unit.

The other candidates of different brands each have their outside air intakes at the bottom side of the units.

I wonder if the first unit older versions revealed some safety problem with the outside air intakes at the top? Somewhere I read of the potential for hot fire exhaust venting out of the cool air intakes. I started thinking, wouldn't this condition more likely occur if such cool air intakes were "on top" as hot air rises?

Then I was left wondering if any cases of that brand's owners had suffered house fires.

Anyone care to weigh in on this whole situation and espouse some theory's or knowledge of actual experience in regard to this?

builderbob
planning for a new home

 

[bmstove.com]

Personally I think reversed draft through an outside air duct is an extremely unlikely event. We have been here 30 years, me for 15 of those and none of us have ever seen this phenomenon occur. IN MY OPINION for this to happen conditions would have to exist that would make wood burning practically impossible no matter where the intakes were located. If hot flue gasses are venting out aluminum cold-air duct, it wont make much difference where they are becuase shortly they will be gone anyway.

Does anyone know where this phenominon orriginated? Does anyone know anyone it actually happened to or is this a "scientific possibility" that worked out on paper and was accepted as a possibility. The only time I have personally heard the point argued is by a popular stove manufacturer who has several popular models that are not outside air adaptable and the subject came up after being asked why they can't make their stoves accept outside air to be mobile home approved or for use in tighter homes that are being built today.

LET THE FLAMING BEGIN! I know my thoughts will have mixed reviews.

Jack

 

[builderbob]

Thanks for the response.

As a newbie to ZC premanufactured units, I found this *possibility* troubling. Picture---a brand new house, our dream home, taking our life savings to build----and our new wood fireplace sends it up in flames Hence my questions about our worst fears.

Nice to know at least one set of 30 years experience has never actually seen or heard of the *actuality*!!

builderbob

 

[Gooserider]

Here are a couple of links to Tom Oyen's "Chimney Sweep's Library" that talk about the issue.

(broken link removed to http://www.chimneysweeponline.com/hooa.htm) and (broken link removed to http://www.chimneysweeponline.com/hooa3.htm)

They offer an excellent discussion of the issue, and I would consider Tom (who is also an occasional particpant here) to be one of the more knowlegable "applied experts" on the subject. In essence I think the outside air "hazard" is basically a non-issue, with little or no actual evidence of the theoretical problems. IMHO the advantages of an OAK are far greater than the potential hazards, especially in new construction.

That said, I wouldn't see anything wrong in trying to further minimze the risk by attempting to locate the vent in a somewhat protected area and make it fire resistant to the extent that it's practical and easy to do (I wouldn't put a huge amount of extra effort into this, just what's easy to accomplish)

Gooserider

 

[builderbob]

All you people are great!! Thanks so much for the good information and links to great discussions. Many of the questions and much of the confusion running through my mind are being put to rest.

I am feeling much better about making an informed decision and being comfortable with it.

Anyone else want to chime in? The more the better.

So, then another question for opinions-----if one has a choice to put a cold air intake to an exterior wall or to the crawlspace, would the crawlspace perhaps have an advantage in terms of being a *protected* intake vent not influenced by winds on one or the other side of the house?

builderbob

 

[Gooserider]

I'm not a construction guru, but I'd go for the crawlspace IF (and only if) it is vented and open to the outside air. If it's a closed off space, you need to go for the wall... Given that there are advantages to closing in the crawlspace (reduced heat losses through the floor, etc.) I wouldn't leave the space open just for an OAK vent, but it might make sense in terms of keeping things neat looking around the stove for you to go down into the crawl space and then out with a vent to the exterior.

If doing that, you'd of course have to pay attention to making sure the vent was big enough to deal with any losses in air speed due to length, etc. I'm not sure how to do that, but it should be in your manual, or be something your installer can look up.

Obviously the crawlspace is theoretically better in terms of wind influence, but in practice its probably not worth the effort just for that one reason... I have also heard of people putting a "T" in the vent and running a branch to opposite sides of the house. Again, theory says that this will cancel out any pressure differences as if one side is low, the other must be high and the OAK will just draw from the high side. - Whether it's worth the extra effort and expense is more debateable...

Gooserider

 

[bmstove.com]

I like the crawl space too. For one it eliminates the hood on the side of the house. Being away from wind is good too. In situations where the fireplace was centrally located in the house above a finished area I have increased the size of the duct to and insulated 6 inch or 8 inch (depending on length) and built it into the ceiling below. This gives the outside ait a low resistance path to the fireplace through a long distance. The insulated ductwork obviously is to overcome any issues with a long run of cool air flowing underfoot to the fireplace which might transfer through standard air duct.

Jack