Rumford Technical Discussion
Why is Exterior Air Required?
7/24/12

Hey Jim, the project continues...

I had a question about the volume of combustible fresh air supply as a proportion of the chimney volume exhaust. As in engine design or ducted fan aircraft propulsions design we try to get the same amount of air into the system as we have in the exhaust out of the system. But, I have noticed fireplace closed systems seem to have much less of a fresh air feeder volume (like 4 inch diameter pipe only) then in the chimney exhaust volume. Is there any reason for this other then convenience?

Thanks Jim for your consultation, Rick.

*****

Rick,

There have not been any first class engineers in the fireplace design business since Ben Franklin and Count Rumford 250 years ago.

The fact is that most of the air that goes up the chimney in an open fireplace comes from the room, not the 4" diameter exterior air source. Hardly any air comes in through the 4" diameter air source - if it's an open fireplace or a fireplace with the doors open. And that's the clue.

The exterior air supply, never required before about 1970, is an idea promoted by the fireplace door manufacturers for cooling purposes. Apparently they do know that glass blocks nearly all the radiant heat. So where, you might ask, does that heat go? Bottled up in the firebox maybe it will overheat the firebox and the long established (before glass doors were thought of) wall thicknesses and clearance to combustibles requirements might not be adequate. "Maybe our glass doors will cause fires or, more importantly, create a liability problem for us." Besides the code required exterior air intake, glass door manufacturers make sure their doors leak like a sieve or about as much as an open damper.

I find this all very delicious, especially recently when the Green Building Code briefly required air-tight gasketed doors before the Hearth Products Association called for an emergency meeting to point out that air-tight doors would be dangerous. Sold to save energy when you close the glass doors on a dying fire, all fireplace doors really do is block all nearly all of the radiant heat and hardly any of the heated room air loss.

So, in answer to your question, five CFM in and 250 CFM out is not a failure in engineering. Even the D students could figure that out. It's a failure in marketing integrity.

Our recommendations vis-a-vis the code-required exterior air supply are at http://www.rumford.com/tech7.html

Warm regards,
Jim Buckley

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