Chimney & Fireplaces/Flue Size

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Expert: - 1/15/2008


Question
QUESTION: I building a fireplace and it is going to have a 48 in damper on it and was wondering what size of Flue do I need to put on it.

This is a full masonry fireplace and someone told me to use an 18"x18" flue,  and someone else told me that would cause a down draft and I would get smoke inside.  

I am wanting to know what size flue I need to use for 48 inch firebox?

Thanks
Bret

ANSWER: Bret,

When you say a 48"inch box, I will assume you are referring to its width? If that is the case, you will want to calculate the height of the box as well, multiply the height and the width and come up with the area of square inches. If you intend on using a round shaped flue, it shouldn't be less than 1/12th of that size. If your flue is rectangle with rounded corners,square with rounded corners or even oval, 1/10th. If its truly square or rectangle, 1/8th. I hope this helps, good luck.

---------- FOLLOW-UP ----------

QUESTION: You helped me out alot, thanks.

My fire box is going to be 48x38 inches.  I am using square flues with rounded corners.  I now understand that I can use a flue as small as 14x14 inches,  If I understand you right?

I am now wanting to know if 18x18 is big?  Will this create some type of a down draft?

Thanks
Bret

Answer
Bret,

14" X 14" with rounded corners would be a good choice. 18" X 18" is probably to big. The larger size of the flue is more difficult to maintain a high flue gas temperature. The cooler flue is a restriction to flow and can result in smoke entry into the living area.

Note that are numerous other consideration in the construction of a new system. I recommend contracting with a qualified mason for help with other potential issues prior to completion. Good luck.

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Randy Brooks

Expertise

Inspection of Masonry, Pre-cast Tilt-up, and Factory Built Fireplaces as well as Free Standing & Insert, Gas, and Wood Appliances. We also provide Hearth Fire Loss Investigations, Litigation Support, Consulting Services, Flue Sweeping, Installation and Repair. Additionally, I’ve been a chimney sweep for 23 years, service and install hearth appliances as well as being recognized by the Superior Court of California as an expert witness.

Experience

Brooks Chimney Sweeping Since 1989 Ojai, CA Owner / President www.brooks-hearth.com

Organizations
NCSG, MEMBER,(National Chimney Sweep Guild), Member since 1990, Board of Directors representing Region 6, (CA, NV, AZ, UT, OR, WA, ID, WY, MT, AK and HI,) 2004-2011, ethics committee chair, 2004-2005, executive committee member, Vice President, 2006-2008, President, 2008-2011. CSIA, BOD (Chimney Safety Institute of America,) Board of Directors, ethics committee chair, 2004-2005, executive committee member, Secretary, 2005-2006, Vice President, 2006-2008, President, 2008-2011. HPBA, MEMBER, (Hearth, Patio, & BBQ Association,) since 2002. NFPA Member, (National Fire Protection Association) Since 2009. Technical Committee member for the 211 Standard.

Education/Credentials
California State Licensed Contractor, C61-D34, #750710. http://www2.cslb.ca.gov C.S.I.A., (Chimney Safety Institute of America,) Certified Chimney Sweep, #2580. Held since 1990 re-certified in ‘93, ‘96, ‘99, ‘02, ‘05, '08,'11. http://www.csia.org F.I.R.E. (Fireplace Investigation, Research & Education Service,) Certified Inspector, #FP-006. Held since 2000, re-certified in ’03, ‘06. http://www.f-i-r-e-service.com Fireplace Investigation, Research & Education Service,) Advance Expert Consulting & Witness, certificate, ’05 N.F.I. (National Fireplace Institute,) Certified Gas & Wood-Burning Specialist, #133851. Held since 2002, re-certified in '05,‘08. http://nficertified.org

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