Thread: Gaps firebox

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That's a great question that I would like to find out the answer to too.

If you need refractory mortars that meet ASTM C-199, you can buy this at most any hardware store in a caulking tube. If you need a 2000F rated insulating mortar, only a chimney sweep would have that (e.g. Chamber Tech 2000).

However, that second pic seemes to show a cracked damper frame itself. These mortars are for masonry cracks. Gaps btw metal and mortar need a high temp. expansion joint such as ceramic fiber insulation (again from sweeps). If you caulk btw metal and masonry, it will crack or cause cracking elsewhere when the metal heats up.

If a metal component such as a damper frame is cracked to where it affects the integrity of the firebox and smoke chamber, then cut it out and replace it with a similar unit or better still, a top damper. Then, using CT2K mortar, smooth up everthing nice and aerodynamic at the throat.
HTH

I believe the real question is,... What caused the cracks? If this steel form damper assembly is in direct contact with the masonry material, (which it appears to be) then expansion during use of this fireplace may result in exactly what we see here. There should be an expansion joint between these two materials.

If we apply sealant, the crack will come back when the fireplace is operated and the material expands and shift again. It is common to apply a repair, but we should first understand the cause.

Dale W. Feb,
Executive Director / F.I.R.E. Service
Lead Investigator / F.I.R.E. Associates
(805) 552-9958 (805) 552-9954
E-mail / FireService@earthlink.net
Fireplace Investigation, Research and Education Service Education
www.FireAssociates.org Investigation