Regular mortar used for relining chimney smoke chamber - is this OK?

Question

Just had our wood burning fireplace chimney relined by a licensed chimney company. They used stainless steel liner down to the smoke chamber. They lined (parged) the smoke chamber itself with regular type-S cement. I understood they should have used a heat resistant mortar like Chambertech, but they insist that it's not necessary. Are they right? If not, how long would regular mortar hold out? The fire will only be used occasionally (say once a week in the winter).

Answer 1

The smoke chamber is not in direct contact with flames. This flame impingement is where the danger lies, particularly if there are issues with clearances to combustibles around the smoke chamber. In most states the authority having jurisdiction dictates what is required here. Usually the NFPA code 211 is referenced, however it applies to fireplaces at the time of construction and most homes are older than the code and therefore do not meet it and are not required to. During a repair or re-line, a technician may decide to thicken up a smoke chamber with a non-combustible material like Ahrens products or form-filling the chamber with cement. The most critical area in the fireplace is the hearth or bottom and the firebox itself, these areas distribute the bulk of the heat into your home. The liner and smoke chamber generally are re-lined repaired because future creosote removals will be impossible if the liner or smoke chamber allow creosote to condense where it will be inaccessible. Creosote is highly combustible and a sixteenth of an inch has the same flash point as a piece of newspaper. My recommendation is to sweep this system after 30 small fires and see if the concrete has cracked, if not - continue to season with small fires another 15 uses and sweep again. After that regular sweeping about every 30- 45 uses.

Answer 2

As far as the parging of the smoke chamber, it is likely that any parging material will work for many years if it will stick. Some times when relining a fireplace chimney the surfaces of the brick or other masonry to be parged are difficult to get clean enough for perfect adhesion of the parging material.  Chambertech is a brand name of course and is only available from those dealers,  and of course those dealers believe it to be the best material.   In many areas of the country other types of mortar and cements are used successfully even if they are not the "perfect" product.  Products like Chambertech are possibly "stickier"  and may not be as mechanically damaged by the intense heat of a chimney fire, however in the event you do have a chimney fire any parging or liner material would need to be replaced to maintain the integrity of the system. Ask your chimney company if they will come back once and fill in any voids in the parging if it doesn't stick. Say  after 1 or 2 seasons?  That should take care of any areas of concern.  And since your chimney will need inspected and cleaned at some point anyway, how can they lose?  It is actually more likely that the smoke chamber area and its parging will be damaged by water coming in from the top/crown, the flashing or directly through the masonry than by the fires you are going to build so make sure that has also been addressed by your chimney company.