NCSG Manual Library
Originally published in the June 2017 issue of Sweeping Magazine
Question: One of our new members we'll call Jim, has a client with a masonry fireplace. It needs a new liner, but the existing clay liner is undersized, and even after the clay liner removal, the chimney won't accommodate a properly sized replacement. Jim wants to know, what are his options?
by Mike Segerstrom
Answer:
One of our new members we'll call Jim, has a client with a masonry fireplace. It needs a new liner, but the existing clay liner is undersized, and even after the clay liner removal, the chimney won't accommodate a properly sized replacement. Jim wants to know, what are his options?
Answer: Let's get the rest of his story before we answer this. Jim says they originally called because the fireplace spills smoke into the home during use. As that is an operation malfunction, he scheduled and performed a level 2 inspection. Inspection results indicated that when the chimney was constructed, the clay liner installed is significantly undersized. Other defects in the system included large voids in the joints between clay liner sections, cracks in areas of the clay liners, corbels in the smoke chamber, exposed brick cores in the corbelling, and improper chimney structure airspace clearances to combustibles in the smoke chamber and flue liner areas. Jim did say that the homeowner does not want to consider an appliance, that they want an open hearth wood burning fireplace.
The original clay liner is a nominal 9" x 13", with an inside dimension of 7" by 11". This gives us a flue cross-sectional area of approximately 77 square inches. The fireplace opening measures 31" high by 42" wide. With a square or rectangle flue liner, this fireplace requires a liner with a cross-sectional area 1/10 the fireplace opening, or approximately 130 square inches. Even a nominal 13" x 13" clay liner with an inside dimension of 11" x 11", would not be large enough.
The brickwork of this chimney was built around the 9" by 13" tile. Jim anticipates that once the tile is removed the inside dimension of the flue passageway will be approximately 9 1/2" x 13 1/2". Improper chimney clearances were observed which requires insulating the liner. Wrapping the stainless steel liner in a 1/2" ceramic blanket insulation system will take up less space than a poured insulation mix 1" thick all the way around the liner. So let's be conservative and say that a rectangle stainless steel liner with an inside dimension of approximately 8" x 12", with a 1/2" wrap, will fit in the flue. This liner gives us a cross-sectional area of approximately 96 square inches, which is still significantly under-sized from 130. Though they may provide better flow, a round or oval liner would be even smaller.
So what are Jim options? There was a time in this industry, before all the advancements that have been made across-the-board, when the attitude at times was simply to install whatever fits, and hope for the best. We have come a long way since then. In this case, there are many things to consider, from performance, function and safety perspectives. Jim has indicated that he will be reconstructing the smoke chamber with a UL-listed parging cement, to smooth the corbels, seal the voids, and make that area of the chimney exterior zero clearance. So let's focus on sizing concerns.
For many years, an accepted practice was to install a smoke guard across the top of the fireplace opening. A 6" smoke guard on this particular fireplace would "simulate" an opening reduction to 25" by 42". An opening that size would require a rectangle liner of 105 square inches. An oval liner with a smoke guard would get us closer, but still not quite there. Since a smoke guard only simulates a reduction, significant turbulence behind the smoke guard and in the throat area and/or smoke residence time may still occur. This may in turn result in rapid and/or excessive soot and creosote buildup. Listed liner manufacturers may also say that this voids the warranty or the listing, as the liner is still technically undersized. And in areas where permits and inspections are required, the AHJ might not approve a smoke guard.
One approach could be to temporarily resize the fireplace opening to meet liner sizing requirements, with a non-combustible material like sheet metal or cement board. If the fireplace vents properly and evidence in the chamber and flue liner support this, then the fireplace opening could be permanently resized with masonry. This would also give the homeowner the opportunity to reface their fireplace for an upgrade to its look.
Another option that was utilized and still has its place, is considering the installation of a mechanical draft inducer/exhaust fan. Manufacturers of these exhaust fans will have charts to match the correct size fan to the fireplace opening and liner size. Though a fan typically guarantees evacuation of smoke and combustion byproducts from the fireplace when properly installed and operated, careful consideration is needed. With today's tight home construction, and renovations like new windows that make older homes tighter, exhaust fan may solve the fireplace problem, but result in other issues. These include, but may not be limited to other combustion appliances in the home experiencing hazardous venting malfunction, drafty areas of the home becoming draftier, and outside air entering the home from places it previously did not. The AHJ would have to approve this installation and sizing calculations, and supplying make-up air or combustion air to the fireplace or other appliances may also be necessary.
Though notably adding to the cost of smoke chamber reconstruction and insulated stainless steel liner, another option for Jim to offer the client is to also replace the firebox with a precast masonry firebox. The benefit here is that these fireboxes have reduced liner size requirements. With a 42-inch-wide fireplace, depending on chimney height, a precast firebox may only require an 8" diameter liner. In addition to reducing liner size requirements, these precast fireboxes also produce more heat from less fuel, and achieve more complete combustion resulting in less soot and creosote in the venting system, and less particulates in the atmosphere. Installation of a UL listed precast replacement firebox is recommended, and would also address chimney clearance to combustibles issues in the firebox area if present.
These are just a few of the options for tackling sizing issues when relining a fireplace with a stainless steel liner. It’s important to understand that construction and repair requirements and codes are minimum standards. At times, even when codes are followed to the letter, there may be performance issues, and the client may not get the results they're looking for. At the same time, specifically with this issue, it is possible, that with an insulated stainless steel liner, and an insulated and aerodynamic smoke chamber, the fireplace may function and vent properly, with an undersized liner. But just because it vents properly, doesn't mean soot and creosote won't accumulate more rapidly.
Jim having a thorough understanding of the options, and making a professional presentation of them to the client is critical in navigating through a situation like this, where the chimney structure itself created a unique but correctable issue. With this particular job, once Jim presented the different options in a professional manner, he was surprised with the results. The client chose to go with the UL listed precast masonry firebox, in conjunction with flue liner replacement and smoke chamber reconstruction. The client told Jim that what sold them on this option, was the efficiency and heat production of it. In the end, the client didn't just want it to work safely, they wanted it to work better, and be better for the environment.