Originally published in the February 2018 issue of Sweeping Magazine
Question: I plan to install 2 UL listed stainless steel chimney liners in the same chimney flue, in a residential single-family home application that will serve two separate appliances. Is this permitted without installing a nominal 4" thick masonry wythe or dividing wall between them?
by Mike Segerstrom
Yes, but there are limitations, restrictions and requirements that must be met. And these will be based on the masonry chimney exterior walls being a minimum nominal 4 in thick solid Masonry.
First we'll look at adopted code requirements. When installing a stainless steel liner, we have to follow adopted local codes. In most states now, for single family homes and residential applications, this will be some version of the International residential code, or IRC. In Chapter 10 of the 2015 IRC there are specifications for "listed materials" as flue liners. A ul-listed stainless steel liner falls in this category.
Section R1003.12.2 states that "the space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance". It then provides an exception that states "this shall not prevent the installation of a separate flue lining in accordance with the manufacturer's instructions".
So this exception permits a second listed liner, however it refers us to the installation instructions of that liner. Most installation instructions for UL listed stainless steel liners do not describe, or specifically allow or prohibit this type of application. it then becomes our due diligence to contact the liner manufacturer, if it's the same brand, or both manufacturers if they are different. Liner manufacturers that do approve this type of installation, will typically provide us with documentation from their engineering department.
We also have to consider insulation requirements, and if the chimney flue is large enough to accommodate any required insulation for one or both of the liners. If a second liner prevents the installation of any required insulation, this would not be acceptable.
If your state or jurisdiction follows the NFPA 211 for chimney work, there is also wording found in the 2016 Edition that states "listed liner system shall be installed in accordance with their listing" (18.104.22.168). So this refers us back to the installation requirements of the liner manufacturer.
We also have to take into account the size of the top of the chimney and flue. We will need to have the necessary room to support both liners and install the required top termination hardware and termination caps. And installing two liners also cannot prohibit making proper connections at the base of each liner.
For applications like this the best practice is to consult with both the liner manufacturer, and the local construction or building official, so that we have approvals from both.