In general, all coffee roasters require positive pressure venting, or Category III. Beyond type, you'll also need to verify that your selection meets the minimum heat rating required by your roaster manufacturer and has a published clearance-to-combustible rating that you can work with in your building.

Check the specifications on the individual roasters – to make things clear, Mill City Roasters calls out the heat ratings on each line in the installation manuals. These temperatures must be accommodated within the heat ratings of the venting you choose.

‘Clearance to combustibles’ is the clear distance that must be maintained between a hot surface (venting, chaff collector, roaster body) and all combustible materials (drywall, wood studs, plywood, insulation, trims, composites, etc.). In building codes, a combustible material is defined as a material or composite that combusts, aids combustion, or adds appreciable heat to an ambient fire. With coffee roasters, however, you should consider the definition of a noncombustible found in the mechanical code, which is more strict in its definition. It takes into account performance during exposure to a constant high heat source and also gives more priority to eliminating fire potential rather than containing the spread of fire.

Yes. Though drywall or gypsum wallboard (GWB) is considered non-combustible per building code as it performs as a component in fire-rated wall assemblies, per the mechanical code it is considered to be combustible. Mechanical code considers performance during exposure to a constant high-heat source, which GWB fails as its papery surface is combustible. Also, the implementation of GWB in preventing fire spread is largely due to the high moisture content of the gypsum core. If you place a radiant heat source (venting or chaff collector) too close to drywall, the radiant heat will cause that moisture to evaporate, and prolonged exposure will degrade the fire rating due to calcination (removal of water) of the gypsum.

TLDR: If you have combustibles in your building envelope or if you have condensation concerns, you're going to need double-wall.

In double-wall ducting, the exhaust air is carried inside the inner duct (often called liner) and there is a second outer duct (often called shield). The space between the two walls insulates the outer shield from the heat carried by the inner liner - the space is typically left empty or sometimes it is insulated with fiberglass to increase the protection. This double-wall configuration allows the duct to be safely placed closer to combustible materials in your walls/roofs without the danger of igniting those materials. How close it can be to a combustible is determined by the ducting manufacturer and will be noted in their specifications as 'clearance-to-combustible' requirement.

Conversely, the outside surface of single-wall pipe (essentially, a liner without a shield) can rise to nearly the temperature of the exhaust air it carries. Due to this potentially hot surface, building code can require six to seven times the clearance to combustibles than for double-wall pipe, resulting in larger and larger openings in walls, ceilings, or roofs to allow single-wall pipe to pass to the exterior.

Even if it is the correct type and heat rating, we do not recommend using flex duct. Roasting exhaust carries chaff dust and oil residue that can quickly build up on the rib structure of flex duct. Because of the difficulty in keeping it clean, flex duct often ends up being a temporary solution, which can be quite costly when properly rated to carry the average exhaust temperatures.

We recommend very minimal, low-resistance, rain caps on vertical terminations and simple, minimal, bird screens on horizontal terminations (no tighter than 1/4” or 3/8” wire spacing). Avoid using caps with insect screens, baffles, or fins. Narrow air passage can promote chaff dust and residue build-up and can easily block air flow. Caps should be rated to handle the heat of the exhaust passing through them.

Condensation will almost always be present inside your stack, but it is typically minor and collects and dries at the bottom of your chaff collector. The quantity of condensate will depend on both your interior & exterior climate and the length of your run. Excessive amounts of condensate in long stack runs can be mitigated by using double-wall venting that is insulated or channeling the condensate out of the line with a collection section in your venting line.

There are three different national building codes (ICBO’s Uniform Building Code, SBCCI’s Standard Building Code and BOCA’s National Building Code) as well as mechanical & fire-prevention subcodes. To verify what regulatory codes apply to your installation, contact a local professional HVAC installer and/or contact your city/town’s building and planning department.

We are not able to provide site-specific direction, but common things to be aware of are clearances to combustibles in your interior, building envelope and exterior; distances between your exhaust termination and doors, windows and overhangs; distances of exhaust termination above roof structures and beyond exterior walls, maintaining minimum path widths for egress; protecting the general public from your equipment, venting, and exhaust; proximity to electric panels, gas meters, and fire suppression units.

Please be advised that your local code official will defer to both your roaster and venting manufacturer’s documentation regarding heat ratings and suggested clearances to combustibles. When choosing equipment for your project, you may want to choose one that has that information available, or it could delay your project or cost more to for individual, on-site certification.

(Just to be clear, you'll likely be subject to health, safety and revenue regulations, too, but these noted above pertain to your venting installation).