/ Field knowledge · IMC 805.6
Matching the outlet diameter is not the temperature class.
A higher-temperature appliance can need a different chimney even when the outlet diameter matches an ordinary heater. Factory-built chimneys serving medium-heat appliances that produce flue gas above 1,000°F at the chimney entrance have to comply with UL 959.
Educational 2024 International Mechanical Code (IMC) 805.6 from Daily Code Talk. Confirm the adopted edition. Distinct from factory-built listing (805) and offsets (805.4–805.5). This is not a chimney selection and not a process-temperature assessment.
- The wording is above 1,000°F, measured at the chimney entrance — not a generic “1,000°F and greater” rule, and not a combustion-chamber or jacket reading.
- The provision concerns medium-heat appliances. Do not turn it into a universal chimney rule for every process or high-heat system. At or below the threshold does not independently approve another classification.
- A temperature-test rating or short-duration listing exposure is not automatically the product’s permitted continuous operating condition.
On the set: appliance classification, temperature basis (measured / calculated / rated / control-limited), and the required chimney standard in the criteria and the schedule. A change in chimney construction can change outside diameter, clearance, weight, support, and shaft space. Resolve that before procurement.
“Typical temperature” is not a source
If only a nominal outlet value is available, request the missing basis instead of assuming cooling along an unspecified connector. Do not drill a listed chimney to insert a probe without an approved access method.
Source: Daily Code Talk — IMC 805.6 · listing: 805 · offsets: 805.4–805.5 · connector clearance: 803.10.6.
What to send
Appliance cut sheet (flue-gas temperature and where it is measured, if shown), chimney listing assumed, and whether this is called medium-heat on the schedule.
If the appliance is hotter than a typical heater
Send the cut sheet. Masterbuild will say whether 805.6 is a UL 959 problem — or the entrance temperature still has no documented location.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. The 1,000°F entrance figure and UL 959 are 2024 IMC 805.6 education from Daily Code Talk — not a chimney selection. This page is not legal advice.