/ Field knowledge · IMC 803.5–803.7
The smaller connector enters higher — not wherever the pipe lands.
Manual dampers do not belong in ordinary appliance connectors. Automatic dampers on oil-fired heating appliances have to be listed to UL 17. Where two connectors enter a common vent, the smaller one enters at the higher level — not wherever the pipe happens to land.
Educational 2024 International Mechanical Code (IMC) 803.5–803.7 from Daily Code Talk. Confirm the adopted edition. Distinct from connector fundamentals (803) and multistory common vent (801.19). This is not a damper selection.
- 803.5: no manual dampers in appliance connectors except chimney connectors serving solid-fuel-burning appliances. A manually closed damper can obstruct the combustion-product path while the appliance operates.
- 803.6: automatic dampers for oil-fired heating appliances listed and labeled to UL 17, installed per the manufacturer. A retrofit damper cannot be added unless the appliance is listed for that installation, the damper is installed under its listing, and the installer’s name and installation date are on the device.
- 803.7: where multiple connectors enter a common vent or chimney, the smaller connector enters at the highest level permitted by available headroom and clearance to combustibles.
On the set: damper type on each connector (none, UL 17 automatic, or solid-fuel manual). For common vents: connector sizes and entry elevations together — smaller above larger, with headroom and combustible clearance shown.
The retrofit damper with no listing
An automatic damper added to an existing oil-fired appliance with no UL 17 basis and no installer identification looks like a line on the schedule. It is not a compatible listed system. A poorly arranged common-vent connection can backdraft between appliances.
Source: Daily Code Talk — IMC 803.5–803.7 · connectors: 803 · pass-through: 803.10.4 · clearance: 803.10.6.
What to send
Whether any damper is proposed, oil vs solid-fuel vs gas, connector sizes into a common vent, and any UL 17 cut sheet.
If someone added a damper, or two pipes hit one chimney
Send the connector detail. Masterbuild will say whether 803.5 still prohibits the damper — or 803.7 has the smaller pipe entering below the larger one.
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. UL 17 and the smaller-enters-higher rule are 2024 IMC 803.5–803.7 education from Daily Code Talk — not a damper specification. This page is not legal advice.