IMC 803.5 through 803.7 address two conditions that directly affect draft and combustion safety: dampers in appliance connectors, and multiple appliances entering a common vent or chimney.
Plain-English Highlights
IMC 803.5 prohibits manual dampers in appliance connectors except for chimney connectors serving solid-fuel-burning appliances - for most fuel-fired appliances, the connector must remain open and unobstructed, since a manually closed damper can obstruct the combustion product path while the appliance operates.
IMC 803.6 requires automatic dampers for oil-fired heating appliances to be listed and labeled to UL 17 and installed per the manufacturer's instructions; an automatic vent damper cannot be added to an existing appliance unless the appliance is listed for the installation, the damper is installed under its listing, and the installer's name and installation date are labeled on the device - the appliance and damper must function as a compatible listed system.
IMC 803.7 requires that where multiple connectors enter a common vent or chimney, the smaller connector must enter at the highest level permitted by available headroom and clearance to combustibles.
Permit Proof Chain
Damper type identified (manual, automatic, or none) -> manual dampers confirmed absent except where the solid-fuel exception applies -> automatic oil-appliance dampers checked against UL 17 listing and manufacturer instructions -> retrofit dampers confirmed to carry installer identification and installation date -> for common-vent connections, connector sizes and entry elevations checked so the smaller connector enters at the higher level within available headroom and clearance.
Where To Show It
M-001: damper and common-vent criteria. M-101: appliance and connector layout. M-501: damper and common-vent details. M-601: connector sizes and venting basis. E sheets: automatic damper controls and interlocks.
Do
Prohibit manual dampers in appliance connectors unless the solid-fuel exception applies. - Specify UL 17-listed automatic dampers for oil-fired heating appliances. - Confirm listing compatibility before retrofitting an automatic damper to an existing appliance. - Show the smaller connector entering at the higher level in any common-vent configuration.
Do Not
Install manual dampers in standard fuel-fired appliance connectors. - Retrofit an automatic damper without confirming the appliance's listing supports it. - Omit installer identification and installation date from a retrofit damper. - Show common-vent connections entering at arbitrary or unverified elevations.
Masterbuild QA Lens
An improper damper can block the vent path and cause combustion product spillage; a poorly arranged common-vent connection can create draft interference, condensation, or backdrafting between appliances - the result can be carbon monoxide exposure, nuisance shutdowns, and failed inspections. Compare connector sizes and entry elevations together before approving a common-vent detail, not damper compliance and vent layout separately.
Drawing / Submittal Check
Confirm the damper type shown on each connector detail matches an allowed condition (none, or a listed automatic damper with UL 17 basis, or a manual damper only where solid fuel applies). For common-vent connections, verify the smaller connector's entry point sits above the larger connector's entry point, with headroom and combustible clearance both shown.
Common Review Risk
A mechanical schedule shows an automatic damper added to an existing oil-fired appliance with no listing reference and no installer identification - the retrofit passes a casual glance but has no documented basis establishing the appliance and damper as a compatible listed system.
When To Escalate
Escalate any manual damper proposed outside the solid-fuel exception, any automatic damper retrofit without a documented UL 17 listing and installer record, and any common-vent configuration where entry elevations, headroom, or combustible clearances are not clearly resolved on the drawings.
Field Tip
Compare connector sizes and entry elevations side by side. The smaller connector should enter higher while maintaining the required headroom and combustible clearances - checking one without the other is how common-vent draft problems get missed at plan review.