IMC 801.18.3 brings existing masonry chimney reuse back to a basic maintenance requirement: can the chimney actually be accessed and cleaned? A chimney that passes on size, passageway condition, and clearance can still fail reuse if the cleanout itself was never verified.
Plain-English Highlights
IMC 801.18.3 requires masonry chimneys to be provided with a cleanout opening complying with Section 801.13. That means the cleanout must have a minimum 6-inch opening height, an upper edge located at least 6 inches below the lowest chimney inlet opening, a tight-fitting noncombustible cover, and access maintained for inspection and cleaning. For plan review, this matters most when an existing masonry chimney is reused for a new or replacement appliance.
The drawings should not only show the appliance connection; they should also show how the chimney will be inspected, cleaned, and maintained. If the cleanout is missing, blocked, undersized, concealed, or inaccessible, the existing chimney condition is not fully reviewable.
Permit Proof Chain
Existing masonry chimney identified for reuse -> cleanout location surveyed -> opening height and elevation below the lowest inlet checked against Section 801.13 -> cover material and fit specified -> access path coordinated with equipment layout, casework, and finishes -> inspection hold point assigned before the chimney is accepted for the new or replacement appliance.
Where To Show It
M-001: existing chimney cleanout notes and Section 801.13 basis. M-101: appliance and chimney location. M-501: cleanout opening, cover, access, connector, and liner details. M-601: appliance schedule with existing chimney venting basis. A/LS sheets: chimney enclosure, wall, shaft, and access coordination. P sheets: boiler, water heater, fuel, and oil coordination.
**Do** - Show the cleanout location and access path on the same sheet as the chimney detail. - Confirm cleanout opening height and elevation comply with Section 801.13. - Provide a tight-fitting noncombustible cover. - Coordinate cleanout access with equipment layout, casework, and finishes before those drawings are finalized. - Treat the cleanout as a permanent maintenance access point, not a one-time construction item.
**Do Not** - Reuse an existing masonry chimney without checking cleanout access. - Hide the cleanout behind equipment, casework, or finished walls. - Omit the cleanout cover material or its fit condition. - Treat chimney cleaning as only a field maintenance issue with no design responsibility. - Assume a cleanout that was compliant before a renovation remains accessible after new construction concealed it.
Masterbuild QA Lens
Add a fourth line to the existing-chimney acceptance record for the cleanout alone: location, opening height, elevation below the lowest inlet, cover material, and access condition. A chimney that passes size, passageway, and clearance can still fail reuse if the cleanout was never actually verified against Section 801.13.
Drawing / Submittal Check
Compare the cleanout location, size, elevation, and cover shown on M-501 against the equipment layout, casework, and finish plans on the same or adjacent sheets. Confirm nothing in the coordinated design encloses or blocks the cleanout after construction and occupancy.
Common Review Risk
An existing masonry chimney is reused for a replacement appliance, and the drawings show only the new appliance connection. The cleanout is undocumented, or it is shown behind new equipment or finishes added by the same project. The reviewer and the inspector cannot verify size, elevation, or access, and the chimney condition is effectively unreviewable until the concealment is corrected.
When To Escalate
Escalate a missing or undersized cleanout on an existing masonry chimney, a cleanout concealed or blocked by new equipment, casework, or finishes, an unclear elevation relationship between the cleanout and the lowest chimney inlet, and any oil-fired appliance chimney reuse where cleanout condition has not been coordinated with the applicable NFPA 31 scope.
Field Tip
On existing chimney reuse, look below the lowest inlet opening. If the cleanout location, access, and cover are not shown or verified, the chimney detail is incomplete regardless of how well the rest of the venting path is documented.