The problem

Recirculating (carbon-filter) range hoods that vent into the shallow gap between a cabinet top and the ceiling usually end up with a butchered 90° elbow squeezed into that space - mine was cut and taped, choking a Ø120 mm duct to roughly half its area. Result: weak suction and a loud, strained-sounding hood.

The part

This is a plenum diffuser cap that replaces the elbow entirely. Instead of forcing air through one narrow outlet, it turns the airflow inside the 35 mm gap and releases it through two large side exits into the soffit cavity:

• Exit area ≈ 8,400 mm² — about 75 % of the Ø120 pipe's cross-section (typical flat elbows manage ~40–50 %)
• Internal splitter ridge and guide vanes divide the rising jet evenly to the left and right
• Solid floor plate seals the 170×170 mm cabinet hole completely — no moist cooking air recirculates into the cupboard
• Wall-mount plate with two front-accessible screw holes (Ø5.2 mm, 140 mm apart) holds the part against the wall; the cap body rests on the cabinet top

Measurements (check before printing!)

• Overall: 157 × 168 × 81.5 mm (fits 180 mm beds)
• Collar: Ø121.5 mm inner / Ø126.9 mm outer, slips over a Ø120 mm flex hose, 25 mm grip for a worm-drive hose clamp
• Requires ≥ 35.5 mm of vertical clearance above the cabinet top (cap is 34.5 mm tall)
• Air exits sideways (left + right) — nothing is blown forward, so it's friendly to decorative boards/pelmets in front of the soffit
• Cabinet panel thickness up to ~18 mm assumed

Printing

• Print as oriented (cap roof flat on the bed, collar pointing up)
• 3 wall loops, 15 % infill, 8 mm brim
• PETG / PCTG / ASA recommended (warm, humid exhaust air) — ~315 g, roughly 15.8 h

Installation

Clamp the flex hose onto the collar first, feed it up through the hole while raising the cap, screw the plate to the wall, connect the hood. Done.

Result on my hood: noticeably stronger suction and much quieter — the restrictive "hiss" is gone.