What plenum means
A plenum is the box that connects the furnace or air handler to the duct system: a supply plenum on the outlet side and a return plenum on the inlet side.
Where it matters on a HVAC job
On a replacement, the new unit rarely matches the old plenum, so a transition or new plenum is a normal line on the quote. Seal it well; plenum leaks waste conditioned air right at the source.
What the model code says
- The code defines a plenum as a chamber forming part of an air circulation system, other than the occupied space. (IRC 2021, Chapter 2)
Check which code edition your building department enforces for plenum before you quote.
What it costs
Jobs that involve plenum are priced in the ductwork replacement cost guide: about $1,250 typical, usually $450 to $2,200. The guide shows its sources and what moves the price.
Work it out
- Cost guide: Ductwork replacement cost
- Licensing: HVAC license rules by state
- Tools, templates and guides: Free HVAC resources
Sources: 2021 International Residential Code, Chapter 2, Definitions (ICC). Checked October 9, 2026.