underfiring

noun4

1. •
   The state of not burning fuel at the desired level, and thus not providing heat efficiently.

   • Gas-burner maintenance should include these measures: […] Adjust gas input if combustion testing indicates overfiring or underfiring. — 2008
2. •
   The state of being insufficiently fired or underfired.

   • After either underfiring (4 minutes at 1,450° F), normal firing (4 minutes at 1,575° F), or overfiring (6 minutes at 1,750° F), the specimens were deenameled with molten sodium hydroxide. — 1952
3. •
   [before a noun] Heating from below.

   • The piers should be a minimum of 8″ high (0.2 m) so that underfiring can be used to heat the pieces from below (and traditional overfiring to heat from above). — 2004
4. •
   Intentional operation of a boiler, furnace, oven, etc., at a low level.

   • The designer believes that by this combination of a combustion-chamber with the underfiring grate the combustion of the fuel is improved. — 1890