underfire

verb4

1. •
   To intentionally operate a boiler, furnace, oven, etc., at a low level.

   • It is generally accepted […] that underfiring of ovens with lean gas (generally blast furnace gas) leads to a greater heat consumption than underfiring with rich gas. — 1989
2. •
   To burn fuel at less than the desired level, thus not providing heat efficiently.

   • If the burner is underfiring, the nozzle may need to be changed or the pressure at the outlet of the fuel pump may need to be adjusted. — 2012
3. •
   To fire at a low (or excessively low) temperature.

   • You no longer need to underfire your products or guess at their high temperature behavior simply because of lag in furnace technology. — 1973
4. •
   To heat from below.