underfiring

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

In natural gas-fired boilers, gas pressure is critical to proper burner operation and efficient combustion. Variations in gas pressure may cause over- or underfiring of the boiler. […] Underfiring may cause flame instability and poor combustion efficiency due to high levels of excess air.

[W]hile there were fully as many under-firing as tube-firing boilers, at work, the majority of explosions took place in boilers of the latter class, and they almost invariably commenced with the collapse of the fire flue.

Hence it is, I must suppose, that while more effects could be produced with rose if one could only be sure of the fire, yet carmine is the more serviceable, as one is at so much less risk of loss from overfiring, which is more common than underfiring.