Definition of "underfire"
underfire
verb
third-person singular simple present underfires, present participle underfiring, simple past and past participle underfired
(transitive, intransitive) To heat from below.
Quotations
It was contended, that the only objection which could be raised against under-firing, was the danger of incrustation, or deposit upon the boiler bottom, of matter held in suspension by the water; but that this rarely, if ever, caused explosions; the utmost injury it occasioned, was causing the boiler plate to be burnt out, and that this effect could not take place, without gross neglect.
1856 May, William Kemble Hall, “On the Causes of Explosions of Steam Boilers”, in John F[ries] Frazer, editor, Journal of the Franklin Institute of the State of Pennsylvania for the Promotion of the Mechanic Arts. Devoted to Mechanical and Physical Science, Civil Engineering, the Arts and Manufactures, and the Recording of American and other Patent Inventions (Third Series; XXXI), volume LXI, Philadelphia, Pa.: Published by the Franklin Institute, at their hall, page 299
(transitive) To intentionally operate a boiler, furnace, oven, etc., at a low level.
Quotations
The large supply of natural gas at Chicago permitted underfiring the batteries of the Chicago By-Product Coke Oven Plant with a mixture of producer gas and natural gas. […] The mixing of by-product coke oven gas with blast furnace gas to correct the varying thermal deficiencies of the latter when used to underfire coke ovens is patented by Becker.
1934, Wilbert J. Huff, “Gaseous Fuels during 1932 and 1933”, in A Survey of American Chemistry, volume VIII, New York, N.Y.: Chemical Catalog Company for National Research Council, page 339
One of the major factors contributing to high particulate emissions from domestic burners is on-off cycling. […] Underfiring consists of using a smaller (undersized) unit which has to stay on longer and, therefore, cycles less. However, no data are available on the effectiveness, cost, and public acceptability (more complex system or inability to heat house as comfortably during very cold periods) of these proposals.
1976 April, G. R. Offen et al., Control of Particulate Matter from Oil Burners and Boilers (EPA-450/3-76-005), Research Triangle Park, N.C.: Office of Air and Waste Management and Office of Air Quality Planning and Standards, Environmental Protection Agency, page 1-19
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, Roger Loison, Pierre Foch, André Boyer, Coke: Quality and Production, 2nd edition, London, Boston, Mass.: Butterworth & Co. (Publishers) Ltd., page 467
(intransitive) To burn fuel at less than the desired level, thus not providing heat efficiently.
Quotations
Often home owners select a [wood] stove that is too hot for the space to be heated. Many stove owners compensate by underfiring the stove, which causes poor performance, increased emissions and potential chimney fires by creosote buildup.
1999 June, “Homes & the Canadian Experience”, in Alexandra R. McKirdy, editor, The Canadian Renewable Energy Guide, 2nd edition, Burnstown, Ont.: General Store Publishing House [for the] Solar Energy Society of Canada, page 16, column 2