"A large portion of the book is based upon the research work in forced-air heating that has been conducted in the warm-air heating research residence, under the joint sponsorship of the University of Illinois engineering experiment station and the National warm air heating and air conditioning association. The results obtained from this cooperative research project ... from 1918 to 1932 have been previously published in a volume entitle 'Gravity warm air heating--digest of research'. This present effort may be considered as a supplementary volume dealing primarily with the research results obtained since 1932."--Pref.