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Cover Page - shares
 6 Months Ended
Mar. 31, 2020
May 01, 2020
Document Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2020  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus Q2  
Entity Registrant Name GENCOR INDUSTRIES INC  
Trading Symbol GENC  
Entity Central Index Key 0000064472  
Current Fiscal Year End Date --09-30  
Entity Current Reporting Status Yes  
Entity Filer Category Accelerated Filer  
Entity Interactive Data Current Yes  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Shell Company false  
Title of 12(b) Security Common Stock  
Security Exchange Name NASDAQ  
Entity Address, State or Province FL  
Entity Incorporation, State or Country Code DE  
Common Stock [Member]    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   12,277,337
Class B Stock [Member]    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   2,308,857