Cover Page - shares	9 Months Ended
	Jun. 30, 2020	Jul. 31, 2020
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Registrant Name	GENCOR INDUSTRIES INC
Entity File Number	001-11703
Entity Tax Identification Number	59-0933147
Trading Symbol	GENC
Entity Central Index Key	0000064472
Current Fiscal Year End Date	--09-30
Document Quarterly Report	true
Document Transition Report	false
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Interactive Data Current	Yes
City Area Code	407
Local Phone Number	290-6000
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Address, Address Line One	5201 North Orange Blossom Trail
Entity Shell Company	false
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Address, State or Province	FL
Entity Address, City or Town	Orlando
Entity Address, Postal Zip Code	32810
Entity Incorporation, State or Country Code	DE
Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		12,287,337
Class B Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		2,318,857