I have a 520 sf attached garage.  I used the heater below and it works great.  Needs a 30a breaker, 220v. Ceiling mounted.  Heats up the room almost instantly.  OK, maybe 5 minutes.
	
	
		
			
				
					
						
					
				
			
			
				
		
	
				
			
					
				Fahrenheat 5,000-Watt Unit Heater-FUH54 - The Home Depot
Provide 5,000 watts of heating power to up to 500 sq. ft. of a garage, basement and warehouse or construction addition using this Fahrenheat 5,000-Watt Portable Electric Heater. A high-level cut-out automatically
				
					
						
					
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