Ford F-650 2015 Owner's Manual page 105

104 Starting and Stopping the Engine
Using the Engine Block Heater
Make sure the receptacle terminals are clean and dry before use. To
clean them, use a dry cloth.
The heater uses 0.4 to 1.0 kilowatt-hours of energy per hour of use. The
system does not have a thermostat. It achieves maximum temperature
after approximately three hours of operation. Using the heater longer
than three hours does not improve system performance and
unnecessarily uses electricity.
2015 F-650/750 (f67)
Owners Guide gf, 2nd Printing, April 2014
USA (fus)