Ford F-Series Super Duty Class A 2019 Owner's Manual page 31

Starting and Stopping the Engine
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.
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Motorhome (TY3) Canada/United States of America, enUSA, Edition date: 201803, Third-Printing-