Why would you leave it on? Hook it up to a timer.

Water.

Roy

It is in the water jacket and shouldn't damage anything.

FMB
('03 CTD)

The heater's  installed in the side of the block, in a coolant passage.  It
warms the engine coolant.  It won't get hot enough to boil the coolant -
it's only about a 600W heating element, trying to heat 6 gallons of coolant,
3 gallons of oil, and about 800lbs. of cast iron, all sitting in suo-zero
temps.

when it is cold out you only need to heat up the block for a couple hours
what i did with my diesel is i used a timer it is a indoor-outdoor timer i
just set it to turn on at 4:30 am and turn off at 8:30 am i leave at 6:00
am for work that is more than enough time to worm up the block the block
heaters were not designed to heat up the water to operating temp they were
to warm up the blocks for easy starting so you dont fry the starter.  when
i lived in a apartment had no place to plug it in every year i would fry a
starter (life time warranty) coldest dam day of the year i am putting a new
one in