Ok well I guess what I was trying to point out is the heater doesn't make the coolant hot enough for the heater to work. And doesn't warm the coolant enough that the glow plug cycle would be shortened. In the book it only says to plug it in if the temp is going to be below -20 F and leave it plugged in for atleast 8 hrs. So 30 - 60 min of being plugged in isn't going to make a difference. My trucked starts just as good at -15 plugged in or not.