>air blowing out the vents when it was 35 below zero and by
>the way, that was with the truck ouside. The wind was
>slightly blowing so the wind temp was below that. I can see
>the added benefit of the tank heater circulating the fluid
>because it can't freeze then.

Wind chill doesn't affect inanimate objects.

If you've got the right mixture of coolant to water it shouldn't freeze anyway. The circulation simply heats the whole system more evenly.