Heat immediately?

[luvbowties]

OK, all we theoreticians, I'd like some answers. Living in the southeast, I seldom face weather in the teens. Not so lately! Suppose one enters his truck in the early AM where a thermometer inside shows temps in the upper teens. Say you assume the coolant must be above this to stay liquid--just assume 20. If you immediately turn on heat to max as soon as you start up, would the 20-degree coolant allow the heater to immediately begin raising the temp inside the truck? I recently argued "No".

And now, where could there be a flaw in answering "Yes"?

I thought it was interesting.
Sam