There's a lot of things that could be leaking, but regardless of the cause, low coolant level will stop it from circulating through the heater core, so the heater will still blow air, but it will be cold air.
With a leak as fast as you described, it should be rather easy to locate. Corroded engine block core plugs and ruptured heater or radiator hoses will leak pretty fast. Water pump leaks usually start out rather slow, and get worse over days or weeks if you ignore it long enough.
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Friday, March 28th, 2014 AT 5:35 PM