Coolant system on 2001 Nissan Sentra

I have no heat in the car until I drive at hyw speed then there is lots. Car does not overheat and I flushed the heater core out with a garden hose and it was clean. Thermostat is new and the rad has also been flushed. If the pump fails, would it not overheat the engine? Thank you

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Not in the winter time in Albany, I have been there and, wow, it gets really, really cold. If the impeller of the water pump gets worn out, the engine has to spin much faster. And, check to make SURE that your radiator flows very nicely, that too will impede the proper circulation of the coolant. I know that you flushed it, but HOW does it flow? And, don't forget to check any of the heater control valves, to make sure they are letting the coolant flow properly.
Where are the heater control valves? Are they vacuum opperated? I seam to be getting a lot of air bubbles coming out of the bleeder, is this common or due I have a bad head gasket? Today it was running hot then cool down to normal, then heat up again and I had no heat at all in the cabin. grrrrrr
Thanks for your help