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So it takes the better part of an hour for the heat to go from 80 to 100. So the core has to be pretty clogged up. Granted it's been single digits to well below zero, but that should not affect this that much in a normal working condition.
If the engine coolant is only at 130F as you stated in an earlier post, it does not make much difference how much you flush the heater core!
The engine is running too cold.
I might have mis spoke. Coolant temp is normal at 190. Air temp in the cab gets up to 130 on a warmer day or with a lot of driving around. Normally with my shorter drives I'm lucky if the cabin air temp gets above 100.