"Rather, I'd guess that you are having an issue with your blend door."
" I interestingly had a similar problem, it ended up fan speed 1 and 2 were burnt out on the variable resistor, . . . "
"I think it's possible that you've got air getting in your "closed" cooling system (and coolant getting out)."
" . . . the impeller on your water pump may be getting worn and need to be replaced . . . "
Courtney Says, "My 2003 Buick Century heater works fine while accelerating but as soon as I release the gas the air begins to cool, swiftly."
Not enough information is leading to guesses. I feel we need some clarification from Courtney. I took her comments to mean that the fan and its functioning were working correctly, but the air temperature was the problem, here.
Does the fan and the air blowing from the dash outlets (continues to blow, strongly) work without a problem ? Is it only the temperature of the air that changes with accelerating/coasting ? Please explain.