i have a 1964 chevy impala and when the heater is off, after it warms up hot air blows out of the heater ducks
On these old rigs there was a foam seal that went around the blend doors, over time the seals disintegrate and you start feeling warm air when the door is closed, there is a modification that you can do, your local parts store should have a cable driven hot water control valve, and a push-pull cable and bracket, you then cut the hot water supply hose and put in the valve, make sure you get the supply, not the return, then find a place where you would like to install the cable assembly and run the cable and attach it to the valve and adjust the throw and your done. You can now turn off and on the hot water to the heater core as you please. Or you can remove the heater box and replace all the seals on the air blend doors inside the box. HTH
If in fact it is the seals,buy some foam insulation,get under the dash and glue in place.Easy fix!!!!!!
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