Yes there will still be radiant heat in the car from hot coolant going through the heater core if you don’t have a valve.
I know I definitely notice the heat in the car on longer drives if I forget to close the temp control slider on the dash that closes the heater valve. I don’t have AC in my car, but it’s really obvious if I forget to reset the temp slider in the spring and the fall. I need the heat in the cool/cold mornings but in the hot afternoons leaving the slider on warm heats the cabin up fast even without the blower going.
