well.
If you have a stock car with the heater box, during the summer when the heat isnt running that brass valve will be shut and not allow the circualtion of coolant to the heater core.. during the winter it will flow thru and the blower motor will draw heat off of it to make your feet nice and toasty..
Just as bruce said, blocking it off is just the same as running it in the summer, there wont be any heat dissipated because it wont flow toward the heater lines... it just gets blocked off...
If you want to bake yourself in the summer, then leave the heater setup so you can turn it on.... when im at the track, i always turn my heater on and get rid of some heat n the staging lanes...
BW