I guess then you can open the doors somehow... and then it will be about 6 months until you need to be worried about closing them.No, not with this design ( if you are talking about a liquid valve ), Chevy of this era has the hot hose direct to the heater core from the intake manifold, from there it goes straight back to the top of the radiator.
The only control for heat / cold is a few blend doors, the heater core is always charged.
when the heater core blend door is closed there is no air flow through the plenum.
these doors get sticky all the time, and the electronic module that controls them cannot overcome the force needed to fully open or close them.
( or the gear set in the modules crack, also common, but this problem has been addressed by replacement. )