1997 chevy tahoe heat

There's a heater control valve on the heater hose that goes into the firewall to the heater core.It works on vacumn.Has a skinny vacumn hose connected to it .Manually push plunger in all the way.This opens water to heater core.Got heat?Switch works on vacumn also.Sounds like a vacumn leak indede.

Make sure the coolant is full. Start the engine, turn the heat on high, and allow the engine to warm up. Once at operating temp, feel both heater core hoses. If both are hot, check blend air door. If only one is hot, (check the heater control valve... but I don't think this vehicle has one). If one is hot, try flushing the heater core. Also, make sure the engine is running at a normal operating temp. If the engine doesn't warm up, you won't get good heat.

As far as the air flow direction, North Polean is correct. There are vent doors that are actuated by vacuum. If there is a leak, default is the defroster. These are small vacuum hoses that run under the dash from the engine's intake. If you are lucky, with the engine running, you may hear a hissing sound from the leak.