[wiswind]

Water ALWAYS flows through the heater core when the engine is running.
The temp control causes a electric motor to move a blend door to determine how much air will go over the hot heater core.

The control that you select defrost, heat, A/C, etc causes vaccum driven control of where the air goes.....with defrost being the default in case the vaccum source is lost (there is a resevoir that holds vaccum for the diverter control when the engine is off).

I read the label on the bottle of Bar's product for head gasket sealing (don't remember for sure if it was called liquid copper or not...but sounds right), give it a good reading.....they tell you to flush all coolant out of the system.....only pure water......put in the product......run....then drain completely....and wait a long time.

If you had steam from the radiator....and you are sure that it was not from running down from the radiator cap.....or the hose connection from the overflow bottle....then I'll bet that you found your problem.
I read that there can be an issue where the plastic tank to radiator connection can leak......does not take much of a leak to cause the problems you have.

You will notice it with the radiator cap ON......such that it builds up pressure.
And....if it is such that you can get a little air into the system.....it can take your overflow bottle right out of the circuit.......so you can be low on coolant....with a full overflow bottle......

So.....I would fill things up.....overflow bottle also.....and put the radiator cap on.....
Then start the engine....and let it idle......keep a really good, constant, watch on the temperature gauge....and shut it OFF if it gets up to HOT......so that you do not damage the engine........
Then, watch what happens.....now....if it has shot coolant out as you said.....you can expect steam to come up if the radiator becomes warm.....as it will be just drying off.......
Once the temperature is up into the warm, normal area...........It will drop down some when the thermostat opens......
But....once things are warmed up....and there is pressure in the system.....you can shut the engine down.....and then look for any signs of leakage......steam rising that has a hiss to go with it......is a sign of leakage.