Volkswagen Repair: No heat - 95 Passat, passat glx, money pit

Question
Heater in my daughters '95 Passat GLX was working fine. When I was checking her oil I noticed the antifreeze level was very low in the ball. I added enough to bring it up to the "full" level while the car was running. Shortly thereafter I realized there was only cold air coming out of the heater although plenty of heat was coming out just an hour earlier. The heater was on and set to hot while I added the antifreeze. That was a few days ago and still no heat. The antifreeze level is where it should be and the car does not overheat. The hose going into the firewall gets very hot but the hose coming out stays cold. What happened? Is there an easy fix to this, like a sensor or switch, without spending another $800. This car has been a money pit. This is Minnesota and it's February 1st!!!

Answer
Unfortunately, you did not state how low was very low,

Why?  Because(if you put in nearly a gallon, the system was empty), and in putting in coollant for an nearly empty system - the coolant is just not put in the overflow tank---it must be done thru the upper rad hose, to pour coolant into the block(the otherside of the thermostat). The thermostat is closed and if there is no coolant on one side of it,  it will not open... as the air pocket there does not heat up enough to open it...resulting in no circulation of coolant.  The engine is not overheating is probably due to the cold air temp outside, and the distance driven.  I could drive my diesels that way in the winter for 10 miles where I stopped, went to work and then went home.
  Blame a possible a possible air pocket first and pull off the rad hose from the rad and pour in coolant(be sure the engine is "cold"-cold otherwise you will warp the head....I dun that, buy putting in what I determined was hot water out of the kitchen tap(not hot enuff)on what I determined to be a warm engine.  

The thermostat has been known to fail, but try doing the coolant thingy first.  You may have to go the thermostat route but this is far easier so try it and no--- it doesn't fill in from some backway cause I was hopin on that too.  How soon will you know?  As the engine heats up there should be a change.
 The fact the one hose was warm also happened with me. And you can get the thermostat from Wal-Mart.
 Be careful on the install and again do the fill first from the rad hose and then in the overflow tank.