AKA the Overflow Tank. Does that help?

There is one other common problem with these vehicles - Older bottom
(suction) hoses often collapse if there is even a small restriction in
the radiator (or an air-lock) - which would cause overheating. Some
such hoses come with wire springs inside to prevent this problem. So,
check all your hoses and if any of them are soft or distorted, replace
them.

You can also check for proper circulation by leaving the overflow tank
cap off and watching for the coolant to move through it. START COLD
(don't open when hot) unless you want a face-full of scalding water.
But as the thermostat opens, you should see movement.

One thing about VWs, most all of them from their very first water-
cooled beasts to the present day do require "Burping" if the coolant
is drained or is lost. So dave is dead on-point with his advice.

Peter Wieck
Melrose Park, PA

Yes I had to bleed the air out of the heater core of a 1999 Passat 1.8t last
week.  I flushed the heater core and backflushed it also.  There was no heat
before, but after I filled it up and allowed the air to escape out of the
heater hose bleed hole until coolant shot out.....there was heat!!!  <g>
I had never seen a bleed hole on a hose before except on some Porsche 924s.
But it worked.  ;-)

Like Peter Wieck you have to get that air out of the bottle (I call it the
coolant expansion tank) or whatever you call it.  Burping the coolant system
usually always has to be done.  Removing the cap or removing the highest
hose to allow the air to escape usually works for me.  Also having patience
as I squeeze the upper radiator hose to help the air escape.