add more coolant. ill literally bet you a 6 pack on this. paypal ready...
And another 6-pack says you are low on coolant as well.
The clear coolant bottle, often incorrectly called an overflow bottle but is actually a more sophisticated degas bottle, located on the passenger side of the engine compartment. Be careful taking the cap off, it is part of the pressurized cooling loop. Fill it to the max line, maybe a little more. Go drive it. No fancy burping needed, keep the level at the max line and drive it is all that is needed.
When you run a little low on water you get a bubble at the highest point in the cooling system, the top of the heater core. At idle the waterpump isn't spinning very fast and can't push through this bubble of air. By filling it with coolant and driving around you work the bubble out. The degas bottle does exactly what it's name says. Remove gas bubbles from the cooling system. but it has to be full to work, or else it allows gas to enter the cooling system.
You probably didn't even notice I never told you to add to the radiator. Simply put that is the wrong place to add coolant to this engine. That is another high point. the radiator cap is just a cap, not a pressure cap. It is also a high point in the cooling system. Take the cap off and the coolant seeks it's own level, and you just put a big air bubble in the cooling system. Don't do that. It is only used when filling an empty system, not for top offs.
So add some coolant and drive it around. Check it a few times and make sure it fills up and stays full. You may have to add it a few times to get it to stabilize and get all the bubbles out. After that keep an eye out for the level and just keep it filled and the heat will work just fine.
ccasion5:. I'll top her off tomorrow morning. My VW had a pressurized reservoir i didn't know the X had one too, so that's good to know. IDK why but my mind always goes for the worst case scenario the water pump in this case. That type "trouble shooting" has cost me a lot of money and time*. 
