Heater doesn't turn off

Heater cores are always "on," in that coolant is always flowing through them. You can block off the coolant hose and eliminate it, keep in mind that will affect fluid capacity so the marks one the reservoir will no longer be relevant. I would not install a valve, only because if it failed it would lead to a sudden loss of coolant. Just install a block off plate where the heater hose connects to the engine. Otherwise, Jetta forums may have some shortcuts and tips on how to replace the blend door, or it's actuator.

Thanks. I don't know how many other brands do this today, but even my 2003 Honda Accord has a valve in the engine compartment. Water is always in the system, of course, but does not flow if that valve is closed. I can't understand why the system would be engineered to have hot water flowing through the heater core when no heat is needed, but that's a design topic, not one for here. Re the block off plate, we would then need to block the return as well, unless I'm misunderstanding block off plate. A valve in the incoming water hose would not affect how much water was in the system and would provide an option to turn the heat back on. Cutting out the heating system entirely with the plate you've suggested would affect the volume of water in the system, but why would this affect the reservoir reading? That should be based on the level of coolant not the absolute amount. Did I miss something there?

My mistake, I do see VWs have that valve. Can you retrofit one to your vehicle if it didn't come with it? Seems straighforward

i'm pretty sure it does not or I would have expected to see people suggesting that as a temporary cure for the vent door problem... but now I want to look. Unfortunately the car is 1,000 miles away.... Yes, cutting in a manual valve in the incoming water hose seems straight forward.

I found a schematic of the cooling system for this Jetta and it does not show any valve between the engine and heater core. I guess it does indeed flow all the time - pretty strange in my view to bring hot water into the cabin when you don't need heat (and are probably running the AC). But it is what it is. The schematic also shows components downstream from the heater core including perhaps some flow back into the block itself. This was my original concern. I would not cutoff this flow for fear of causing problems in those downstream components. The only option would be to bypass the heater core, connecting the inflow and outflow hoses in the engine compartment. I'm not prepared to do that level of changes. It would be a lot more than just cutting in a valve and would make it doubly hard to then restore heat at a later time. For now, I'm marking this as not a good solution for the problem. It does explain why this idea hasn't surfaced in any of the threads I've seen about the airflow door problem in Jettas. Thanks for the input.