More on Volvo MD2030 heat exchanger

[Marc S]

I’m assuming the problem centers on the heat exchanger. Here are the symptoms:
1. Raw water has apparently totally replaced the fresh water (antifreeze) in the system.
2. At temperature water flows in a steady stream through the pressure overflow tube.
3. Much corrosion near the exhaust riser elbow.
4. Engine does not overheat and exhaust water flow is good.

I assume that raw and fresh water mingling can only happen with a bad heat exchange element(?). Is there any chance that a bad thermostat could be responsible?