Problem one, if you remove the heat exchanger, you can clean it out with kettle descaler, you will also be able to check the condition of the tubes.
Problem two, check the thermostat isn't stuck.
The two systems are separate and from what you have said I think you may have a head gasket problem. Easy check is to remove the pressure cap and fill the header tank, start the engine and if you have air bubbles coming up its more than likely it's head gasket.
God luck with this.
Born to cruise...........once forced to work!