[Tapani]

Did you get your system properly cleaned? If not.....

What ever it is (probably calcium based stuff from tap water) you have it everywhere in the system.

All your engine water jackets have it. Your heater core has it.

Naturally it will have a substancial effect on heat transfer from engine castings to the coolant and from coolant to the heater core to the cabin air.

The deposits in the water jackets will make your engine parts run hot w/o you noticing it (only coolant temp is controlled).

/T