[Andrei_ierdnA]

I have a suggestion if you cannot get to the bottom of this dilemma:

* sell your large 1kW block heater and with that money buy a small block heater 125-250 W.

So instead of heating your coolant in 20 min and having a localized boiling point, you will have to plug into a timer and let it heat the coolant for several hours overnight. Same end result, same electricity cost, but with a lower and more gradual heating of the coolant and without any localized boiling.