Finally, I got the Eberspächer 6kW electric heater installed and up and running! It sure is a beast that needs to be tamed :) With my current water flow rate it starts boiling the water with anything higher than 2.5kW. When the blower of the car is off, bubbles even start circulating the system with 1.5kW. I equipped my GEVCU-Extension with a SW-CAN transceiver. It communicates via single wire CAN bus with the heater and controls its power output from 0 to 6000 Watts. The module of course can be configured to a desired maximum power output. It also measures the temperature of the water via a NTC resistor built-in the heater (via A0 of GEVCU transmitted via CAN to GEVCU Extension).
With the current settings, it heats with 1.5kW until the water reaches 40°C. Then it de-rates the power to 0 Watt at 70°C. This reduces the bubbling to a minimum.
I'll need to hook-up the already installed flow meter to get an idea of how many liters per minute travel through the system.
If you're interested in the code, you'll find it at GitHub. (Note that not all Eberspächer heater use SW-CAN. Many operate PWM controlled).
After 4 weeks driving at freezing temperatures (inside the car), windshields that fog up and even start to freeze over while driving, feet that won't warm up again and catching one cold after another, I'm almost looking forward to the Monday morning commute where I can hop in a pre-heated and de-iced car.
PS: Of course I didn't drive with iced/fogged windows.. I stopped and used a 12V 120 Watt heater and wasted a lot of time pointing the cheesy air flow at the windshield to get clear vision.