Prusa i3 XL: heatbed leveling issue

Hi Guys!

Again I've got a problem with my Prusa i3 with bigger heat bed (200x300mm, thus it's called XL). I have a 12V PCB heatbed with 3mm borosilicate glass on top. Even though the PCB is 12V I run it from 24V with 2/3 cycle setup in Marlin. I use it this way because the resistance of the bed on 12V pads is incorrect and with regular 12V PSU I wasn't able to reach 100C (even 24V need 30 mins to reach this temperature). Anyway the bed is heating this way and this is my setup. I attach a photo so you can see it:



I have a problem with leveling the bed: I'm calibrating it when the bed is at 100C and nozzle 220C. These are my normal printing conditions for ABS. If I level the corners of the bed, the center of the bed is higher than the corners and I cannot print anything in the middle of the bed. In the attached photo you can see red lines on the bed where the nozzle travels without any problems and even if parts are heated I can put A4 paper sheet between the nozzle and the heatbed, but If I try to do the same in the center of the bed (yellow cross) the nozzle seems to be too close to the bed.

Could you please help me answering why it happens this way? Is it because the bed is damaged? The PCB doesn't heat evenly and there may be 10-15 degree difference between the center of the bed and the edges. The thermistor is installed near the edge, because there were pads to solder it.

Sounds like the heat bed is warped.
What is it like if you put a steel ruler on edge across the bed? Does it show that it's warped?

Yes, of course - when the heatbed is warmed to 100C the blue line in the below picture is raised against the corners.



For me it looks like the area around the blue line is heating more than the rest of the bed.

Is it perfectly flat when cool or at room temp?
What about at 70c?

Yes, it's perfectly flat when in a cool room (2C) and in room temp (25C). For me, it looks like the heat distribution in PCB and in the borosilicate glass is poor and thus when the bed heats up in the center more than in the corners the heat is not distributed evenly. I thought of applying a thin sheet of aluminum (2mm) between the bed and the glass. This could help me at least to check if it isn't a matter of heat distribution. As I mentioned my thermistor is located at the edge of the bed, next to the power supply pads of the heatbed, so it shows me the temperature from that region. I've measured that when the thermistor is reaching 75-77C the center of the bed reaches 110-115C. Is it normal?