Heat bed sagging

[printbus]

How much is the sag?

I assume you have glass on top of the heater. Have you tried flipping the glass over to see if the sag remains or turns into a hump?

Keep in mind that what we're dealing with here is the distance between the nozzle and the print surface. A variance could be in the print surface, or in the vertical location of the nozzle as the carriage moves in X and Y. What has been done to rule out a height variation in X Y movements of the carriage? This can be tough to do, but people ALWAYS assume the print bed is the problem (and maybe it usually is), but it can be more than that.