Do you have orifice or wedge? I’m confused by your last post.
If you have orifice plates,
+ continuous purging the taps
+ orienting the taps at 2 and 10 o’clock (free draining)
+ eccentric orifice plates
Assymetry, seems possible. Coke fines could preferentially build up in 1 leg. Can’t say that I have seen this before but I think almost all the heaters I have experience with have symmetric feed pass piping.