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then goes back up to a right turn towards the standpipe. It then goes at a slight upward angle to connect to the standpipe.
These bothere me. EVERYTHING should slope towards the sewer. There should be no "goes back up" or ":slight upward angle" anywhere in that line. That's my opinion on the installation. However I may be able to help the backups. Your new washer might have a more powerful pump that's overpowering the drain line. The solution would be to convert to a closed system, ( had to do that myself.) It's easy! Simply connect the washer hose to the standpipe using a compression fitting or a rubber coupling, (see image) or you can just duct tape it shut. I did that to my system and no more backups. Good luck, Tom