Can't Identify Return Line

[jabo1]

I have an older inground pool with spa and inground cleaning system. I am drawing up a diagram for future reference and have identified everything except one return line.

I have five return lines. They are
1. Spa Return
2. Cleaner return
3. Spa Water Feature
4. Skimmer Return
5. ????????

cant identify what number 5. is. Need help!!

 

[Texas Splash]

Can you post a pic? Main drain?

 

[Richard320]

Look around the pool for any mystery ports without a standard eyeball. There may have been a pressure side cleaner at one time.

Or it could be a line to discharge excess water, especially if you have a cartridge type filter.

Ultimately, you may just have to close everything but the unknown valve and turn on the pump and watch the pressure gauge. If it skyrockets, the unknown port is capped. If it stays normal or goes up some but there's plenty of flow in the pump basket and the skimmers, you just have to watch for water movement somewhere. And also watch the water level, if the return flow isn't obvious. You could be emptying the pool.