By Joecus-- Cause of rusted heater?

Hello Everyone - I've read these old post and need some help. Last year and this year, I noticed the white PVC water outlets and inlets turned brownish color. I did some research and put some of the "metal out" chemical in my pool. Within a few minutes, the brown turned to white and I thought my problem was solved. Everything was going fine for a few weeks and then I went away for two days. I did not run the filter while I was away. When I returned, I turned my filter on, and brown water shot out of the filter outlets. It appeared to be rusted water along with little brown particles.. Now, all my white PVC outlets are brown again. I have a Hayward gas heater, in ground 10,000 gal pool with vinyl. Both 5 years old. I had a low PH issue last year that went un-addresed for quite a while until I found this forum. Please let me know your thoughts on what to do. I cannot figure out how I can get rust with a pool heater that should be copper. Any help, please.