12-29-2010, 06:31 AM
Many years ago I started having a similar issue. The pipes were pex. The first culprit was thought to be the older water heater. Replacing it did reduce the problem a lot. But it reappeared months later and after some investigation, it turned out that in the massive array of piping for the domestic water, heat exchanger, water heater, etc., two things were found. One was a defective backflow preventer and a piece of iron pipe that dead ended.
The BFP allowed some reverse siphoning of the yard plumbing which could have sucked back some dirt. One of the landscape lines feed an area with flood irrigation, it was underwater occasionally for a day or two. The iron pipe appeared to have been where a pressure gauge was attached to the plumbing but when removed the pipe was left behind and capped. It should have been replaced with a piece of copper or capped at the tee. It looked good on the outside but when removed, the inside was a rusted mess.
Both were replaced at the same time and didn’t have anymore issues but can’t say which one was the real cause.