what is this rustic water come from--New tankless water heater

[wassermeister]

Please see the picture in the begining post. There are two water outlet, one is right before the heater(at the cold water line) and the another is right after the heater (at the hot water line). The water always clean before the heater. The water after the heater will become rust-color if it is not used for a while (may be 12 hours, the more time the more rust-color stuff) even the heater is shutted off ( temperature of the water comes out from heater is not heated, in order to test if the heating will create some changes).

The tankless water heater contains some water that can settle overnight or during the day when you are not at home. It will collect at the outlet of the unit.

[LLigetfa]

I have to wonder if the tankless unit has some sort of galvanic action that is acting as a catalyst to oxidize dissolved iron causing it to precipitate.

[wassermeister]

I have to wonder if the tankless unit has some sort of galvanic action that is acting as a catalyst to oxidize dissolved iron causing it to precipitate.

Interesting thought - the heat exhanger is made of copper....not sure how this would oxidize....I think it is sediment. The water flow stops, it sits for sometime, the sediment settles in the water heater: you start with sediment when the water heater kicks off again.

[yan2hua]

Hi, Every One, Thanks for all the thoughts. Now I think it is not the dediment, since I have another water heater without the problem. And the cold water line is always clean. Please see the attached picture, Will this "Union" create the problem. It is not a brass/copper Union for both ends. Remember the rusty color water only keeps about 10 seconds. Thanks

[master plumber mark]

Hi, Every One, Thanks for all the thoughts. Now I think it is not the dediment, since I have another water heater without the problem. And the cold water line is always clean. Please see the attached picture, Will this "Union" create the problem. It is not a brass/copper Union for both ends. Remember the rusty color water only keeps about 10 seconds. Thanks

yes, they will rust up over time ,
especially if there is a galvonic reaction going on...
they will rust up Big Time on a water heater....

dialectric unions cause more problems than they actually solve

get rid of them and hook the unit up with copper female adaptors
and see if that does not solve the problem

[yan2hua]

Hi, Thanks for everyone ! The problem was gone After replacing the Union with tankless valve kit ( all are mady by brass) The problem was generated by the Union even it does not directly touch the copper pipe, but the water conducted the elelctrolyte-ion migration and the electricity (tankless water has electricity connected)accelerated the electode action.

[Terry]

I'm glad you figured out the problem.
I quit using dielectric unions years ago. Some cities around here like to use a 6" brass nipple between galvanized pipe can copper for that reason. I've found that a galvanized dielectric union takes very little time to close up when attached to copper.