Rusty water from Rinnai Tankless Water Heater

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Hi Everyone,
I need help diagnosing rusty water coming out of my new Rinnai Tankless Water Heater. Here's the info and "symptoms".
- I have 2 units RL94IN (indoor) and RL94EN (outdoor), installed about 1 month ago.
- When I filled my bath tub today, suddenly light brown water came out. It continued on for a few minutes, then became clear again. However when I drained the "seemingly clear" water, it left brown residue on the tub bottom (see pic). It is a very fine brown powder and it was kinda "dissolved" when I mixed the water.
- The brown residue appeared in both hot and cold water. However there is more brown residue in hot water than cold.
- I checked my second bathroom, which is connected to the other Rinnai. SAME issue.
- We use water frequently during the day. No long period of no water usage.
- We use the brass hot and cold valve kit that came together with the unit. Brass shouldnt rust (see pic of valves and all fittings/connections).
- My neighbor has no issue, meaning it is not due to the water source.
- It is unlikely that the parts inside the water heater can rust. Rinnai would be bombarded with complaints if that is the case lol.
- I just replaced my water supply with copper pipes. (I am in a middle of home remodeling)

I found this thread from another person back in 2012 who had the same issue. However he only had rusty water from the hot water. He found out that Dielectric Fittings was the cause and once he replaced it with the brass valve kit from the manufacturer, the problem went away. I am already using the brass valve kit from Rinnai.

What could be the cause then?

Thank you so much for your help. I really appreciate it.


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Peace valley missouri
Rusty water out of both hot and cold do have a pressure reducing valve? Do have a hose bib before the prv test water there. Compare before and after.


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Ontario, Canada
I just replaced my water supply with copper pipes. (I am in a middle of home remodeling)
  • City water or private well?
  • What was the original pipe material that was replaced with copper?
  • Are the new copper plumbing lines now larger than they were previously?
  • Were globe valves replaced with less restrictive ball valves?
If the new copper lines are larger ID than the previous lines, the maximum flow rate will be likely greater. A bath tub faucet is usually the highest flowing faucet in a home, and a tub spout is often not equipped with an aerator screen which will usually collect solid sediment and debris.

If the new plumbing is now allowing a higher low rate, debris and sediment that may have collected in the home's feed line from the city or well could become dislodged and carried to your fixtures. Suggest shutting off the gas supply to each tankless unit and flushing the home plumbing lines by opening all faucets at the same time for several minutes. This should dislodge and flush out any possible remaining debris from within the feed line. After several minutes of clear flow, remove and clean each faucet aerator and any inlet screens to each tankless unit.
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Hey, wait a minute.

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