Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Water is hot, then cold for a couple minutes, then hot again

So the weirdest thing. When my wife turns on the faucet in her bathroom, the water is initially warm/hot (it might take some time to heat up in the morning), then it turns cold for 2-3 minutes, then it turns hot again and stays that way. This also happens in the shower in the same bathroom from time to time, but it's not as bad as the faucet situation. This affects exclusively her bathroom and does not happen to any other bathroom/faucet/shower in the house.

We have a Rinnai RUR160in tankless water heater. Previously, a thermal bypass valve was installed under her sink and I thought it was causing some weird feedback issues, or was straight out broken. So a couple days ago I went ahead and removed the bypass valve and also installed a new faucet for good measure. That did not help. I also know that there is an anti-scalding valve installed in the shower, and from what I read a broken anti-scalding valve can cause similar symptoms, but I'm not sure if this would affect the faucet in the sink.

My original theory was that recirculation was broken in some way, so I turned it off completely in the heater. That did not help, other than the fact that now we have to wait a minute or two for the water to turn hot when we turn it on first thing in the morning. (That's how I know that I did, indeed, turn the recirculation off.) So then I thought the thermal bypass valve was defective, so I got rid of that too (planning to install a new one later). I was so sure that's what was causing the problem that now I'm at a loss. Any ideas what could cause this weird problem?

Thanks in advance.


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,294
    Hi, I think a bit more pinpointing can be done. When she draws water, you keep an eye on the heater and see if it fires continuously or not. If not, it’s possible flow isn’t activating the heater properly.
    Yours, Larry
  • AMservices
    AMservices Member Posts: 610
    Every Rinnai I've installed over the last 4 years, has had a different, but common problem.
    Sometimes the water pressure sensor is gummed up and the boiler isn't reading pressure right, or the flow switch is gummed up and not sensing flow.
    The gum is what looks to be this really greasy dope used in the factory.
    Those are problems I would see on start ups, but I think your issue lies in the thermostatic mixing valve inside that unit. sounds like the element in that valve might be sticking then stars hunting for the right temperature.
  • gregsparrow
    gregsparrow Member Posts: 2
    Thanks for your responses, I'll do some testing to determine whether the heater fires continuously when the faucet is open. For the thermostatic mixing valve, is there any diagnostics I could run to determine that this is the culprit, or is my only option to call the installer?

    Above all else, I'm so confused as to why this would only affect just one bathroom. We literally never ran into this issue elsewhere. Could piping layout also be part of the problem?
  • AMservices
    AMservices Member Posts: 610
    "Above all else, I'm so confused as to why this would only affect just one bathroom. We literally never ran into this issue elsewhere. Could piping layout also be part of the problem?"

    That makes me think it's more of a flow switch problem.
    If you don't have enough flow, the burner doesn't stay on.
    Let us know what you see When testing.
  • SeanBeans
    SeanBeans Member Posts: 520
    Does that one have a built in buffer tank?
  • Intplm.
    Intplm. Member Posts: 1,949
    The problem is just in that one bathroom? Are you sure you have verified this by checking yourself or have you taken this from the word of others.
    This is very rare that this is happening as you describe in just one bathroom. This as you describe should show up at all points of use if it was just your water heater. Could be your mixing valve is malfunctioning.

    You are certainly right that shower valves will do this. But only at the shower valve.

    I would check if you have a cross connection that is causing this. This type of thing has shown up when a garden hose, or faucet hose on a utility sink has a temporary screw on type of valve or other device that can make this happen. Hose connections can cause this type of ghosting due to pressure fluctuations with the hot to cold, cold to hot. This can show up in one area but not migrate to another area.

    I would check for something as I've described.
    Think about what type of changes have taken place in the home. New construction or renovation etc. ? Seems you are on the right track.

    Retrace your steps and make a list of what you have done. Check off what has been added or eliminated.