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.

Rennai tanlkless heater - water becomes cold after running hot and then becomes hot again

elp Member Posts: 1
We have the rennai topline tankless water heater. we like it very much except for the following.
It takes a long time to get hot water (plumber said that is because how long the water has to travel - understandable). Next, we noticed that in kitchen faucet, after hot water is running for a while (say 3-5 min), it will start to get cold for no reason - no other faucets in the house are open. It will become completely cold and after several minutes of that it starts to become hot again. As we checked other faucets, this is happening in kitchen and in master bath (one of the two showers). Plumber can not figure out and we re not sure why this is happening.
Any ideas? Thanks for your help.


  • icesailor
    icesailor Member Posts: 7,265
    If it worked well when it was new, and now is doing what you describe, your "plumber" needs to get certified in Rinnai and other instantaneous water heaters and how to service them.

    Sounds like it needs a good cleaning and you have funky water.

    A Buffer tank is a Band-Aid on an infection. If you are using water and it is hot while you are continuously using it, and it goes cold, the unit is overheating and having to shut off, cool down, and restart again. That's how it was designed to safely work. That's not how it works in "normal" operation.

    The intent is that once flow is initiated, the water flow switch closes and initiates the burner to start. It is supposed to heat water to the setting of the controller for 60 seconds/ one hour, 24/7/365 or as long as required. When the water stops flowing, the flow switch stops the burner, until the next call.