A homeowner shared a baffling water heater problem: a shower that turned lukewarm right after a plumber got it to produce scalding hot water, proving everything was working.
What happened?
In a post on Reddit's r/Plumbing, a user said that their Rinnai RUR199iN gas tankless water heater seemed to cooperate only when a plumber was standing there to test it.
"I don't get hot water when the plumber isn't here," they wrote. After working properly when the homeowner moved into a newly built house in 2020, the unit later developed problems. They noted that a water softener was added in 2021, with the first trouble appearing in 2024.
The homeowner said repair efforts had included cleaning the filters, condensate trap, and air screen as well as replacing the thermistor, PCB board, and mixing valve multiple times.
Even so, the problem apparently kept recurring. The plumber got the unit running "scalding hot," but "three hours later, I try it and we're back to lukewarm."
Why does it matter?
Reliable hot water is a basic household need, and repeated service appointments, part replacements, and no lasting solution can drive up both costs and stress.
Some homeowners are moving away from combustion-based appliances when it comes time for replacements. Heat pump water heaters can reduce household pollution, avoid some of the complexity associated with gas combustion systems, and lower monthly utility costs.
What can I do?
If your water heater behaves erratically, documenting the timing, changes, and usage patterns can help a technician narrow down the cause. Homeowners can also ask a licensed pro to check recirculation settings, sensors, flow rates, and warranty coverage before sinking money into repairs.
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For people ready to compare options, Cala offers customizable smart heat pump water heaters that help homeowners reduce their energy bills by heating water only when needed. That means the system can better match a household's routine, rather than constantly working harder than necessary. It also gives homeowners another way to think about comfort: not just whether the water gets hot, but whether it does so efficiently and predictably.
If repeated fixes on a gas unit still don't solve the problem, compare long-term operating costs, incentives, and maintenance needs to see if replacement is the smarter move. Smart heat pump models can be especially appealing for households seeking more control over when and how water is heated, and Cala is one company that uses this approach.
"I'm on my last straw," the original poster wrote. In conclusion, they asked a question that likely sounds familiar to plenty of homeowners who have dealt with stubborn appliances: "Has anyone run into this or have any ideas or even a whisper of an idea?"
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