re: Weird plumbing issue in brand new house with tankless water heater

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Literally sometimes the water will be lukewarm and stay that way until I turn it off then right back on. Then it's steaming hot. I thought this might be a quirk of tankless water heaters.

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The tankless heater fires up when it detects water movement/ flow. I have two of them and sometimes during very cold outside temperatures mine do what you’re describing. If i open and close the faucet once or twice, it picks up the flow and water gets appropriately hot. Doesn’t usually happen unless temperatures are in low thirties or below. My tankless units are in the attic.

I’ve replaced one and both the older unit (10 years) and newer unit (2 years) have done it at times.