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Why does my tankless water heater go hot, then cold, then hot again?

When a tankless swings hot and cold, it’s almost always one of three things: scale buildup inside the unit, water flow dropping too low to keep the burner lit, or the “cold water sandwich” you get after short back-to-back uses. In Midlands homes, scale is the one we find most.

This goes for Rinnai, Navien, any brand. The physics doesn’t care about the logo.

The usual suspects, ranked

1. Scale buildup. Mineral deposits choke the flow inside the unit and confuse the temperature control, so it hunts: too hot, backs off, too hot again. If your unit has never been descaled, start here.
2. Flow dropping too low. A tankless needs a minimum flow to keep the burner going. Clogged faucet aerators or a dirty inlet filter can let the flow dip mid-shower. Burner cuts out, you get cold, flow recovers, it relights.
3. The cold water sandwich. Someone rinses a dish, hot water sits in the line. Next shower gets that leftover hot, then the cold slug behind it, then steady hot. Brief, mostly normal, worst with short repeated uses.
4. A worn faucet cartridge. A failing single-handle faucet can let cold sneak into the hot line. The telltale: it only happens at that one faucet.
5. A unit that’s too small for two showers and a washer at once. The fix is timing, or honestly, a bigger unit. We’ll tell you which.

What’s safe to try yourself

Clean the aerators on the affected faucets. Run one fixture at full hot and see if it still swings. And pay attention to when it happens: one faucet or every faucet, short uses or mid-shower. That one detail hands us half the diagnosis before we knock.

When to call

It’s happening at every fixture, error codes keep showing up on the display, or the unit hasn’t been descaled in a couple of years. We can usually look at it the same day.

Quick answers