Why does my tankless water turn cold when I barely open the tap?

MGI’s recommendation: Have the hot-water flow checked before replacing the heater. A small draw may be too small for that model to keep heating. The useful fix is one that lets your normal sink or shower work reliably, without making you waste water.

Rinnai’s homeowner FAQ explains that its tankless gas heaters require enough hot-side flow to start and remain in operation. Falling below the required flow can stop heating. Have the technician verify the actual requirements for your model, rather than applying a generic threshold. Rinnai tankless flow FAQ

Notice what happens during ordinary use. Does a steady, fuller hot-water draw stay warm? Does the problem occur only at one tap, or throughout the home? Does the burner indicator stop as the water turns cold? Record any code. These observations help the technician reproduce the problem; they do not prove which part is bad.

A model-specific assessment should separate three questions:

• Is the actual flow through the heater below its operating range?
• Is a fixture, inlet filter, or water-side restriction limiting that flow?
• Does the heater stop despite having enough flow, indicating a different fault?

Ask the qualified tankless technician for the measured flow at the point of failure, the model’s documented requirement, and the reason for the proposed correction. If the restriction belongs to a fixture or the home’s plumbing, identify the qualified plumbing provider who will handle that work.

A lower approved temperature setting may be worth discussing if you currently mix a lot of cold water into very hot water. The provider should explain the effect on usable hot-side flow and check the home’s temperature and mixing arrangement. Avoid changing installer parameters or defeating a safety control.

Until the visit, use the fixture at a stable setting that already works, if one exists. Check the water before bathing, and pause use if it swings to unexpectedly hot temperatures. Running another faucet indefinitely is an observation, not the lasting solution we would choose.

Send the model label, the affected fixture, your usual temperature setting, and a short description of when the water changes. Confirm that the appointment covers that exact tankless model and a flow diagnosis.