Why does my new tankless heater alternate between hot and cold?

MGI’s recommendation: Ask the installing contractor to return and demonstrate steady hot water under the conditions that are failing. A new installation deserves a completed performance check, rather than an explanation that every temperature swing is normal.

A recirculation system uses a pump to move water through a return path so hot water reaches distant fixtures sooner. The actual return arrangement and controls must fit the installed heater. Professional circulation explanation.

Start by describing the pattern. A brief change after you stop and restart a draw is different from repeated cold periods while the shower runs continuously. Also note whether the problem appears at one fixture or several, with one person using water or during simultaneous use. That distinction gives the installer a specific test to perform.

Navien’s NPE-2 user manual lists hot-water flow, inlet-filter condition, error codes, and an external recirculation system among checks for water that turns cold. These are conditional clues for its listed A2 and S2 models, not a diagnosis of yours. The installer also needs the applicable installation instructions. Navien NPE-2 troubleshooting guidance

Ask for a visit that recreates the actual complaint. The work record should show the flow through the heater, its outlet temperature during the swing, and the temperature at the affected fixture. Request a check of the water connections, supply pressure under use, approved settings, and any mixing or recirculation arrangement. For a gas unit, the qualified installer should also verify the fuel supply and combustion installation against the exact model’s instructions. You should not adjust those items yourself.

Here is why those measurements matter: if the outlet stays steady while the shower changes, the investigation needs to follow the water path to that fixture. If the heater itself stops or its outlet changes, the technician needs to establish why. Neither observation alone justifies replacing a control board or adding a larger gas line.

Photograph the model label and any displayed code. Note the time to the first cold period, which taps were running, and whether the burner indicator changed. Avoid making several settings changes before the callback, since that makes comparison harder.

While waiting, use an already reliable fixture if available, and check the temperature before bathing. Do not leave someone in a shower with unpredictable hot and cold swings.

Ask the installer for the findings, the correction, and a repeat test using the same conditions. If fixture or plumbing work falls to another provider, have the quote name that owner and explain how the final performance check will be coordinated.