Why do tankless on demand natural gas water heaters such as Certain water heaters not have a stable shower water temperature?

My experience with gas tankless water heaters is that they are quite stable when the temperature is not set too high.

Most people are familiar with tank storage water heaters, and the behavior there is to turn up the temperature in the tank and mix more cold water in at the fixture to moderate the temperature. The higher temperature in the tank reduces the amount of hot water used per minute, which extends the usage of hot water in common residential usage.

When one uses this same strategy with a tankless water heater it often backfires because of how a tankless water heater works. As other answers have pointed out, a tankless water heater is highly dependent on the water flow through the unit, and there is a minimum flow rate below which it will just shut off. A typical showerhead in the US is 1.5 - 2.0 gallons/minute (gpm) (6 - 8 liters/minute, lpm), and most tankless water heaters require a minimum flow rate of 0.5 gpm (2 lpm). If too much cold water is mixed in at the shower valve to moderate the temperature, the flow rate through the heater can fall below the necessary flow rate causing the water heater to shut off. If there are fluctuations in water pressure (as are common with a private well system) and the flow rate fluctuates around the activation rate, the water heater will cycle on and off as the pressure (and flow rate through the heater) goes up and down.

• The solution is to set the temperature on the tankless no higher than 120°F (49°C). You want to maximize the flow rate through the tankless water heater to ensure it remains in continuous operation while in use.

The default temperature of a tank water heater is 120°F (49°C). I commonly see them turned up to 130°F (54°C) or higher. Those higher temperatures are dangerously high and can cause nearly instant scalding. In other words, there's no reason for a person to use a higher water temperature than 120°F.

Another possibility the questioner may be referring to is the modulation of the tankless. Lower quality (less expensive) tankless units have fewer, larger modulation steps, so the difference in power output from one step to the next can cause small temperature fluctuations as the burner goes from too much to not enough because the flow rate is on the edge. This is usually a problem when the flow rate through the unit is minimal (<1 gpm/4 lpm). Again, the solution is to increase the flow rate through the unit by reducing the set temperature on the heater.

TL;DR If you keep the set temperature on the tankless water heater at or below 120°F/49°C, that should address the issue. Or buy a higher-quality/more expensive tankless unit that has a lower activation rate and more, smaller burner modulation steps (and keep the temperature set at or below 120°F).

Most instantaneous water heaters are quite reliable. It sounds like something has gone wrong. It could be that the wrong specification was chosen, the installation was faulty, or the temperature setting is incorrect. Alternatively, the “anti-scald device” on the shower diverter might be malfunctioning, or there could be another cause!