The FVI12 has an over temperature sensor to prevent the water heater from running at excessive temperatures. While in operation, if the outgoing water or internal components achieve excessive temperatures, the over temperature sensor will automatically shut down operation and the water heater will display an E5 error code.
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This is typically resolved by using the knob on the front of your water heater to decrease the amount of gas being used and also decreasing the water temperature knob.
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Overheating is also directly related to an inadequate amount of water flow to absorb the heat being generated. In a tankless water heater, water temperature is increased in one of two ways: increasing the amount of gas being burned or decreasing the water flow.
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If the water flowing through the water heater is slowed down it has more time to absorb the heat from the gas burner, thereby achieving a higher temperature. The water temperature knob functions by restricting the flow of the incoming water to increase water temperature.
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If you look at the possible settings of the water temperature knob you will notice a series of blue marks. By setting the knob at the smaller marks you increase the amount of water flow and the temperature will decrease. By setting the knob at the larger marks you decrease the amount of water flow and the temperature will increase.
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Water flow can easily be affected by the size of your pipes, the amount of water your faucets use, or blockages in the water lines or filter screens located inside of your faucets. If you only have an issue with overheating at one faucet, you will need to check to see if it has a dirty filter screen and remove any efficiency items which may have been installed inside of it from factory.
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Lastly, a faucet that does not have a separate hot and cold knobs can create issues with tankless water heaters which do not self-adjust based upon water flow. The reason for this is that there is no way to only add cold water. When you attempt to add cold water, you are also reducing the demand for hot water, so the temperature of the water heater will increase. This can also happen with faucets that have separate hot and cold knobs when you attempt to mix cold water. If this occurs consistently, you may need to set water temperature using the water heater controls and only using hot water at that fixture.
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The FVI12 is manually controlled, meaning that it always uses the exact amount of gas and reduces water flow by the amount the knobs are set to. Because it operates in this manner, a faucet with low water flow will run at a higher temperate than one with a higher volume of water flow. The FVI12 cannot be set to maintain an exact temperature regardless of which fixture you use. If your water flow varies between fixtures, or if it is changed while the water heater is in operation, the temperature will change accordingly.