# Instantaneous water heater

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**Performance**

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An instantaneous water heater, also known as a tankless water heater, is a popular choice for homeowners looking to save on energy costs and space in their home. These water heaters heat water as it flows through the unit, rather than storing and heating a large tank of water.

When considering purchasing an instantaneous water heater, it is important to calculate the unit’s flow rate, or the amount of hot water it can produce per minute. This can vary depending on factors such as the size of the unit, the inlet water temperature, and the desired output water temperature.

To calculate the flow rate of an instantaneous water heater, use the following formula:

Flow Rate (gallons per minute) = (Temperature Rise (°F) x Gallons per Minute per °F Rise) / (1.1 x Efficiency)

Temperature Rise is the difference between the inlet water temperature and the desired output water temperature. Gallons per Minute per °F Rise is a factor specific to the unit and can typically be found in the product specifications. Efficiency is the percentage of energy used by the unit that is converted to heat, and is also included in the product specifications.

For example, if the inlet water temperature is 60°F, the desired output water temperature is 120°F, the Gallons per Minute per °F Rise is 4, and the Efficiency is 0.82, the flow rate would be:

(120 – 60) x 4 / (1.1 x 0.82) = 4.87 gallons per minute

It is important to note that this calculation is based on ideal conditions and may not accurately reflect real-world usage. Additionally, it’s better to consult with a professional before taking any action.

In conclusion, calculating the flow rate of an instantaneous water heater is an important step in determining the appropriate unit size and ensuring that hot water needs are met. By using the formula above and consulting product specifications, homeowners can make an informed decision when purchasing a tankless water heater.