Determining the right size for a tankless water heater involves considering your hot water needs and usage patterns. Here's how to calculate the appropriate size for your specific situation:
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Calculate Flow Rate (GPM): Start by determining the maximum simultaneous hot water usage in your home. This includes all fixtures and appliances that use hot water simultaneously, such as showers, faucets, dishwashers, and washing machines. Each of these fixtures has a flow rate measured in gallons per minute (GPM).
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Determine Temperature Rise: The temperature rise is the difference between the incoming cold water temperature and the desired hot water temperature. For most residential applications, a hot water temperature of 120°F (49°C) is common. To find the temperature rise, subtract the average groundwater temperature in your area from 120°F (49°C). The average groundwater temperature varies by region and season, so you may need to check with your local utility or use local climate data.
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Calculate Total Demand: Multiply the flow rate (GPM) of each fixture by the temperature rise (ΔT) to find the demand in BTUs (British Thermal Units) per minute. For example, if you have a shower with a flow rate of 2 GPM and a temperature rise of 70°F, the demand would be 140,000 BTUs per minute (2 GPM x 70°F).
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Add Up Total Demand: Add up the total BTU demand for all fixtures and appliances that may be used simultaneously. This gives you the maximum BTU demand at peak usage.
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Select the Appropriate Tankless Unit: When selecting a tankless water heater, look for a unit that can provide the required BTU output to meet your peak demand. Tankless units are typically rated in BTUs per hour (BTU/hr). If the total peak demand is 140,000 BTUs per minute, you would need a tankless unit that can provide at least 140,000 BTU/hr.
It's important to choose a tankless water heater with a BTU rating that matches or exceeds your calculated peak demand to ensure you have a sufficient supply of hot water during periods of high usage.
Keep in mind that while determining your peak demand is essential, you may not always operate all hot water fixtures simultaneously. For most homes, it's advisable to select a tankless unit that can meet about 70-80% of the calculated peak demand to provide a comfortable margin and ensure hot water availability even during occasional simultaneous usage.