- 1). Determine the busiest time of the day for your hot water usage.
- 2). Determine how much hot water you need at the busiest time of the day. The flow rates are listed in the owners manuals for the water devices throughout the home. Most showers use about 2.5 gallons per minute and most faucets use about 0.75 gallons per minute.
- 3). Total the different usage amount of water during the busiest time of day. For example, if one person is taking a shower and another person is brushing her teeth at the same time, the total water usage would be 3.25 gallons per minute. This would be the minimum size for your tankless water heater.
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