Sink faucet: 1 gpm Bathtub: 3 gpm Shower: 2. 5 gpm Dishwasher: 3 gpm Washing machine: 3 gpm
Let’s say your peak usage happens after dinner when you’re running the dishwasher, washing a load of clothes, and taking a shower simultaneously: 3 gpm + 3 gpm + 2. 5 gpm = 8. 5 gpm. Your tankless water heater needs a minimum 8. 5 gpm flow rate. That way, your heater’s minimum flow rate can easily support your peak usage.
It’s important to know the water temperature in your house, also known as the ground water temperature, because that’s the temperature the water will be before the heater starts working. The lower the ground water temperature, the more power the heater will need to use get the water hot.
Let’s say your cold water temperature is 62 °F (17 °C): 110 - 62 = 48. The required temperature rise for your tank is 48 °F (9 °C). The new water heater will need to heat your water 48 °F (9 °C) in order to reach 110 °F (43 °C).
For example, if your peak flow rate is 8 gpm, buy a tank that’s 8 gpm or higher. 10 gpm would be a good choice. Try to stay reasonably close to your gpm; there’s no benefit to vastly overshooting your needs. If your peak flow rate is 8 gpm, you don’t need a heater that can support 15 gpm. The higher the flow rate, the more expensive the device will be.
Gas-powered heaters are usually a bit more powerful. If your minimum flow rate is on the high side (8 GPM or higher), gas may be better a better choice. If you go with an electric tankless water heater, you’ll also need to know how much electricity is required to support your flow rate (measured in kW). Use this handy chart to figure out your kW needs.
