How to Size a Tankless Water Heater

Choosing the right size tankless water heater is crucial for ensuring you have enough hot water to meet your household’s needs. Unlike traditional water heaters that store and heat a set amount of water, tankless models heat water on demand. This means the size of the heater directly impacts how effectively it can meet hot water demand when needed. Use our step-by-step guide to help you size a tankless water heater for your home.

Calculate Household Flow Rate

The first step in proper sizing of a tankless water heater is calculating your household’s total flow rate, measured in gallons per minute (GPM), which is the amount of water you use at peak demand. To calculate your household water flow rate:

1. List all water fixtures and appliances. Make a list of any hot water that could be used simultaneously, such as showers, sinks, and running the dishwasher and washing machine.
2. Determine the GPM for each fixture. For example, a standard showerhead uses about 2.5 GPM, while a faucet may use around 1.5 GPM.
3. Add the GPM rates together. Add up the GPM for all the fixtures that may be used simultaneously. For instance, if you typically run two showers and a dishwasher at the same time, the combined flow rate would be about 7 GPM (2.5 + 2.5 + 2).

Determine Max Temperature Rise

Once you know the household flow rate, the next factor to consider is the temperature rise. This is the difference between the temperature of the incoming cold water and the desired hot water temperature.

• Check your local groundwater temperature. This is the temperature of the water entering your home, and it can vary depending on your location and the season. In colder regions, groundwater temperatures might be as low as 40°F, while in warmer areas, they could be around 60°F.
• Determine your desired hot water temperature. Most households aim for a temperature of 120°F for comfortable use.
• Subtract the groundwater temperature from the desired temperature. For example, if your incoming water is 50°F and you want your water heated to 120°F, you need a tankless water heater that can handle a 70°F temperature rise.

This calculation helps ensure that your heater consistently delivers hot water at the right temperature, even during the colder months when groundwater temperatures are lower. For example, if your household flow rate is 7 GPM and your temperature rise is 70°F, look for a tankless unit that can deliver 7 GPM at that temperature rise.

Get Expert Help With Tankless Water Heater Sizing and Installation

Sizing a tankless water heater can be complex, but with the proper calculations, you can choose the right size system to provide continuous hot water for your home. If you need help with sizing or installing a tankless water heater, the professional plumbing team at Lynch Plumbing is here to assist. Contact us today for expert tankless water heater maintenance installation and repairs in the San Fernando Valley and throughout Los Angeles and Ventura counties.