Sizing a gas tankless water heater primarily involves determining the required flow rate (measured in Gallons Per Minute or GPM) needed for your household's peak hot water demand and considering the necessary temperature rise based on your climate.
Determining Your Peak GPM Needs
The most crucial step is to estimate how many hot water fixtures (like showers, faucets, dishwashers, washing machines) you might use simultaneously. Proper sizing is important prior to selecting and installing a tankless unit. While specific fixture GPMs vary, general guidelines help estimate the total demand.
Based on typical usage, here are average rules for sizing:
- 1-2 fixtures simultaneously: Roughly 3.5 GPM required.
- 2-3 fixtures simultaneously: Roughly 5 GPM required.
- 3-4 fixtures simultaneously: Roughly 7 GPM required.
Source: Navien Tankless Water Heater Sizing Guide
You can also estimate your peak GPM demand by listing potential simultaneous uses:
- Shower (2.5 GPM) + Sink (1 GPM) = 3.5 GPM
- Shower (2.5 GPM) + Shower (2.5 GPM) = 5 GPM
- Shower (2.5 GPM) + Dishwasher (1.5 GPM) + Sink (1 GPM) = 5 GPM
- Shower (2.5 GPM) + Washing Machine (1.5 GPM) + Sink (1 GPM) + Shower (2.5 GPM) = 7.5 GPM (requires a unit slightly above the 7 GPM threshold)
Identify your likely maximum simultaneous usage to find your peak GPM requirement.
Considering Temperature Rise
Tankless heaters also have a maximum temperature rise they can provide at a given flow rate. The required temperature rise is the difference between your incoming cold water temperature and your desired hot water temperature.
- Colder climates have lower incoming water temperatures, requiring a larger temperature rise.
- A higher temperature rise capability typically means a larger, more powerful unit is needed to maintain the desired GPM output.
A heater rated for 8 GPM with a 40°F temperature rise might only provide 4 GPM with an 80°F temperature rise (common in colder regions). Therefore, you need a unit capable of meeting your peak GPM demand at the required temperature rise for your location.
Sizing Process Summary
- Estimate your peak simultaneous hot water usage to determine the required GPM. Use the general rules or list individual fixtures and their GPMs.
- Determine your required temperature rise based on your location's cold water temperature and your desired hot water temperature.
- Select a gas tankless water heater that is rated to deliver your required peak GPM at your specific temperature rise.
Choosing a unit slightly larger than your calculated peak demand can offer a buffer, especially if your simultaneous usage estimates are conservative or if you anticipate future needs.
