Climate
Groundwater temperature varies by region. Warmer water needs less heating – lets a smaller unit deliver more GPM.
120°F is the recommended setting for most homes – hot enough for showers and dishes, safe for kids and seniors. Higher temps risk scalding and waste energy.
Peak simultaneous usage
Select the fixtures that might run at the same time during your busiest morning or evening. The unit must handle these simultaneously.
Fuel type
Gas tankless units handle higher flow rates and are more efficient. Electric whole-house tankless requires 200A service and is rare in cold climates.
Sizing result
Peak demand
0
GPM simultaneous
Temperature rise
0°F
from cold to target
Recommended unit
0
GPM rating at your rise
BTU input needed
0
gas unit size
Tankless water heater GPM ratings are based on temperature rise – a unit rated at 7 GPM with a 35°F rise will only deliver 4-5 GPM in cold climates with a 70°F+ rise. Always size for your worst-case incoming water temperature. Add 20% margin if you may add fixtures in the future.