How to Size a Tankless Water Heater: Complete Guide with Calculator
Learn how to properly size a tankless water heater. Step-by-step guide with tankless water heater calculator, flow rate calculations, and BTU requirements.
Calculate required flow rates and BTU/kW capacity for tankless water heater sizing.
Open CalculatorProper tankless water heater sizing ensures adequate hot water supply during peak demand. Undersized units cause cold water, while oversized units waste energy and increase costs.
Step-by-Step Guide
Determine maximum simultaneous hot water demand. Add flow rates of all fixtures that may run together: Shower (2.5 GPM) + Kitchen Sink (1.5 GPM) + Dishwasher (1.0 GPM) = 5.0 GPM peak demand.
Calculate required temperature rise: Desired temp (120°F) - Inlet temp (varies by region: 45-70°F). For 120°F desired and 50°F inlet: 70°F temperature rise needed.
BTU = Flow Rate (GPM) × Temperature Rise (°F) × 500. For 5 GPM at 70°F rise: 5 × 70 × 500 = 175,000 BTU. For electric: kW = BTU ÷ 3,412 = 51.3 kW.
Choose tankless unit that meets or exceeds calculated BTU/kW. Common sizes: 140,000-180,000 BTU for gas, 18-27 kW for electric. Always size up slightly for safety margin.
Ensure gas line can supply required BTU. Most tankless units need 3/4" or 1" gas line. Use our Gas Line Sizing Calculator to verify adequate gas supply.
Common Mistakes to Avoid
- Not accounting for all simultaneous fixtures
- Using wrong inlet temperature for region
- Ignoring temperature rise requirements
- Not verifying gas line capacity
- Sizing based on tank water heater gallons
Frequently Asked Questions
For family of 4 with 2 bathrooms: Peak flow typically 5-7 GPM. At 70°F temperature rise: 175,000-245,000 BTU needed. Use our Tankless Water Heater Flow Rate Calculator for exact sizing.
Add flow rates of all fixtures that may run simultaneously. Standard flow rates: Shower 2.5 GPM, Sink 1.5 GPM, Dishwasher 1.0 GPM, Washing Machine 2.0 GPM. Peak is not all fixtures at once.
Temperature rise is the difference between inlet water temp and desired output temp. Higher rise requires more BTU/kW. Cold regions (45°F inlet) need larger units than warm regions (70°F inlet).
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