8 Tips for Sizing a Tankless Water Heater Correctly
Tip 1: Calculate Your Peak Simultaneous Demand, Not Average Usage
Most homes need a tankless heater sized for the moment of peak demand – like running a shower while the washing machine heats water. Calculate how many gallons per minute (GPM) you need at maximum simultaneous use. A typical shower uses 2.5 GPM, a washing machine uses 2-3 GPM, a dishwasher uses 1.5-2 GPM. Add them up for your peak demand, not average daily usage.
Tip 2: Groundwater Temperature Affects Heater Sizing
A tankless heater in cold climates (where groundwater is 40-50 degrees F) needs more power to reach 120 degrees than one in warm climates (where groundwater is 60-70 degrees F). If you’re in a cold region, size up from the calculator’s recommendation. Cold groundwater requires the heater to work harder and heat more water per minute.
Tip 3: Incoming Water Temperature Is Critical to the Calculation
Know your incoming water temperature (test it by running cold water for a minute and measuring). If your calculator uses the national average of 58 degrees and your water is actually 45 degrees, you’ll undersize the heater. Most calculators let you input your actual incoming temperature for accuracy.
Tip 4: Gas Tankless Heaters Deliver More GPM Than Electric
A gas unit can typically deliver 3-5 GPM while maintaining temperature. An electric unit typically delivers 1-2 GPM. If you have high simultaneous demand, gas is the better choice. If you only need one high-flow point (like a shower), electric might suffice and is cheaper to install.
Tip 5: Factor in Future Expansion or Additional Fixtures
Don’t size exactly for current needs. If you might add a bathroom, upgrade your shower, or install a water-heated radiant floor in the future, size up now. Undersizing means replacing the heater in 5 years. Oversizing by one category is cheaper than replacement.
Tip 6: Efficiency Rating (EF) Affects Real-World Performance
A higher Energy Factor (EF) rating means better efficiency, but it also means the unit delivers consistent temperature more reliably. A lower-efficiency unit might fluctuate in temperature, especially during peak demand. Don’t just buy the cheapest option – efficiency affects usability.
Tip 7: Water Pressure Affects Flow Rate and Performance
Low water pressure (below 40 PSI) can cause tankless heaters to underperform or not ignite at all. Check your home’s water pressure. If it’s below 40 PSI, install a pressure booster or consider a tank-style heater instead. High pressure (above 80 PSI) requires a pressure regulator to protect the heater.
Tip 8: Installation Location and Venting Add Cost and Complexity
Indoor installation with proper venting is expensive. Outdoor installation is cheaper but requires weatherproofing and freeze protection in cold climates. Factor installation complexity into your decision. A slightly larger unit that vents easily might be cheaper than a smaller unit requiring complex venting.
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Where to Go From Here
Now that you understand peak simultaneous demand, groundwater temperature, incoming water temperature, gas vs. electric, future expansion, efficiency ratings, water pressure, and installation complexity, use our Tankless Water Heater Size Calculator to determine the right unit for your home. Enter your peak simultaneous GPM demand, incoming water temperature, and climate zone, and the calculator will show you the recommended heater size and type.
Frequently Asked Questions About Tankless Water Heater Sizing
What if I size my tankless heater too small?
You’ll experience temperature fluctuations or insufficient hot water when multiple fixtures run simultaneously. A shower might go cold when the washing machine kicks on. This is frustrating and leads to regrets about the purchase. It’s better to oversize slightly than undersize.
Can I use one tankless heater for a whole house and a radiant floor?
Possibly, but you’d need to oversize significantly. Radiant floor heating uses enormous amounts of hot water. Most homes need either a separate heater for the radiant system or a much larger primary heater. Consult a professional before attempting this combination.
What’s the difference between point-of-use and whole-house tankless heaters?
Point-of-use units (2-3 GPM) serve one fixture like a shower. They’re cheaper and easier to install but don’t work for simultaneous multi-fixture use. Whole-house units (4-10 GPM) serve the entire home but cost significantly more. Choose based on your actual usage pattern.
How long does a tankless heater take to deliver hot water?
Typically 5-10 seconds if installed near the fixtures. Longer delays (20-30 seconds) indicate the unit is far from the fixture or has slow-heating pipes. Install as close as possible to the furthest fixture for faster hot water delivery.
Should I install a tankless heater if I have very hard water?
Hard water requires more frequent descaling (flushing with vinegar or descaling solution). Tankless heaters are more susceptible to mineral buildup than tank heaters. If you have hard water, consider a whole-house softener or budget for frequent maintenance. It’s not impossible, just requires more upkeep.