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Tankless Water Heater Sizing: Why Most Systems Underperform

Most tankless water heaters don’t fail, they’re just installed wrong.


And the one issue we see across Cobb, Cherokee, Paulding, Forsyth, Fulton, and Bartow County is improper sizing.


At Hot Water Solutions, we specialize in water heaters, and proper sizing is one of the biggest reasons our systems perform the way homeowners expect, from Marietta and Kennesaw to Canton, Dallas, Alpharetta, and beyond.

Tankless water heater gas sizing

How Do You Size a Tankless Water Heater?

Tankless water heater sizing depends on:

  • Flow rate (GPM)

  • Temperature rise

  • Gas line capacity

👉 All three must be correct for proper performance.


Tankless Water Heater Sizing: What You Need to Know

If you’re researching tankless water heater sizing, there are three key factors that determine whether your system will perform correctly:

  1. Flow rate (GPM)

  2. Temperature rise

  3. Gas line capacity


Miss any one of these, and your system may struggle to keep up.


Flow Rate (GPM)

Tankless systems are sized based on how much hot water you need at one time.


Typical usage:

  • Shower: 2–3 GPM

  • Sink: 1–2 GPM

  • Washer: 3–5 GPM


👉 Example:

Two showers + a sink = 5–8 GPM demand


Temperature Rise

Your system has to heat incoming cold water to your desired temperature.


If winter water temps drop to 45°F, to reach 120°F, a 75°F rise is required.


The higher the temperature rise, the harder the system has to work and the lower the output capacity.


BTU Calculation

Tankless systems need enough energy to heat water quickly.


Formula:

GPM × Temp Rise × 500 = BTU/hour

Example: 10 × 75 × 500 = 375,000 BTU/hour


👉 This is why tankless systems require significantly more gas than traditional tanks.


Gas Line Sizing (Critical)

This is one of the most overlooked parts of tankless water heater sizing.


Your existing gas line may have worked fine for a tank, but tankless systems require much higher gas flow.


We evaluate:

  • Existing gas line

  • Total BTU load

  • Distance from meter


👉 If needed, we upgrade the line so the system can perform properly.


Common Mistakes

  • Choosing a unit that’s too small

  • Not accounting for winter water temperatures

  • Ignoring gas line limitations

  • Assuming all homes need the same setup


Why Proper Sizing Matters

When tankless water heater sizing is done correctly, you get:

  • Consistent hot water

  • Reliable performance

  • Full system efficiency

  • Long-term satisfaction


When it’s done wrong, you get:

  • Lukewarm water

  • Flow limitations

  • Frustration


Bottom Line

Tankless systems work incredibly well, but only when they’re sized correctly from the start.


👉 If you’re considering tankless in Acworth, Smyrna, Roswell, or nearby areas, we’ll make sure it’s done right. Call or text 678-540-6164.

 
 
 

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