1. What is a Tankless Hot Water Heater Size Calculator?

Definition: This calculator determines the required size (in BTU/h) of a tankless water heater based on flow rate, temperature rise, and system efficiency.

Purpose: It helps homeowners and plumbers properly size tankless water heaters to meet hot water demand.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( \text{Size} \) — Heater capacity (BTU/hour)
• \( \text{sum\_gpm} \) — Total flow rate of all fixtures (gallons per minute)
• \( \Delta T \) — Temperature rise needed (°F)
• \( \text{efficiency} \) — System efficiency (0-1, typically 0.82 for gas units)

Explanation: The formula calculates the energy required to heat the specified water flow by the desired temperature increase, adjusted for system efficiency.

3. Importance of Proper Sizing

Details: Correct sizing ensures adequate hot water supply while avoiding energy waste from oversized units or insufficient heating from undersized units.

4. Using the Calculator

Tips:

• Sum the flow rates of all fixtures that might run simultaneously
• Determine ΔT as the difference between incoming water temp and desired output temp
• Typical efficiency: 0.82 for gas, 0.98 for electric units

5. Frequently Asked Questions (FAQ)

Q1: What's the 500 factor in the formula?
A: This constant converts between gallons, minutes, degrees Fahrenheit, and BTUs (8.33 lb/gal × 60 min/h × 1 BTU/lb·°F ≈ 500).

Q2: How do I determine total flow rate?
A: Add the gpm of all fixtures that might run simultaneously (e.g., shower + sink + dishwasher).

Q3: What's a typical temperature rise?
A: For 120°F output with 50°F incoming water, ΔT = 70°F. Adjust based on your needs and local water temperature.

Q4: Why is efficiency important?
A: Less efficient units need higher BTU input to deliver the same heating capacity.

Q5: Should I round up the calculated size?
A: Yes, select the next available unit size above your calculation to ensure adequate capacity.