1. What is a Tankless Water Heater Size Calculator?

Definition: This calculator determines the required heating capacity (in BTU/h) for a tankless water heater based on flow rate, desired temperature rise, and unit efficiency.

Purpose: It helps homeowners and plumbers properly size tankless water heaters to ensure adequate hot water supply.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( \text{Size} \) — Required heater capacity (BTU/h)
• \( Q \) — Flow rate (gallons per minute)
• \( \Delta T \) — Temperature rise needed (°F)
• \( \text{Efficiency} \) — Unit efficiency (0-1, typically 0.9 for 90%)

Explanation: The formula calculates the energy needed to heat water at a given flow rate through the specified temperature rise, adjusted for heater efficiency.

3. Importance of Proper Sizing

Details: Correct sizing ensures sufficient hot water supply, energy efficiency, and prevents overworking the unit which can reduce its lifespan.

4. Using the Calculator

Tips: Enter the flow rate (gpm), desired temperature rise (°F), and unit efficiency (default 0.9). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is the 500 factor in the formula?
A: This constant represents the energy needed to raise 1 gallon of water by 1°F (8.33 lb × 60 min/hr ÷ 1.0 BTU/lb-°F ≈ 500).

Q2: How do I determine my flow rate?
A: Add up the flow rates of all fixtures that might run simultaneously, or measure with a bucket and stopwatch.

Q3: What's a typical temperature rise?
A: For most homes, calculate ΔT as desired output temp (usually 120°F) minus incoming water temp (varies by location).

Q4: What efficiency should I use?
A: Standard units are about 0.82-0.85, while high-efficiency models reach 0.90-0.98.

Q5: Should I round up the calculated size?
A: Yes, it's recommended to round up to the nearest available unit size for safety margin.