1. What is a Tankless Water Heater Size Calculator?

Definition: This calculator determines the required capacity (in BTU/h) of a tankless water heater based on your maximum flow rate and desired temperature rise.

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:

• Size — Required heater capacity (BTU per hour)
• max_flow — Maximum flow rate (gallons per minute)
• ΔT — Temperature rise needed (°F difference between incoming and desired output)
• 500 — Constant factor (BTU per gallon per °F per minute)

Explanation: The formula calculates the energy needed to heat water at your maximum flow rate by your desired temperature increase.

3. Importance of Proper Sizing

Details: Correct sizing prevents undersized units (that can't meet demand) and oversized units (that waste energy and cost more).

4. Using the Calculator

Tips:

• Determine your peak hot water demand in gpm (add up all fixtures that might run simultaneously)
• Calculate ΔT: desired output temp minus incoming water temp (typically 50-60°F in winter)
• Example: For 2.5 gpm shower needing 105°F output with 50°F incoming water, ΔT = 55°F

5. Frequently Asked Questions (FAQ)

Q1: Why is the constant 500?
A: It's based on water's specific heat - 8.33 lbs/gallon × 60 min/hour × 1 BTU/lb-°F ≈ 500.

Q2: What's a typical flow rate for showers?
A: Standard showerheads use 2.0-2.5 gpm, while low-flow models use 1.5-1.8 gpm.

Q3: How do I find my incoming water temperature?
A: Measure with a thermometer or use local averages (45-50°F in northern winters, 60-70°F in southern summers).

Q4: Should I add a safety factor?
A: Many professionals add 10-20% to the calculated size for safety margin.

Q5: What if I have multiple units?
A: For multiple heaters in parallel, divide the total flow between them when calculating each unit's size.