1. What is a Tankless Water Heater Size Calculator?

Definition: This calculator determines the required capacity (in BTU/h) for a Rheem tankless water heater based on your hot water demand.

Purpose: It helps homeowners and plumbers properly size a tankless water heater to meet household hot water needs.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( \text{Size} \) — Required heater capacity (BTU/hour)
• \( \text{GPM} \) — Gallons per minute of hot water needed
• \( \Delta T \) — Temperature rise needed (°F difference between incoming and desired water temperature)
• \( 500 \) — Constant (1 BTU raises 1 lb of water by 1°F; water weighs 8.33 lbs/gal)

Explanation: The formula calculates the energy required to heat a specific flow rate of water by a specified temperature rise.

3. Importance of Proper Sizing

Details: Correct sizing ensures adequate hot water supply while maximizing energy efficiency and preventing undersized units that can't meet demand.

4. Using the Calculator

Tips:

• Determine your peak GPM by adding flow rates of all fixtures that might run simultaneously
• Calculate ΔT by subtracting your incoming water temperature from desired output temperature
• Typical ΔT values range from 45°F to 70°F depending on climate and usage

5. Frequently Asked Questions (FAQ)

Q1: What's a typical GPM for household fixtures?
A: Shower: 1.5-2.5 GPM, Kitchen faucet: 1.0-1.5 GPM, Washing machine: 1.5-3.0 GPM.

Q2: How do I find my incoming water temperature?
A: Measure with a thermometer at a cold water faucet, or estimate: 40°F (cold climate) to 70°F (warm climate).

Q3: What if my calculated size is between standard unit sizes?
A: Round up to the next available size to ensure adequate capacity.

Q4: Does this account for multiple simultaneous uses?
A: Yes, if you enter the total GPM of all fixtures that might run together.

Q5: What's the 500 constant in the formula?
A: It combines the weight of water (8.33 lbs/gal) and minutes-to-hours conversion (60 min/hr) ≈ 500.