How to Size Tankless Water Heaters Calculator

Tankless Heater Size Formula:

\[ \text{Size} = \text{Max GPM} \times \Delta T \times 500 \]

Size (BTU/h):

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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 and temperature rise.

Purpose: Helps homeowners and plumbers properly size tankless water heaters for efficient and adequate hot water supply.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \text{Size} = \text{Max GPM} \times \Delta T \times 500 \]

Where:

\( \text{Size} \) — Required heating capacity (BTU/h)
\( \text{Max GPM} \) — Maximum flow rate needed (gallons per minute)
\( \Delta T \) — Temperature rise required (°F)
\( 500 \) — Constant (1 gallon of water requires about 500 BTU to raise temperature by 1°F)

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

3. Importance of Proper Sizing

Details: Correct sizing ensures:

Adequate hot water supply for simultaneous fixtures
Energy efficiency (avoiding oversized units)
Longer equipment lifespan
Prevention of cold water "sandwich" effect

4. Using the Calculator

Tips:

Determine your peak hot water demand (GPM) by adding flow rates of all fixtures that might run simultaneously
Calculate ΔT as the difference between your desired output temperature and incoming groundwater temperature
For whole-house systems, consider a safety factor of 10-20%

5. Frequently Asked Questions (FAQ)

Q1: What's a typical ΔT for residential applications?
A: Most systems are designed for 70°F rise (from 50°F groundwater to 120°F output), but this varies by climate.

Q2: How do I determine my max GPM?
A: Add flow rates of all fixtures that might run simultaneously (e.g., shower + dishwasher + faucet).

Q3: What's the 500 constant in the formula?
A: It's the approximate BTU required to raise 1 gallon of water by 1°F (8.33 lbs × 60 min × 1 BTU/lb°F).

Q4: Should I round up the calculated size?
A: Yes, choose the next available size from manufacturers (they typically come in set BTU increments).

Q5: What if I have very hard water?
A: Consider a slightly larger unit as scale buildup can reduce efficiency over time.