How to Calculate Tankless Water Heater Size

Tankless Water Heater Size Formula:

\[ \text{Size} = \text{sum\_fixture\_flows} \times \Delta T \times \text{constant} \]

Required Heater Size:

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1. What is a Tankless Water Heater Size Calculator?

Definition: This calculator determines the required capacity of a tankless water heater based on your hot water demand and temperature requirements.

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

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \text{Size} = \text{sum\_fixture\_flows} \times \Delta T \times \text{constant} \]

Where:

\( \text{Size} \) — Required heater capacity (BTU/hr or kW)
\( \text{sum\_fixture\_flows} \) — Total flow rate of all hot water fixtures (gallons per minute)
\( \Delta T \) — Temperature rise needed (inlet water temp to desired output temp in °F)
\( \text{constant} \) — Conversion factor (500 for BTU/hr)

Explanation: The formula calculates the energy required to heat the total water flow by the desired temperature increase.

3. Importance of Proper Sizing

Details: Correct sizing ensures:

Adequate hot water supply during peak demand
Energy efficiency (avoiding oversized units)
Longer equipment lifespan
Cost-effective installation

4. Using the Calculator

Tips:

Calculate total flow by adding flow rates of all fixtures that might run simultaneously
Determine ΔT by subtracting your groundwater temperature from desired output temperature (typically 120°F)
Use 500 as the constant for BTU/hr calculations (or 8.33 for kW)

5. Frequently Asked Questions (FAQ)

Q1: How do I determine total fixture flow?
A: Add the flow rates (gpm) of all fixtures that might run simultaneously (e.g., shower + sink + dishwasher).

Q2: What's a typical temperature rise (ΔT)?
A: For 120°F output with 50°F groundwater, ΔT=70°F. Adjust based on your local groundwater temperature.

Q3: Why use 500 as the constant?
A: 500 comes from (60 min/hr × 8.33 lbs/gal × 1 BTU/lb°F) / 1.0 (efficiency factor).

Q4: Should I add a safety factor?
A: Many professionals add 10-20% to the calculated size to account for variations in groundwater temperature.

Q5: How do I convert BTU/hr to kW?
A: Divide BTU/hr by 3412 to get kW (shown in the results).