Tankless Water Heater Flow Rate Calculator

Flow Rate Formula:

\[ GPM = \frac{BTU/h}{500 \times \Delta T} \]

Flow Rate (GPM):

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1. What is a Tankless Water Heater Flow Rate Calculator?

Definition: This calculator determines the maximum flow rate (in gallons per minute) that a tankless water heater can support based on its heating capacity and desired temperature rise.

Purpose: It helps homeowners and plumbers properly size tankless water heaters for different applications and flow requirements.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ GPM = \frac{BTU/h}{500 \times \Delta T} \]

Where:

\( GPM \) — Flow rate in gallons per minute
\( BTU/h \) — Heating capacity of the water heater
\( \Delta T \) — Temperature rise needed (°F)
500 — Constant (BTU/gal/°F for water)

Explanation: The formula calculates how many gallons per minute of water can be heated by the specified temperature rise given the heater's BTU capacity.

3. Importance of Flow Rate Calculation

Details: Proper flow rate calculation ensures your tankless water heater can meet your hot water demands without temperature fluctuations or insufficient heating.

4. Using the Calculator

Tips: Enter the heater's BTU rating (found on the unit) and your desired temperature rise (difference between incoming water temp and desired output temp). Default ΔT is 70°F (typical for 50°F incoming to 120°F output).

5. Frequently Asked Questions (FAQ)

Q1: What's a typical temperature rise for residential use?
A: Most homes use 70°F rise (50°F incoming to 120°F output), but this varies by climate and personal preference.

Q2: How do I find my heater's BTU rating?
A: Check the manufacturer's specifications on the unit or in the manual. Residential units typically range from 140,000 to 199,000 BTU/h.

Q3: Why is the constant 500?
A: It's based on water's specific heat (1 BTU/lb/°F) and weight (8.33 lb/gal), rounded for practical calculation.

Q4: What if I have multiple fixtures running simultaneously?
A: Add up the flow rates of all fixtures (typically 1.5-2.5 GPM each) and ensure your heater can handle the total at your desired ΔT.

Q5: Does this account for efficiency losses?
A: No, for more accurate results, multiply the BTU input by the unit's efficiency rating (typically 0.8-0.98) before calculating.