Tankless Gas Water Heater Size Calculator

Tankless Water Heater Size Formula:

\[ BTU = \frac{Q \times \Delta T \times 500 \times 60}{Eff} \]

Flow Rate (Q):

gallons per minute

Temperature Rise (ΔT):

°F

Efficiency (Eff):

decimal (e.g., 0.85 for 85%)

Required BTU:

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1. What is the Tankless Water Heater Size Calculation?

The tankless water heater size calculation determines the required heating capacity in BTU (British Thermal Units) per hour to meet your hot water demands. Proper sizing ensures adequate hot water supply while maximizing energy efficiency.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ BTU = \frac{Q \times \Delta T \times 500 \times 60}{Eff} \]

Where:

\( Q \) — Flow rate in gallons per minute (GPM)
\( \Delta T \) — Temperature rise in °F (desired output temperature minus incoming water temperature)
\( 500 \) — Weight of water per gallon (approximately 8.33 lbs) × specific heat of water (1 BTU/lb-°F)
\( 60 \) — Minutes to hours conversion
\( Eff \) — Efficiency factor (decimal value, e.g., 0.85 for 85% efficiency)

Explanation: The formula calculates the energy required to heat water from the incoming temperature to the desired temperature at the specified flow rate, adjusted for the unit's efficiency.

3. Importance of Proper Sizing

Details: Correct sizing is crucial for optimal performance. An undersized unit will not provide enough hot water, while an oversized unit wastes energy and increases operating costs.

4. Using the Calculator

Tips: Measure your actual flow rate using a bucket and stopwatch. Determine your temperature rise based on local groundwater temperature and desired output temperature. Efficiency values typically range from 0.80 to 0.95 for modern tankless heaters.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical flow rate for household fixtures?
A: Shower: 1.5-2.5 GPM, Kitchen faucet: 1.5-2.2 GPM, Bathroom faucet: 0.5-1.5 GPM. Add flow rates for simultaneous use.

Q2: How do I determine the temperature rise?
A: Subtract your local groundwater temperature (usually 40-60°F) from your desired output temperature (typically 120°F).

Q3: What efficiency should I use?
A: Modern tankless heaters range from 80-95% efficiency. Check manufacturer specifications for your specific model.

Q4: Can I use this for electric tankless heaters?
A: This formula is primarily for gas heaters. Electric heaters use kW ratings (1 kW = 3412 BTU/hour).

Q5: What about altitude adjustments?
A: Gas heaters at high altitudes may require derating. Consult manufacturer guidelines for altitude compensation factors.