Hot Water Heater Size Calculator

First Hour Rating Calculation:

\[ FHR = \text{Tank Size} \times \text{Recovery Efficiency} + \text{Tank Capacity} \]

First Hour Rating (FHR):

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1. What is First Hour Rating?

First Hour Rating (FHR) is a measure of how much hot water a water heater can supply in the first hour of use. It's a critical factor in determining if a water heater can meet a household's peak hot water demand.

2. How Does the Calculator Work?

The calculator uses the First Hour Rating formula:

\[ FHR = \text{Tank Size} \times \text{Recovery Efficiency} + \text{Tank Capacity} \]

Where:

\( \text{Tank Size} \) — Water heater tank capacity in gallons
\( \text{Recovery Efficiency} \) — The percentage of tank capacity the heater can recover per hour
\( \text{Tank Capacity} \) — The actual amount of usable hot water in the tank

Explanation: The equation calculates how much hot water the system can deliver in one hour by combining the tank's stored hot water with the amount the heater can heat during that hour.

3. Importance of Proper Water Heater Sizing

Details: Proper water heater sizing ensures you have enough hot water for daily activities while maximizing energy efficiency. An undersized unit will run out of hot water quickly, while an oversized unit wastes energy and increases operating costs.

4. Using the Calculator

Tips: Enter the tank size in gallons and recovery efficiency as a percentage (e.g., 70% as 70). Both values must be positive numbers for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is a good First Hour Rating for a typical household?
A: For a family of 4, a FHR of 50-70 gallons is typically sufficient. Larger families or homes with high hot water demand may need 80+ gallons.

Q2: How does recovery efficiency affect water heater performance?
A: Higher recovery efficiency means the heater can heat more water per hour, resulting in better performance during periods of high demand.

Q3: Should I consider energy source when sizing a water heater?
A: Yes, gas water heaters typically have higher recovery rates than electric models of the same size, which may affect your sizing decision.

Q4: How does temperature affect First Hour Rating?
A: FHR is calculated based on a 90°F temperature rise. Actual performance may vary with different inlet water temperatures and thermostat settings.

Q5: Are tankless water heaters rated differently?
A: Yes, tankless water heaters are rated by flow rate (GPM) at a specific temperature rise rather than First Hour Rating.