Tank & tankless water heater sizing tips for pros
Correctly determining water heater sizes is a critical skill for pros aiming to eliminate callbacks and maximize system efficiency. Questionable sizing can lead to performance failures, so for contractors, mastering tankless water heater sizing is especially vital. This guide outlines the technical procedures for sizing storage tanks, tankless water heaters, and high-efficiency units, ensuring peak performance and customer retention to keep your business efficient and your customers with hot water.
Storage tank sizing: First hour rating (FHR)
Selecting the appropriate water heater sizes for storage systems depends entirely on the first hour rating (FHR), which is the approximate maximum amount of hot water a water heater can supply within an hour. It is not enough to solely rely on just tank capacity — the FHR accounts for the tank's volume plus its recovery rate.
To ensure you’re sizing a system that can handle peak demand, calculate the total hot water demand during a household’s single busiest hour of use. This determines the required FHR for a new storage tank water heater, ensuring the system can meet demand without running short.
Calculating peak hour demand
First, work with your customer to identify the single busiest hour for hot water use in their household. For example, a family of four getting ready for work and school may use the most of their hot water during the hour of 7 AM to 8 AM.
Next, approximate the number of times showering, handwashing, and similar hot water activities typically occur during that single peak hour.
Then, determine the total gallons used by taking each of those numbers and multiplying them by the average hot water usage for each activity.
Finally, take the sum of each activity’s total gallons added together. Now, you have your peak hour demand.
Let’s put this calculation in practice with the table below:
| Hot water activity | Avg. hot water usage (gal.) | Times used in 1 hour | Total gallons | ||
|---|---|---|---|---|---|
| Shower | 12 | x | 4 | = | 48 |
| Hand washing | 2 | x | 8 | = | 16 |
| Dishwasher | 6 | x | 1 | = | 6 |
| Clothes washing | 7 | x | 2 | = | 14 |
| Peak hour demand | 84 |
Finding the right size water heater is a matter of ensuring the selected unit’s FHR on the EnergyGuide label meets or exceeds the calculated 84 gallons the household demands.
Pro Tip: Always calculate for the "worst-case scenario" (e.g., the dishwasher running while the shower is in use) to ensure the replacement unit or component is rated for the specific load requirements of the household.
How to size a tankless water heater
Tankless water heater sizing is not based on volume, but on the ability to provide a continuous flow of hot water at a specific temperature. To calculate size of tankless water heater requirements, professionals must evaluate two primary variables:
Flow rate (GPM)
Total the flow rate of all fixtures intended to operate simultaneously.
| Fixture | Flow rate |
|---|---|
| Standard showerhead | 1.75 GPM |
| High efficiency dishwasher | 0.5 GPM – 1.5 GPM |
| Standard dishwasher | 2.5 GPM |
| Faucet | 1 GPM |
| High efficiency washing machine | 1 GPM |
| Standard washing machine | 2.5 GPM |
Temperature rise
The temperature rise is the most critical calculation for tankless water heater sizing because it determines how much work the heating element must do to reach the desired temperature at the tap.
Here is a breakdown of the components:
Output temperature
This is the desired temperature of the water coming from the faucet (typically set to 120°F to prevent scalding).
Inlet temperature
This is the temperature of the cold water entering the house from the ground. Of course, this varies significantly by geography and season. Take a look at what kind of inlet temperatures you can expect in each region.
Rise
This is the total number of degrees the unit must add to the water as it passes through the heat exchanger.
The equation in practice
The most common way to express this calculation is:
Rise = Output temp - Inlet temp
To show the importance of accurately calculating temperature rise when sizing a water heater, let’s take Florida and Maine — two regions with significantly different climates.
| Location | Output temperature | Inlet temperature | Required rise |
|---|---|---|---|
| South Florida | 120°F | 72°F | 48°F Rise |
| North Maine | 120°F | 37°F | 83°F Rise |
Note, the capacity of a tankless unit is not fixed. As the rise increases, the flow rate decreases.
Common sizing pitfalls
One of the most frequent mistakes in water heater sizing is a failure to account for the specific infrastructure requirements of the chosen fuel source. Here are some key sizing and installation pitfalls by fuel type:
| Gas water heater | Electric water heater | High-efficiency (condensing) | |
|---|---|---|---|
| Fuel & power mistakes | Using an undersized gas line for high-BTU units starves the burner and leads to frequent error codes. | Failing to verify if the home's electrical panel can support a new 30-amp or 60-amp draw often results in tripped breakers. | Low gas supply pressure prevents these sensitive burners from firing correctly or reaching their rated energy efficiency. |
| Venting failures | Installing horizontal vents without a proper upward slope allows carbon monoxide to linger and moisture to pool in the pipes. | Neglecting to leave sufficient clearance around the unit makes it impossible to access thermostats or replace heating elements later. | Reusing old metal chimneys is a critical error because the cool, wet exhaust from these units will quickly rot metal piping. |
| Drainage errors | Failing to pipe the Temperature & Pressure (T&P) relief valve to a safe discharge point risks scalding and basement flooding. | Omitting a discharge line for the T&P relief valve creates a safety hazard if the tank ever reaches dangerous internal pressures. | Forgetting to install a condensate neutralizer allows acidic runoff to eat through cast-iron sewer pipes and concrete flooring. |
Regardless of whether you choose gas, electric, or high-efficiency, the most common code violation is omitting the thermal expansion tank. In modern homes with check valves, water has nowhere to go as it expands during heating. This creates pressure spikes that cause premature tank failure and can void warranties. Always verify if your local municipality requires an expansion tank to meet plumbing codes.
Adhering to these sizing methods, you can provide systems that deliver consistent comfort while maintaining efficiency — and partnering with Ferguson ensures you select the correct water heater sizes every time. With an industry-leading inventory and the expert support needed to accurately calculate size of tankless water heater requirements, Ferguson helps pros master water heater sizing, eliminate callbacks, and maximize profitability.