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Water Heater Sizing Guide for Tampa Homes

The #1 question we get from Tampa homeowners is “What size water heater do I need?” This article provides step-by-step instructions for determining your home’s hot water needs and choosing the right size water heater for your home.

Sizing a Storage Tank Water Heater

To size a storage water heater for your home, follow this simple two-step process:

Step 1: Estimate your home’s peak hour hot water demand

Step 2: Select a water heater with a first hour rating that matches your needs

Simple, right? Below, we break down those steps in detail so you can pick the best water heater.

PRO TIP: Want to skip the math? Our friendly Tampa water heater installers can do the math for you and recommend water heater options that meet your needs. Call 813-486-4038.

Estimate Your Peak Hour Hot Water Demand

Use this worksheet to estimate the amount of hot water you need during your home’s peak hot water usage time.


 Shower 10 x =
 (.05 gallons per min.)
2 x =
 Hand dishwashing or food prep
(2 gallons per min.)
4 x =
 Automatic dishwasher 6 x =
 Clothes washer 7 x =
Total Peak Hour Demand =

Step 2: Select a Water Heater with a First Hour Rating That Matches Your Needs

To do this, check the water heater’s first hour rating. But what does this mean?

The first hour rating is the number of gallons of hot water the heater can supply per hour (starting with a tank full of hot water). Your water heaters first hour rating depends on the capacity of the storage tank (measured in gallons), the heat source (burner or element), and the size of the burner or heat element.

If you’ve ever looked at your water heater (some homeowners never have!), you’ve probably noticed the EnergyGuide label stuck to your unit. All new conventional storage water heaters must have this label.  It lists the first hour rating of the water heater in the top left corner as “Capacity (first hour rating).”

PRO TIP: When selecting a water heater, look for a model with a first hour rating that matches within 1 or 2 gallons of your peak hour demand – that is, the daily peak 1-hour hot water demand for your home.


Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. To size a tankless water heater, you need to determine the flow rate and the temperature rise you’ll need.

But first, you’ll need to answer a few questions:

  • How many hot water devices do you expect to use at any one time?
  • What is the flow rate (gallons per minute) of your hot water devices?
  • What temperature rise do you want? This is the difference between the incoming water temperature and your desired water temperature.

PRO TIP: Call us for help sizing a tankless water heater in Tampa. Call 813-486-4038.

How to Calculate Your Home’s Flow Rate

This simply means calculating how much hot water you need for each of the appliances you’ll be running. For example, the flow rate of a typical kitchen sink is 1.5 gallons per minute (gpm). A washing machine flow rate might be around 2.5 gpm, and your shower may have a flow rate of 2.5 gpm, too.

Add these flow rates together to calculate your home’s total flow rate.

How to Calculate the Desired Temperature Rise

In Tampa, groundwater is around 72 degrees. Calculate the temperature rise by subtracting this number from the desired temperature setting of your water heater.

For example, if you set your water heater to 115 degrees:

115 degrees – 72 degrees = 43 degrees temperature rise

Review Tankless Water Heater Specifications

Most tankless water heaters are rated for a variety of inlet temperatures and flow rates. Some types of tankless water heaters can vary their output temperature depending on the water flow rate of the fixture (faucet, shower, etc.) and the inlet water temperature.

PRO TIP: If you’re shopping big box stores or reviewing tankless water heaters online, check the unit’s specifications to make sure it’s rated for the flow rate your home needs and the temperature rise you need. A water heater only rated for one of these factors could fail to produce enough hot water when you need it.

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