What Size Water Heater Do I Need? A Tampa Plumber Answers

September 26, 2018

This is one of the first questions Tampa homeowners ask when they’re shopping for a new water heater.

And it’s a good question, too.

You see, if you get a water heater that’s the wrong size, you risk issues like:

  • Never getting enough hot water
  • More water heater repairs (due to overworked components)
  • Higher energy bills

To avoid those issues, we’ll share how to properly size 2 types of water heaters:

  • Tank water heaters
  • Tankless water heaters

Let’s start with tank water heaters…

Want a professional to size your new water heater?

How to size a tank water heater

Follow the steps below to calculate tank water heater size.

Step 1: Find your peak hour demand

Peak hour demand is how many gallons of hot water your household uses during the busiest hour of the day.

To calculate your home’s peak hour demand, use the worksheet pictured below.

Source

Step 2: Choose a water heater with a “first hour rating” that matches your peak hour demand

First hour rating (FHR) is the maximum number of gallons of hot water the unit can provide in one hour (starting with a full tank of hot water).

You’ll want to find a tank water heater that has an FHR within 1–2 gallons of your peak hour demand. This means your water heater will provide enough hot water for your household when the hot water demand is highest.

You can find a tank water heater’s FHR on the unit’s yellow Energy Guide label or online under its product specifications.

Where to find the FHR on the Energy Guide label. Source.

Where to find the FHR in online product specifications. Source.

How to size a tankless water heater

Follow the steps below to calculate tankless water heater size.

Step 1: Estimate your home’s flow rate

Flow rate refers to how many gallons a plumbing appliance uses per minute (gpm).

To find your household’s total flow rate, add up the flow rate of all the plumbing appliances you use at once.

Below is a list of common hot water appliances and their average flow rates:

  • Kitchen sink: 1.5 gpm
  • Dishwasher: 1.5 gpm
  • Washing machine: 2.5 gpm
  • Shower: 2.5 gpm
  • Tub: 4.0 gpm

For example, if you use your shower, washing machine and kitchen sink all at the same time, you’d calculate your flow rate like this:

  • Shower: 2.5 gpm
  • Washing machine: 2.5 gpm
  • Kitchen sink: 1.5 gpm

Average household flow rate = 6.5 gpm

Step 2: Note the temperature rise you need

Temperature rise refers to the difference between the incoming cold water temperature and your normal hot water temperature.

In Tampa, the groundwater temperature is 72° F.

Let’s say you set your tankless water heater to 120° F. The temperature rise you would need is 48° F.

Step 3: Search for a unit that matches the flow rate and temperature rise you need

Now that you know your home’s flow rate and the temperature rise you need, you can look for a tankless unit that will deliver enough hot water for your household.

To find a tankless unit’s flow rate and temperature rise, look at the product specifications of the unit. You can find the product specifications online, as seen in the image below.

Screenshot taken from a Google Shopping results page.

Want a Tampa plumber to find out what size water heater you need?

We’ll do all of the calculating for you, so you can rest assured you’re getting an accurately sized water heater for your home.


Similar Articles: