What Size Tank Water Heater Do I Need?

May 14, 2021

Has the time come for you to invest in a new tank water heater? It may seem tricky to determine the ideal size for your home, but a few simple calculations can help you choose the right size for your new tank water heater.

Every home is unique, so having your property assessed by a professional plumber will ensure that you get the guidance you need to help choose your new water heater. However, in this article, we’ll help you explore what you should consider as you’re shopping:

  • Calculate your peak demand
  • Determine your first hour rating & capacity
  • Tank water heater considerations

Advanced Air has provided 5-star service across Ft. Myers for over 30 years. Schedule an appointment with one of our expert plumbers to replace your water heater.

Calculate Your Peak Demand

Your instinct may be to equate water heater size with your home’s size, but the ideal appliance for your home depends more on how many people live there and how much hot water you’ll need at any given moment than on your square footage.

This is where the “peak demand” calculation can help. To determine your peak demand, let’s figure out the maximum amount of hot water you’ll need at any given time for your household using this table from DOE Energy Saver.

Estimating Peak Hours

Let’s consider a hypothetical scenario in which your household starts a laundry load, takes two showers and runs the dishwasher all within the same hour. Here’s the hot water peak demand for that hour:

  • Laundry (7 gallons) x 1 = 7 gallons
  • Shower (10 gallons) x 2 = 20 gallons
  • Dishwasher (6 gallons) x 1 = 6 gallons
  • Total peak demand hour: 33 gallons

Determine your first hour rating & capacity

Tank water heaters fill their tank with pre-heated water and then tap into that water as needed. So, you’ll need a unit that’s big enough to meet your peak demand hour without running out of the hot water reserve.

You’ll see two common sizing metrics as you shop for your new water heater:

  • First hour rating (FHR): This is how many gallons of hot water a unit can provide in the first hour of hot water demand.
  • Capacity: This is how many gallons the water heater can hold. Common capacities range between 30-80 gallons. Typically, you’ll want a water heater that has a capacity of 1-5 gallons more than your peak demand.

Let's revisit the hypothetical 33-gallon peak demand that we calculated above. In this example, you’ll want to find a unit with an FHR of 33 gallons and a capacity of about 38 gallons.

Now, calculate the peak demand for your home and you’ll know which unit size you’ll require. You’ll want to take the above grid and figure out your typical hot water usage and what will be the most amount of hot water your household will ever use at once.

Tank Water Heater Considerations

There are a few other things to consider beyond just the size of your new tank water heater.

What’s your power source?

Some homes have a gas heating source and others have an electric heating source.

Gas heaters typically cost more upfront but have lower monthly utility bills than electric heaters. However, one great way to lower your monthly bills with an electric heater is to consider a hybrid unit, which can be more energy efficient.

What’s the warranty like?

Gas tank water heaters typically last 8-12 years and electric units typically last 10-15 years. Various appliances will come with diverse warranty lengths, so in addition to choosing your water heater based on its size, power source, and price, there’s value in considering the length of the manufacturer’s warranty as well. You will have to pay more for an extended warranty.

Ready for your new water heater? Advanced Air is here to help!

When you need a new water heater, you need it fast! We provide same-day installations so call us to schedule service and install a new water heater today.


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