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What Size Tankless Water Heater Do I Need? (With Calculator)

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Once you decide to purchase a tankless water heater, the next thing you need to determine is what size tankless water heater you need.

This handy sizing calculator will help you find the right size tankless for your home. Simply answer the questions and it'll do the math for you! 

Tankless Sizing Calculator


Tankless Water Heater Sizing Calculator

Use our  Sizing Calculator to determine your: 

Flow Rate  and Temperature Rise

Where are you located?

Ground Water Temperature Map

What Size Tankless Do I Need for a Family of 5?

We wish this was an easy answer, but you really need to work through the calculator to determine your individual needs.

Where you live, how many fixtures in use during your peak hour, and whether you'll be running laundry or other appliances will all play a role in determining the correct tankless size.

But it's fair to say, you'll need to purchase a larger tankless heater. In most cases, you'll need a tankless heater that can deliver at least 7 GPM. The tankless sizing calculator above will help you determine the right size for your home.

Gas tankless water heater with box

What is a Tankless Water Heater?

A tankless water heater is often called an on-demand water heater because they heat water only when there's a demand.

When a hot water faucet is opened, the tankless appliance turns on as the cold water begins to flow through the water heater. As the water passes through a heat exchanger, it's heated to the desired temperature.

With the water now hot, it enters your household plumbing as it travels to the open hot water faucet. Once the hot water faucet is closed, the tankless water heater shuts down until there's another demand for hot water.

Unlike a traditional tank-style water heater that's limited by the size of its tank, a tankless water heater can deliver an endless stream of hot water.

Do you want to learn more about the differences between tankless water heaters and traditional heaters? Click Here 

Glasses with Water

What Does it Mean to Size a Tankless Water Heater?

Sizing a tankless water heater is industry speak for determining the correct amount of hot water your household requires so you can purchase a tankless appliance to meet your hot water needs.

In order to properly size a tankless water heater you'll need to know a few things:

  • Temperature Rise (this is determined by where you live)
  • Peak Hour of Use (the hour of day when you'll need the most hot water)
  • Demand (how many hot water fixtures used during the peak hour)

Once you answer these questions, our tankless water heater sizing calculator will determine the GPM for your household needs. Armed with the GPM, you're ready to begin shopping for your new tankless water heater.

Neighborhood Houses

Why is Where I Live Important?

Simply put, where you live is important to determine the necessary temperature rise.

The ground temperature varies based on where you live, and the ground temperature determines the incoming water temperature.

One of the critical things you need to know in order to properly size a tankless water heater is the temperature rise. If the incoming water is 50°F and you have your tankless set to heat the water to 110°F, the temperature rise will be 60°F.

If you live in the Northern Region the ground temperature is much cooler than in the Southern Region. This means that you'll need to purchase a more powerful tankless water heater in order to deliver the amount of hot water your household requires.

Don't worry, our tankless water heater calculator takes this into account. 

Pocket Watch

What Does Peak Hour Mean?

Tankless water heaters need to be able to deliver enough hot water to your household when the demand is the highest.

When sizing a tankless water heater, the term peak hour is used to indicate the hour when you'll require the most hot water. In most cases, the peak hour is in the morning.

Consider what your morning (peak hour) routine looks like. 

Does anyone shower? Do you do laundry? Run a dishwasher? Use a hand sink?

All of these activities require hot water. If they're all happening at the same time, your tankless water heater will need to be powerful enough to service them all. An incorrectly sized (under powered) tankless would deliver lukewarm water, thus making your shower less than desirable.

Water flow in the shower

What is GPM?

GPM stands for gallons per minute and it's the measurement used to determine the flow rate of a tankless water heater.

The flow rate is how many gallons per minute (GPM) of hot water the tankless can heat every minute.

The higher the GPM, the more hot water the tankless can deliver. In other words, a tankless that can deliver 8 GPM will be able to deliver more hot water than a tankless with a flow rate of 5 GPM.

GPM is determined by how many fixtures (showers, hand sinks, etc.) your household will be using during the peak hour of use, taking into account the temperature rise. 

It's important to note that you only need to focus on the number of fixtures used during the peak hour at the same time. If you have two showers, but only use one during the peak hour (or they are not in use at the same time), you'll only use one for your calculations.

Gas flame and electric coil

Should I Purchase a Gas or Electric Tankless Water Heater?

Once you've decided to switch to a tankless water heater, the next big question is the power source. Both gas and electric tankless water heaters have their pros and cons, and you should definitely do your research.

We highly recommend reading our article, Gas vs Electric Tankless Water Heater: Which is Best. This article compares both fuel sources head-to-head. It looks at operational costs, installation costs, maintenance requirements, and other important considerations.

As a general rule of thumb, you can expect a gas fueled tankless water heater to deliver a higher GPM, be more expensive to purchase and install, and require more maintenance.

Where an electric tankless water heater is easier to install, requires less maintenance, and has a lower upfront cost.

Selecting Quality

Which Brand of Tankless Water Heater Should I Buy?

There's a lot of quality tankless water heater brands on the market today, there's also a few less than stellar brands. Ultimately the manufacturer you choose will come down to the model that best meets your needs.

It's hard to go wrong with a Rinnai, Rheem, or Stiebel Eltron tankless water heater. EcoSmart has a large selection of budget friendly models. Nortiz, Navien, and Bosch are also well respected brands.

If properly care for, a tankless water heater can be delivering hot water to your household for 20+ years, so it's important to buy your appliance from a reputable manufacturer. It'll also be easier to find a repair person to work on your appliance.

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