What size tankless water heater do I need to replace a 50 gallon water heater?

Read our comprehensive sizing guide here

You can’t size a tankless water heater by the standards of the storage water heater. They don’t work that way. With a tankless, you can have endless hot water, so it doesn’t matter how big your storage was.

Instead, if you want to replace your old 50-gallon storage water heater, you’ll need to find a tankless water heater that would be able to heat enough water to a specific temperature per minute to keep up with the demand. It’s the flow rate and the temperature rise that matter.

In our ultimate sizing guide for tankless water heaters, we explained how to determine the total flow rate and find out what the temperature rise should be in your region.

If you don’t want or don’t have time to read the whole piece, here are the main points:

Count all the hot water fixtures in your house that you want to run simultaneously. 

That’s pretty easy to do. Remember that it doesn’t matter how many bathrooms your house has. What matters is how many of them you’re going to use at the same time.

Add up their flow rates. 

The flow rate is the amount of water that goes through the fixture per minute in gallons. If you don’t know the exact numbers, run a test.

  1. Take a 1-gallon bucket. 
  2. Measure how fast a faucet, showerhead, bathtub, or any other fixture is going to fill a quarter of the bucket.
  3. Count the flow rate by the formula:
FLOW RATE = 15 / Seconds to fill a quarter of the bucket

Determine the temperature rise for your region.

It’s relatively easy to do with our map of groundwater temps. You can find it in our sizing guide.

To determine the temperature rise subtract your groundwater temperature from the temperature you want to have in the shower.

Pick the right model that meets the requirements

It’s not an easy thing to do, I must say. With so many trusted brands and models on the market, you can easily get confused. 

Knowing what requirements it should meet helps to narrow down the list to several brands and models for different power sources and various budgets, of course.

We’ve tried and come up with a list of the best models for most use cases. But if none fits your situation, let us know in the comments, please.

A sizing example

For example, you live in Texas. You want to be able to run two showers plus let a washing machine do its job. And all of this should be running at the same time.

Now, let’s do the math. 

The flow rate of an average showerhead is 2-2.5 GPM (gallons per minute). Two of them would be 4-5 GPM. A washing machine consumes about 2 GPM on average.

The groundwater temp in your region is between 60°F and 67°F. That results in a temperature rise of about 110°F – 60°F = 50 °F.

Now go to our list of the best tankless water heaters and see which one fits your needs best. It should be able to provide 6 to 7.5 gallons of hot water per minute at the temperature rise of 50°F.

We can see that Rinnai V-Series models would be the best choice for your needs in this case. 

Rinnai RL Series HE+ Tankless Hot Water Heater: Indoor Installation:
  • Fuel Type: Natural Gas or/and Liquid Propane
  • UEF (Efficiency Rating): 0.81
  • Product dimensions: 14 x 9.3 x 23 inches
  • Flow rate: 7.5 GPM
  • Energy Star certified (may be available for rebates)
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