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Gas Tankless Water Heater

Hi-How do I know what size of gas tankless water heater to buy.  There are about three of us women in the house?

My plumber has never installed one, but he moved and put in new regular hot water tank outside for us before.  If he looks at instruction at Home Depot, do you think he can do it?
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Posted 2018-05-07T15:23:00+0000  by KayDJ KayDJ

Hi KayDJ,

Here are the water heater selection perimeters straight from the experts at Home If you need professional installation just visit your local Home Depot store. We can complete the entire project for you.

First, identify your fuel type: natural gas, propane or electric. Next, use the calculations below to determine the size you need.


1. Calculate how much hot water you need at any time.

Use this chart to calculate the gallons per minute (GPM) of hot water each of your fixtures and appliances needs and add up these amounts to get a total GPM for your household. 

For example, if you typically have one shower running, plus a dishwasher, and the clothes washer, you have a flow rate of 3.5-7.5 GPM.  

Fixture/ApplianceTypical Flow Rates

Bathroom Faucet

0.5 – 1.5 GPM

Kitchen Faucet

3.0 – 7.0 GPM


1.0 – 2.0 GPM


1.0 – 2.5 GPM

Clothes Washer

1.5 – 3.0 GPM

Calculate Temperature - Water Heater

2. Determine your home’s required temperature rise

Calculate the difference between your ground water temperature and your desired hot water temperature – typically around 120 degrees Fahrenheit. This is your required temperature rise. 

For example, if you live in Atlanta, you have a ground water temperature of 62 degrees Fahrenheit. To reach an ideal hot water temperature of 120 degrees Fahrenheit, subtract 62 from 120. You have a required temperature rise of 58 degrees Fahrenheit. 

Use the EPA’s map for average groundwater temperature for guidance.  

3. Pick your water heater unit

Use your GPM and required temperature rise to determine which units will meet your needs. Each product information page on The Home Depot website includes this information, as do labels on the units in stores. 

Feel free to ask a Home Depot associate questions about your purchase, or contact our Installation Services team for help and installation.


Look for the Energy Factor (EF) rating. The higher the EF, the more energy efficient the unit is. 

Most water heaters offer increased efficiencies to meet federal standards. The EF rating measures how efficiently a unit converts energy into heat as well as how much heat is lost during storage. 

Look for EF ratings as close to 1 as possible. Electric heaters tend to have the highest EF ratings.


  • Be sure the location you choose for installation meets ventilation requirements. 
  • The ideal location is on an exterior wall near a gas supply line, water supply line and electrical power source. This is also the easiest and most cost-effective way to run the venting.

  • We urge you to consider professional installation for tank water heaters for your safety. 
  • The unit should have ½-inch clearance on the sides, 12 inches on the front and 18 inches off the floor.
  • Hybrids offer a narrow 21-inch diameter for access into smaller locations.

Good luck with your project.

Posted 2018-05-08T02:05:48+0000  by Mike_HD_OC
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