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Customized Electric Tankless Water Heater Sizing

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Determine the maximum number of devices you want to run and their total flow rate. Then, add up their flow rates (gallons per minute).
Determine required temperature rise.

Make is simple and use a sizing calculator that will do the work for you.

Sizing Example: An average shower will be between 104–106° and uses 2.6 gallons of water.

Given the variable water inflow temperature, it’s rather hard to pinpoint exactly how big a tankless hot water heater you need with out more information.  Here is a conversion calculator that will give you a comparable tankless replacement.
Gas tankless water heaters are able to produce a larger temperature rise per gpm than electric models. Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70°F water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric water heaters. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Examples:
FIXTURE
AVERAGE FLOW GPM
AVERAGE TEMP.
Tub 4.0 GPM 102°F
Shower 2.5 – 3.0 GPM 104°F
Washing Machine 2.0 GPM 120°F
Dishwasher 1.5 GPM 110°F
Kitchen Sink 1.5 GPM 110°F
Depending on use and fixtures this can vary.  It’s best to seek the advise of a plumbing professional to properly size your project.  Or, use a sizing calculator that will gather your project information and make recommendations based on your specific needs.
Determine the maximum number of devices you want to run and their total flow rate. Then, add up their flow rates (gallons per minute). Determine required temperature rise. Make is simple and use a sizing calculator that will do the work for you.