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How To Size A Tankless Water Heater

If you are considering replacing your old tank style water heater with a new tankless water heater system it is important to choose the correct size.

Note: Tankless water heaters and on-demand water heaters are the same product with a different name.

tankless water heater
Figure 1 - Tankless water heater

Tankless water heaters can be powered by propane, natural gas or electricity.

It should be noted that it is highly unlikely that you can replace your current tank water heater with a tankless product without changing the supply lines or the size of the wiring that powers the system.

Although tankless water heaters are much more energy efficient than tank style water heaters, they require more energy for short periods of time.

One of the most common installation errors is to use the same natural gas or propane lines that supplied the energy for the tank water heater to power the tankless system.

Sizing a tankless water heater:

There are two determining factors when sizing a system:

  1. Temperature Rise:
  2. The temperature rise is the difference between the temperature of the water entering your home and the temperature of the hot water you require. If you do not know the exact temperature of the incoming water you can use 50 degrees Fahrenheit. The optimum temperature for the hot water in your home is 120 degrees Fahrenheit. Hence you would require a hot water heater that has the capability to produce a temperature rise of 70 degrees Fahrenheit. In some instances, for instance a dishwasher that does not have an internal heater, a homeowner may want to have the outlet water temperature at 140 degrees Fahrenheit.

    Most tankless water heaters are rated based on inlet temperatures. With natural gas or propane powered units a rise of 70 degrees Fahrenheit at 5 gallons per minute are common. Electric tankless water heaters will usually be in the order of a 70 degree Fahrenheit temperature rise at 2 gallons per minute.

  3. The gallons per minute of hot water consuming appliances, faucets and shower heads.
  • Kitchen and Bath faucets require 0.75 to 2.5 gallons per minute.
  • Low-flow shower heads require 1.5 to 2 gallons per minute.
  • Standard shower heads require 2.5 to 3.5 gallons per minute.
  • Dishwashers and Washing Machines require 1 to 2 gallons per minute.
  • By adding the faucets, shower heads and appliances you have in your home you can determine the maximum amount of hot water you could possible use at any one moment in time.

Use some common sense in the sizing calculation. It is normally impractical to think that all of the potential requirements for hot water would occur simultaneously. So consider the worst case scenario in your home.

Selection of hot water tanks.