Tankless water heater (C) Carson Dunlop AssociatesHot Water Heating Capacity of Tankless Water Heaters
Demand Water Heaters or Instantaneous Water Heaters or
Endless Hot Water Systems

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Hot Water Heating Capacity of Tankless Water Heaters: this article explains the hot water heating capacity of tankless water heaters, also called instant water heaters or "on demand" water heaters.

The quantity hot water is delivered can be expressed in gallons per minute of hot water at a particular temperature rise over the incoming water temperature.

We also discuss the question of what "endless hot water" really means.Our page top photo illustrates three Bosch instant water heaters for sale in a Home Depot in Celaya, Mexico in 2012.

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Typical Tankless Water Heater Capacities

Rheem tankless water heater (C) D FriedmanA typical small point of use electric tankless water heater may be able to increase incoming water temperature by about

A larger electric tankless water heater may be able to increase incoming water temperature by about

Some tankless water heaters are rated at 9.8 GPM but be sure to check the temperature rise that the heater can provide at that flow rate (35 deg.F.).

We haven't seen a tankless water heater providing more than 5 gpm flow rate at high temperature rise (over 65 degF) rating.

If your building needs higher hot water flow rates you may need to use a conventional hot water tank or a gang of tankless water heaters.

If you're trying to provide 100° water, a 35 deg.F. temperature rise assumes an inlet water temperature of 65°F - not something you can count on in cold climates.

In our OPINION, even 100° temperature rise may not be high enough if the system is being used with a mixing valve or on long piping runs that we described earlier.

Does a Tankless Water Heater Provide Endless Hot Water?

Tankless water heater (C) Carson Dunlop AssociatesWell yes, no, and sort-of.

Limitations on Hot Water Production Rate of Tankless Water Heaters

Each model of tankless water heater is rated to provide a specific water temperature rise, for a given water flow rate.

[Sketch of a point of use tankless water heater courtesy of Carson Dunlop Associates]

Typically a tankless water heater can provide 2 – 5 gallons (7.6 – 15.2 liters) of hot water per minute.

For example, a heater may be capable of increasing the temperature of incoming water 60 deg. F. at 3 gallons per minute flow rate.

The temperature rise that a tankless water heater can provide and the water flow rate that it can support depend on the heater's BTU output capacity which in turn is a function of the fuel used (gas-fired or electric) and the efficiency and the size of the heater.

Also see tankless water heater Problems & Defects where we describe the Tankless Water Heater Cold Water Sandwich Effect. Readers should also
see ALTERNATIVE HOT WATER SOURCES to distinguish among these types of hot water producing systems.

The original text of this article was provided courtesy of Carson Dunlop - that text has been edited and may not entirely reflect CD's views.

Our OPINION is that readers looking for plenty of hot water heated efficiently should also consider the heater we describe


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