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• A guide to the heating capacity of tankless water heaters - hot water on demand
• Does a tankless water heater make endless hot water?
• Tankless water heaters = demand water heaters = instantaneous water heaters

Here we discuss 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. Page top sketch of a point of use tankless water heater courtesy of Carson Dunlop.

Also see tankless water heater Problems & Defects where we discuss 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 at Indirect-fired Water Heaters.

© Copyright 2009 Daniel Friedman & Carson Dunlop Associates, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left.

Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Typical Tankless Water Heater Capacities

A typical small point of use electric tankless water heater may be able to increase incoming water temperature by about

• 55 deg.F. at 1.5 gpm
• 35 deg.F. at 2.25 gpm
• 25 deg.F. at 3.0 gpm

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

• 90 deg.F. at 2.25 gpm
• 80 deg.F. at 3.0 gpm

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?

Well 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.

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.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

  Tankless Water Heaters
    Guide to Tankless Water Heaters
    Operation & Characteristics
    Problems & Defects
    Hot Water Delivery Speedup
    Installation Requirements & Costs
    Should I Convert to a Tankless Heater?
    Calculate Water Heater Requirements
    Capacities of Tankless Water Heaters
    Life Expectancy & Applications

• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Consumer Reports offers an article on the efficiency versus the economy of tankless water heaters - see http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/water-heaters/tankless-water-heaters/overview/tankless-water-heaters-ov.htm
• Bosch Tankless Water Heaters - http://www.boschhotwater.com/
• Chromolite Tankless Water Heaters
• Bradford White Everhot Tankless Water Heaters - Bradford White offers this step by step hot water retrofit installation file (large, slow PDF) - www.bradfordwhite.com
• Eemax Tankless Water Heaters - http://www.eemaxinc.com/
• Noritz Tankless Water Heaters - http://www.noritz.com/
• Rheem Tankless Water Heaters - http://www.rheemtankless.com/content/
• Rinnai Tankless Water Heaters - http://www.rinnai.us/tankless-water-heaters/
• Stiebel Eltron Tankless Water Heaters - http://www.stiebel-eltron-usa.com/
• Takagi USA Tankless Water Heaters - http://www.takagi.com/
• Titan Tankless Water Heaters - http://www.titanheater.com/
• U.S. Department of Energy on Tankless Water Heaters - http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820

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