 Hot 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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.
[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 at Indirect-fired Water Heaters.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- [1] Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- [3] HREF="http://www.consumerreports.org/">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
- [4] Bosch Tankless Water Heaters - http://www.boschhotwater.com/
- [5] Chromolite Tankless Water Heaters
- [6] Bradford White Everhot Tankless Water Heaters - Bradford White offers this step by step hot water retrofit installation file (large, slow PDF) - www.bradfordwhite.com
- [7] Eemax Tankless Water Heaters - http://www.eemaxinc.com/
- [8] Noritz Tankless Water Heaters - http://www.noritz.com/
- [9] Rheem Tankless Water Heaters - http://www.rheemtankless.com/content/
- [10] Rinnai Tankless Water Heaters - http://www.rinnai.us/tankless-water-heaters/
- [11] Stiebel Eltron Tankless Water Heaters - http://www.stiebel-eltron-usa.com/
- [12] Takagi USA Tankless Water Heaters - http://www.takagi.com/
- [13] Titan Tankless Water Heaters - http://www.titanheater.com/
- [14] U.S. Department of Energy on Tankless Water Heaters - http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820
- ...
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
- Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
er- Plumbing Diagnosis & Repair: Water supply, drainage, septic systems, water testing, water contamination, defective plumbing materials & products.
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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