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 Guide to Indirect-Fired Hot Water Heaters
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Indirect water heater properties: here we define and explain indirect-fired hot water heaters, describing the characteristics & advantages of this method for making domestic hot water where a gas or oil fired heating boiler is installed.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How & Why to Install an Indirect-fired Water Heater - get more hot water
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 Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.
Below we describe some alternative ways to make hot water, either to replace or to supplement an existing hot water supply system. After knowing what the hot water problem really is, there are steps we can take to get more hot water or to increase hot water pressure.
As we explain at TANKLESS COILS, a conventional tankless coil system imbeds a finned copper tube coil inside of the hot heating boiler water, running domestic cold water through the tankless coil where physically separated boiler water heats the coil and thus heats water passing through the coil.
An indirect-fired water heater also uses a finned-copper tube coil in a different approach to providing more domestic hot water.
In our photo at left, the white Super Stor™ tank is an indirect-fired water heater. Water in this tank is heated by a plumbing loop circulating hot boiler water through a heating coil in the bottom of the Super Stor™ water heater tank.
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Indirect fired water heaters: In some other buildings domestic hot water is produced by cycling hot water from a hydronic or steam boiler through
a loop inside of a steel tank. The water in the tank is heated by the water in the coil.
Modern systems
using this approach use the term indirect-fired water heater and such systems are sold by various companies
such as the SuperStorTM unit shown here as the white tank to the right of the heating boiler.
You may also notice the WellXTrolTM blue water tank off to the left of the boiler as well as
in the left foreground covered partly by a cardboard box we have a white-yellow fiberglass close to the thin blue vertical water softener -
water treatment equipment in the same photo. And just to be complete, the little tank over the boiler next to our service technician
is the expansion tank for this heating boiler.
Indirect fired water heater tanks for domestic hot water, such as the SuperStorTM are usually located close to the heating boiler and will have both cold and hot water lines leaving the tank to supply the
building with domestic hot water and a loop of piping that runs between the [usually the] bottom of the tank and a nearby heating boiler. Follow
the pipes to see which pipes are performing which function.
The difference between Range Boiler Water Heaters and Indirect-fired Water Heaters is in the details. The heat exchanger that heats
water in the range boiler is in or at the heating boiler. The water in an indirect fired water heater such as the
SuperStorTM unit is heated by a finned copper coil located inside the hot water tank. The
internal coil is in turn heated by circulating water inside the coil to and from the heating boiler. The range boiler is an old
concept in use for about 100 years. Indirect fired water heaters are a modern system and are in current sales and use.
Plumbing Connections for an Indirect Fired Water Heater
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Where an indirect-fired water heater is installed, the coil is inserted not in the heating boiler, but rather in the bottom of a hot water tank. You can see this design in our photo at left (the blue tank is the indirect fired water heater) and also in our sketch at page top, provided courtesy of Carson Dunlop Associates.
Water from the heating boiler (not domestic cold water) is circulated from the boiler, through the coil, and back to the heating boiler. The coil, in turn, heats water stored in the separate hot water tank.
This installation is a bit fancy: the blue indirect fired water heater is heated by the gray Weil McLain™ oil fired heating boiler.
The blue water heater tank is in turn being used in this case to provide a big reservoir of hot water to a water-to-air heating coil installed in the light gray/tan air handler at far left in the photo. This home uses that air handler for both central air conditioning and also for heat. Other pipes connected to the Weil McLain boiler may indicate that that boiler also provides hot water heat to some building areas. (Click on the photo for details).
But the principle is the same. Hot water from the blue indirect-fired water heater tank could have just as well been used to provide domestic hot water for washing and bathing as well as for heating the water-to-air heating system, as well as serving both purposes.
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Controls on Indirect Fired Water Heaters
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 An indirect fired water heater such as the Super Stor™ or Extrol™ water heater (shown at left) unit will include a temperature control that is usually mounted on the water tank itself.
When the hot water temperature drops below the control set point, the control will turn on a circulator pump that will cycle hot boiler water through the heating coil inside of the Super Stor™ tank.
The indirect-fired water heater does not itself turn on the heating boiler. Rather, when temperatures inside the separate heating boiler drop below a low limit (or hot water maintenance level) set on the heating boiler's own control, that system will turn on its burner to re-heat the boiler water directly.
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Efficiency of Indirect Fired Water Heaters
An indirect-fired water heater is a more efficient way to use a home heating boiler to make domestic hot water. Why? Because the heating boiler will run less often, and when it does run, it will have a longer "on" cycle (as it has to heat up the tank of hot water). Both of these conditions are a more efficient way to burn home heating oil than using a tankless coil.
In sum, during the heating season, when a heating boiler is likely to be "hot" most of the time anyway because it is heating the home, a tankless coil installed on a heating boiler is a reasonable way to obtain some domestic hot water from the system, though in most installations a tankless coil will not be able to produce as much hot water for as long a time period as what we can obtain from a separate water heater.
But an indirect-fired water heater always makes more efficient use of the home heating boiler, year round. And since the consumer can decide how big a hot water tank she wants, 50 or even 100 gallons of hot water can be kept in reserve. More than we can normally obtain from a conventional domestic tankless coil on a heating boiler.
Readers should also read Range Boiler Water Heaters where we describe an older type of indirect-fired water heating system installed at many older homes.
The characteristics of various water heaters such as life expectancy, cost, safety, and capacity are discussed at WATER HEATER PROBLEM DIAGNOSIS
WATER HEATER PROPERTIES
Ways to improve hot water quantity, pressure, and flow are discussed beginning at HOT WATER IMPROVEMENTS and continuing at HOT WATER QUANTITY IMPROVEMENT.
Frequently Asked Questions (FAQs) about indirect-fired hot water heaters
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
 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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 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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- Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
- Carbon Dioxide Gas Toxicity
- Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
- Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
- Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
- Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
- Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
- National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
- The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
- Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
- The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
- Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
- "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
- "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
- "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
- Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
- Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
- Installation Guide for Residential Hydronic Heating Systems
- Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
- The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
- ...
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
- 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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