Guide to Indirect-Fired Hot Water Heaters InspectAPedia® -
Description of Indirect-Fired or "remote" Hot Water Heaters - water tanks heated by a connection to a heating boiler or other heat source.
How to obtain more hot water quantity, pressure, flow, at less cost
Options for replacement of tankless coils as domestic hot water source
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Here we discuss indirect-fired hot water heaters, explaining the characteristics & advantages of this method for making domestic hot water where a gas or oil fired heating boiler is installed.
Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.
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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.
Installing an indirect-fired water heater
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.
Plumbing Connections for an Indirect Fired Water Heater
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.
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.
Controls on Indirect Fired Water Heaters
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.
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 discuss 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 PROPERTIES
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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 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.)
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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. 
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