Questions & answers about diagnosing and fixing leaks on hot water heating systems & heating boilers
Here we explain how to detect, diagnose, find, & evaluate leaks in or on hydronic heating boilers with focus on residential heating boilers, including leaks leading to loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.
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Readers should also see BOILER LEAKS, HOW TO LOCATE. This website answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect
residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.
Our photograph at page top shows severe rust at the tankless coil on a steel heating boiler - this boiler may be beyond repair even though the rest of the unit looks pretty good.
When a tankless coil-mounting surface on the boiler itself is severely rusted it may be impossible to get an adequate seal when
bolting on a new hot water coil, making it necessary to replace the entire heating boiler. See TANKLESS COILS for more about this device.
It is important to recognize and accurately report the significance of rust like this on any heating boiler.
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Even significant boiler water leaks at a tankless coil (left and at page top) may never show up on the building floor as a wet spot, leading an owner to ignore a condition that can lead to very expensive damage. When observing evidence of leaks on a heating boiler, keep these points in mind:
Even serious leaks may never show up as "wet" spots: A boiler may be leaking but you may see no actual water: during the heating season the boiler may always be hot, causing small leaks on the boiler or on heating piping to simply evaporate. But such leaks will usually be visually very evident: look for a build-up of corrosion, green or white or other colored mineral salts, or look for rust or water stains on the equipment.
Surface rust, light, superficial rusting, is generally repairable. Clean the area and fix the leak when the boiler is next serviced and monitor for any future leaks.
Exfoliation, or thick flaking rust on any boiler but particularly on a steel heating boiler is very serious, possibly not repairable, and risks loss of the boiler as well as sudden loss of heat in the building. The boiler shown at left might be repairable. The boiler shown at page top was so badly rusted that simply replacing the coil and its mounting plate might not possible - the boiler might have been ruined by this unattended leak.
Warning about corroded leaky heating system and plumbing parts: Don't Poke at Leaks and Corrosion on heating equipment.
Heating service technicians may be more daring, if they are prepared to immediately replace a ruined boiler or a catastrophic leaking part. We watched a heating company rep poke his pencil right through a badly rusted boiler.
But a home owner or home inspector should look but don't touch corroded leaky parts on a boiler. Too often the mineral build up from corrosion is all that's holding back a serious leak that could, by chipping away the deposited gunk, result in a rapid leak so severe that the heating equipment has to be immediately shut down - not a good idea in freezing weather.
Internal heating boiler leaks: Some critical heating boiler leaks may be internal and not visible by simple inspection, such as a leak inside the boiler heat exchanger (photo at left) which may pass water into the combustion chamber.
A service technician or a skilled home inspector should be able to spot evidence of these leaks by inspecting for subtle clues (rust at the bottom of the boiler jacket) or by removing the boiler external covers (as in our photo).
Leaks related to temperature or pressure: Some leaks occur only at peak operating temperature: e.g. at relief valve.
On some heating boilers such as some cast-iron units, leaks may occur between boiler sections when the system is cold - on these models some technicians prefer to keep a little heat in the boiler year-round to avoid this problem. Leaks between boiler sections may be repairable but if left unattended can destroy the equipment.
Leaks anywhere in the hot water heating system can cause more than pressure loss in the heating system. Leaks can also allow air back into the heating system as it cools, leading to excess air, even abnormally high pressure (less common), or noisy heat piping or circulator pumps.
A Catalog of Common Heating System Leak Points - Where to Watch for Heating System Leaks
Tankless Coil mounting plate - see rust stains below and around plate. See Tankless Coil Leaks and TANKLESS COILS for a guide to diagnosing, evaluating, and repairing leaks at the tankless coil.
Pipe fittings at face of coil plate - mineral salts and rust stains - See Tankless Coil Leaks for photos of this type of leak stain
Leaks around bolt openings - suspect hidden damage - See Tankless Coil Leaks for an example of severely rusted coil mounting bolts that are probably beyond repair
Leaks between cast iron boiler sections of a cast iron boiler, possibly occurring where the nipples or seals join the water passages of individual cast iron boiler sections together. A photo showing cast iron boiler sections is just above at Internal heating boiler leaks.
Leaks at a crack or split in a cast iron boiler section, also leaks at a crack, split, or rust perforation of a steel boiler.
Leaks at the circulating pump mounting flanges - see Circulator Pumps & Relays. Our photo (left) shows a closeup of an active leak at a heating system circulator pump.
Leaks at the boiler temperature/pressure relief valve. This leak may be very dangerous as corrosion from water passing through the valve may prevent its safe operation in an emergency. The boiler pressure/temperature relief valve may be leaking because the system pressure or temperature is too high - it's working properly; or the relief valve may be leaking because its internal gasket has failed or because it has become jammed partly open by corrosion and mineral debris build-up.
Prompt expert inspection and repair are needed. Watch for leaks below the valve's mouth or discharge pipe (a pipe should extend from the relief valve to a few inches from the floor) or watch for corrosion at the tip of the discharge pipe. Gently feel inside the tip of this pipe to see if it's wet. DO NOT TRY TO TEST or open or operate the relief valve itself. See RELIEF VALVES - TP VALVES
Leaks at air bleeder valves - at the boiler or remote where such bleeders are placed on heating piping or baseboards or radiators. See Air Bleeder Valves and see Air Scoops Purgers Separators.
Leaks due to improperly-set boiler controls - temperature set too high - see Limit Switches, Boilers
Leaks at boiler control valves, boiler drains, zone service valves, zone drain valves, & radiator control valves - see LEAKS at BASEBOARD, CONVECTOR, RADIATOR
Leaks at poorly-soldered copper pipe fittings on finned copper baseboard heating systems
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Additional technical contributors & reference sources for this article are listed below.
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"The Fight Against Corrosion - A Study of the Nature of Corrosion and its Problems in Water Services and Heating Systems", Daniel Davies, Research and Development Services, Stansted Mountfichet, Essex, England, World Plumbing Conference-IV, "Plumbing and the World Environment, Compendium of Workshop Papers, October 3-6, 1996, Hyatt Regency Chicago, Chicago, IL", [personal correspondence, DJF - Author, July 2011]
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.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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.)
Links to our list of additional information on heating system inspection, repair, maintenance
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