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How to Diagnose & Evaluate Leaks in or on Heating Boilers
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Here we discuss How to Diagnose & Evaluate Leaks in or on Heating Boilers with focus on residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs. 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.

The photograph 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. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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How to Evaluate Heating Boiler Leaks & Corrosion

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.
  
  
• Internal heating boiler leaks: Some critical boiler leaks may be internal and not visible by simple inspection, such as a leak inside the boiler heat exchanger 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.
  
  
• 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.
  
  
• 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.

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 sections of a cast iron boiler
  
  
• Leaks at the circulating pump mounting flanges
  
  
• 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. 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.
  
  
• Leaks at air bleeder valves - at the boiler or remote where such bleeders are placed on heating piping or baseboards or radiators
  
  
• Leaks at radiator control valves
  
  
• Leaks at poorly-soldered copper pipe fittings on finned copper baseboard heating systems
  
  
• Leaks due to frozen and burst piping or in extreme cases, frozen and burst heating boilers themselves

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• 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.)

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