Photograph of a burning-up and possibly dangerous oil-fired heating boilerHeating Boiler Troubleshooting
How to Recognize & Diagnose Heating Boiler Problems

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Heating boiler defects & operating problem troubleshooting:

Here we explain how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.

We identify different types of building heating systems and outline how to diagnose and repair heating system problems.

This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs.

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How To Recognize Heating System Defects: the Heating System Inspector Must Know the Equipment

Backpressure Burn at an oil burner (C) Daniel FriedmanIf you have no heat and need help diagnosing the cause,
see HEATING LOSS DIAGNOSIS-BOILERS - forced hot water heating baseboard, radiators, convectors, radiant heat

or HEATING LOSS DIAGNOSIS-FURNACES - forced warm air heat

The photograph at page top shows a heating boiler which is burning through it's steel outer jacket - this condition is a potential fire hazard and requires immediate inspection and repair.

There is probably a combustion gas leak at this heater. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

If you don't know whether your heat is provided by a furnace (hot air) or boiler (hot water)

Article Series Contents:

Types of Defects and Problems with Hot Water Heating Boilers - Hydronic Heating Systems, Loss of Heat, Noises, Leaks

What sort of defects should you look for when inspecting a heating system?

  • Unsafe conditions on the heating system such as signs of burn-out of the combustion chamber liner, unsafe relief valves, unsafe flue gas venting, fuel leaks, combustion gas leaks. Burn marks on boilers or furnaces, evidence of leaky or stuck pressure relief valves, evidence of tampering with the safety controls, abnormal heater control settings, evidence of chimney, flue, or flue gas venting defects, are all safety concerns.
  • Loss of heat - boiler or furnace won't operate, or system operates but heat is not produced in the living area - we provide a guide to diagnosing the reasons for "no heat" beginning
    at NO HEAT - BOILER / FURNACE DIAGNOSIS. If your hot water heating system is running (the steam or hot water boiler is making noise and seems to get hot) but some of your radiators or baseboards are cold,

  • Improper Heating System Function: OIL BURNER NOISES, burn marks on the furnace or boiler, unreliability, inadequate heat distribution. Soot production by gas fired heating equipment is an immediate, serious hazard that risks potentially fatal carbon monoxide poisoning. Turn such equipment off and call for expert advice. Severe smoking and soot puffback from oil fired heating equipment is also dangerous and requires immediate action.
  • Improper heating system maintenance, such as failure to adequately clean and inspect the heating flue, furnace, or boiler, unsafe heating safety devices (e.g. relief valves), evidence of sooty, noisy operation, leaks in fuel, air, water. An example of an easy-to-spot clue that an oil fired heating system needs service and cleaning is illustrated
  • Improper Heating System Construction/Installation
  • Leaks in the hot water heating system -

  • Loss of heat - no heat - is discussed just below
    at DIAGNOSE LOSS OF HEAT With Oil-Fired Hot Water Heating Systems
  • Missing HEATING SYSTEM COMPONENTS or missing or damaged heater
    See BOILER SAFETY CONTROLS operating aids
  • Noises in heating systems are discussed at DIAGNOSE NOISES during Heating System & Oil Burner Operation
  • Heat won't turn off:
    see HEAT WON'T TURN OFF - Stop Unwanted Heat for an explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat
  • Heating system operating pressure or temperature are abnormal -
  • Heating system reliability - Unreliable heating systems or heating systems at or near the end of their useful life. Home inspectors and qualified heating inspectors should not give a heating system a passing grade just because heat is entering the building. The heating system needs to be safe, and sufficiently reliable that the owner can reasonably expect to leave the building without a likely loss of heat and related building damage. If a heating system has less than a year of remaining reliable and safe operating life, it is in unacceptable condition.
  • Heating System Damage: leaks, mechanical damage, combustion chamber damage [Photo]
  • Heating System oil burner noise, smoke odors are discussed at Oil Burner Noise, Smoke, Soot, Odors - Diagnosis & Cure
  • Clues of potential other heating system defects: LEAKS CORROSION STAINS, abnormal temperature or pressure, or

How to Diagnose Loss of Heat With Oil-Fired Hot Water Heating Systems

  • Does the heating equipment have electric power?: make sure that electrical power is on for all of the heating system components.

    Check local switches at the boiler, wall mounted boiler switches at the stairwell down to the basement or utility room or in other areas, and check the circuit breakers or fuses in the electrical panel.
  • Is there fuel for the boiler ? check the oil tank gauge; if the boiler or furnace is fired by natural or LP gas check that no one has shut the supply valves.

    If a system has been "shut down" and you don't know when or by whom this step was taken, beware of turning the heating system back on without an inspection and safety check by an expert technician. If you're out of oil call your oil company and request an emergency delivery. Usually the delivery person will also need to enter the building to bleed air from the oil lines in order to get the oil burner operating again.
  • Does the oil burner start but then shut off? Is the system "off on reset"? if so the red "reset" button will have popped up on the oil burner control (or possibly on an electric motor operating the oil burner) and the system has an operating problem and needs service.

    It may be possible to re-start an oil burner that has turned itself off by pressing the red reset button ONCE.
  • If the system re-starts and runs acceptably, you can have heat while waiting for the service technician but the system still needs inspection, service, and possibly other repairs before it can operate safely and reliably. If you push the reset button and the burner does not start and keep running for five minutes or more, DO NOT try again as you may flood the system with oil and make restarting difficult and dangerous. Also
  • Boilers-Hydronic Heat: Is hot water water circulating?
    • Check the hot water circulator: Test to see if the circulator is operating Feeling the hot water distribution piping (Watch out it should be HOT!). Feeling the piping can indicate if circulator is running or not -- a quick change in temperature means the circulator works; slow change in temperature may occur if the circulator does not work but hot water is circulating by convection.
    • Check hot water baseboards: if some baseboards are hot and others cold, are they all on the same heating zone? If not, are all of the thermostats turned up? If some heating baseboards are hot and others cold and we're sure that they're on the same zone, then the system is probably air-blocked.

      A heating circulator pump is capable of pushing water around in the loop of heating baseboard but is often not capable of overcoming a section of baseboard that contains a large bubble of air. It's necessary to remove such air blocks. If air blocking has been recurrent a previous owner may have installed air bleeders at strategic points.

      The bleeder can be opened slightly and carefully, to permit air to escape. As soon as water starts coming out of an air bleeder valve it can be closed. Automatic air purge devices are available and are usually installed right at the heating boiler but sometimes additional ones are needed at higher levels in the building.
    • Check hot water radiators or convectors: if some radiators or wall convectors are hot and others are cold, and provided that the individual feed valves located at each of these devices is open (turned counterclockwise) the radiator or convector may be air blocked.

      Usually an individual air bleed valve is located on each of these devices. As we suggested above, if your hot water heating system is running (the steam or hot water boiler is making noise and seems to get hot) but some of your radiators or baseboards are cold,
      see RADIATORS


How to Diagnose Noises during Heating System & Oil Burner Operation

Complete details about all types of heating system noises, causes, cures, are found

Noises specific to an oil burner used at heating boilers or water heaters are discussed
at OIL BURNER NOISE SMOKE ODORS. Excerpts below focus on some of the most common heating system noises.

  • Noises during oil burner startup - a "bang" or puffback which blows soot into the room through the barometric damper or through other equipment openings: the oil pump may not be shutting down properly at the end of an oil burn cycle, leaking incompletely burned oil into the combustion chamber. That oil ignites at startup causing a potentially dangerous puffback. Immediate service and repair are needed.
  • Noises during oil burner startup - a "rumbling" sound (which usually continues all during operation" or a "stumbling" sound in the combustion chamber probably indicates that the system needs inspection and cleaning very soon. Some noise is normal however, but the normal sounds tend to be more smooth and continuous.
  • Noises during oil burner shut-down - a stumbling or rumbling after the oil burner motor has stopped, indicate that oil is continuing to leak into the combustion chamber and risks a dangerous puffback - see "Noises during oil burner startup" above. Immediate service is recommended.
  • Noises of shrieks or grinding coming from the electric motor or oil pump on the oil burner or more rarely at a circulator pump mean that prompt service is needed - probably a bearing is failing.
  • Startup problems: noises and clues of puff back: if you see flapping at the barometric damper or if you see or hear vibrations in the system, prompt service is needed -
  • Noisy hot water heating system piping or circulator pumps: can be caused by air in the piping

    or by (more rarely) a defect in the circulator pump itself
  • Noises from radiators or heating baseboards:
    • Clanking heating system pipes or sharp snapping noises may be heard as a normal consequence of expansion of metals during the heating cycle. These noises can often be eliminated or reduced by careful routing of piping and by allowing room around heating pipes for expansion, but probably not eliminated in the case of hot water baseboards.
    • Bubbling or rumbling heating system noises in hot water heating piping can be caused by air in the heating lines. If the amount of air becomes excessive the heating system may be unable to circulate hot water and extra steps to bleed unwanted air will be required.
    • Hissing heating system sounds such as air escaping from radiators or the piping where air bleeder valves are installed are normal but should be brief and uncommon. If you constantly hear air hissing from radiator bleed valves double check that you understand what kind of heat you have - hissing from bleeder valves on steam heat radiators as heat is coming up in the building is normal.
      See RADIATORS.

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
  • Pipe fittings at face of coil plate - mineral salts
  • Leaks around bolt openings - suspect hidden damage
  • 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

How to Recognize & Diagnose Inadequate Combustion Air, Sooting, or Burn Marks at Oil-Fired Heating Systems

Backpressure sooting at an oil fired furnace (C) Daniel Friedman Backpressure Burn at an oil burner (C) Daniel Friedman

Lack of adequate combustion aircan be indicated by or can result in these heating system operating and heating system safety worries:

  • Improper oil burner system operation such as noises, rumbling, etc. discussed above.
    See BOILER NOISE SMOKE ODORS for the conditions listed here.
  • Oil burner sooting or dirty operation (the photo at above-left) may be due to improper adjustment of the oil burner or draft, a blocked chimney, or other problems. Also
    see DRAFT INDUCER FANS as their use may indicate a chimney, draft, or venting problem.
  • Burn marks on the boiler (photo at above right), furnace, or water heater, (these conditions may be caused by a blocked exhaust flue and inadequate venting).

    Safety Warning: burn marks on a heating boiler or furnace (shown in our page top photo) can also be due to collapse or damage to the combustion chamber liner - a serious fire hazard needing immediate attention. Notice that in our photo at right, the brown burn marks around the oil boiler combustion chamber inspection port (that rusty round door above the oil burner) have been cleaned-off.

    The presence of these burn marks does not necessarily mean that the oil burner backpressure problem remains - in this case the system has been cleaned and adjusted, but no one has re-painted the front of the boiler. But if you see fresh peeling paint or soot in such an area further investigation is needed.

    That's why we recommend that after repairing a back-pressure problem at heating equipment the service technician should clean the boiler or furnace exterior - to remove confusing debris and to make it easier to see if the problem recurs.
  • Heating equipment located in a small utility room with no provision for combustion air intake. When the service technician adjusts the system she probably worked with the utility room door open, but when the service tech left the job he may have closed the door - completely changing the availability of combustion air for the equipment. We need about one square inch of un-louvered (unobstructed) combustion air intake per 1000 btuh of the oil fired heating boiler, furnace, or water heater.

  • Increased heating system operating cost, -
  • Damage to oil burner components (backpressure heat can destroy an ignition transformer),
  • Loss of building heat - the heating system stops working,

  • Noises at the heating system when the boiler, furnace, or water heater starts, is operating, or is shutting down. See How to Diagnose Noises during Heating System & Oil Burner Operation
  • Smoke indoors,coming out of the heating equipment or its draft regulator.
    See BOILER NOISE SMOKE ODORS for the conditions listed here.
  • Carbon monoxide or combustion gas and flue gas leaks: potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous.



Also see Unsafe Air Conditioning or Heating Duct Openings which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present, and also see CARBON MONOXIDE - CO and CHIMNEY INSPECTION DIAGNOSIS REPAIR

A Guide to Heating Equipment Malfunctions & Their Implications

  • Noises & soot buildup can lead to a potentially dangerous puffback which can damage the heating equipment and blow soot and smoke throughout the building.
  • Leaks on hot water heating systems are never acceptable, anywhere.
    • Leaking pressure/temperature relief valves need immediate attention and repair (Leaking relief valve could be due to water-logged expansion tank, improper control settings (temperature too high), improper automatic water feeder operation (pressure too high), or defective valve (leaky). True, you only have to report the valve.
    • Did you miss problems at the other components?)
    • Corroded relief valves also need to be test by a service technician and repaired or replaced as they may be unsafe and may fail to open when necessary
  • Relief Valves: Do not touch the heating boiler or water heater temperature or pressure relief valve - it may open and fail to shut down. A relief valve which does not work is dangerous since the system is missing an important safety device. Should an over pressure or over temperature condition develop for other reasons, the boiler could explode.
  • Corrosion on heating system parts: Do not pick at corrosion as you may start a catastrophic, un-stoppable leak requiring total system shut-down. Leaks left un repaired can destroy heating equipment.
  • Tankless coils for making hot water:
    • If there is different water pressure/flow observed in the kitchen in which the hot water pressure is significantly less than cold, even before examining the building's mechanical systems you may speculate that a tankless coil installed may be installed.
    • What other clues suggest that the local water supply may be hard in minerals and that there may be a risk of clogged piping?
    • When the water supply is high in minerals, the hot water pipes or tankless coils clog up before cold water piping. Is a water softener installed? Is there known "hard" water--have mineral deposits clogged the coil?
    • Are there "cleanout" plumbing fittings on the tankless coil piping? Does this suggest a history of clogging and acid-flush treatments?
    • A clogged tankless coil can be repaired or replaced.
    • How severe was that rust you observed at the tankless coil mounting plate? Severe rust may mean that the boiler must be replaced.

Examples of Heating System Defects with Important Implications




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