LARGER VIEW of heating furnace parts and air flow, showing the heat exchangerFurnace Heat Exchanger Leak Tests
Heat Exchanger Testing & Safety Inspection Procedures & Standards

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Furnace heat exchanger inspection, troubleshooting, and leak testing guide: This heating system test article describes how to inspect furnace heat exchangers for leaks.

We compare and evaluate the reliability of all of the various furnace heat exchanger testing methods, we explain just how much leakage is "acceptable" by industry standards, and we conclude with recommendations for reliable heat exchanger testing and inspection.

Here photographs show clues indicating leaky, dangerous furnace heat exchangers. Furnace heat exchanger leak testing, A Complete List of Methods Used to Find or Test for Leaks in a Furnace Heat Exchanger.

13 leak detection procedures for furnace heat exchangers. Heat Exchanger Testing & Test Devices: Who's Right? Recommendations for reliable furnace heat exchanger testing procedures .

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to inspect a furnace heat exchanger for damage or leaks & carbon monoxide CO gas hazards

Photograph of furnace rust damage.Gas utility, gas furnace, & oil furnace service technicians use a variety of procedures in attempts to identify potentially dangerous leaks in furnace heat exchangers. The procedures are typically used in response to service calls for

[Click to enlarge any image]

In a literature review and a survey of utilities conducted first in the 1980's, more than thirteen different procedures [for testing furnace heat exchangers for leaks] were identified. When used alone, many of these methods (described in this article) do not give reliable results, thus often more than one method is used. Some test methods are corrosive to the heat exchanger.

Others are performed under unrealistic conditions (such as increased pressure in the heat exchanger), or are so sensitive that they indicate leakage even in new heat exchangers.

Here we provide a catalog of the various methods used to check or test furnace heat exchangers for unsafe leakage of combustion gases - a potential safety hazard in buildings. These include the text from historical articles on methods used for testing furnace heat exchangers for leaks, and the allowable or standards for heat exchanger cracks, holes, leaks, or carbon monoxide hazards from such leaks.

Visual Inspection of the Furnace Heat Exchanger can Detect Some Leaks but Not All

Photograph of rust on a furnace warning of unsafe heat exchanger.

Watch out: while visual inspection may discover obvious furnace damage and / or safety concerns, you should never rely on visual inspection alone to determine the safety of a furnace heat exchanger.

Watch out: Dangerous carbon monoxide gas leaks, potentially fatal, can be present intermittently depending on variations in heating system operation and building conditions.



Burns & soot in a heat exchanger (C) Daniel FriedmanMore about carbon monoxide - CO - is at CARBON MONOXIDE WARNING.

Those considering using instruments to test heat exchangers for leaks should review RECOMMENDATIONS for GAS LEAK TEST INSTRUMENTS & gas detector tubes for indoor gas level tests.

In response to a reader who wrote that they have a G14Q3, installed in 1988, Lennox Pulse Furnace and who was wondering if there are any visual inspections for signs of a defective heat exchanger we provide the furnace heat exchanger inspection suggestions described in this article.

The photographs shown just above are two examples of rust and flame marks on a furnace that would be a basis for further inspection of the condition of the heat exchanger for cracks, rust perforation, or other unsafe conditions.

But heat exchanger leaks can occur in a variety of locations and parts, including:

Inspection Points & Tests for Furnace Heat Exchanger Damage & Leaks

TIF 5000 refrigerant gas leak detector for halogensThe following is a catalog of all known methods for testing residential heating furnace heat exchangers for dangerous flue gas or carbon monoxide leaks. Contact us to add methods or critique. The list is arranged alphabetically rather than in any order of recommended procedure.

Watch out: SAFETY WARNING: Any evidence of furnace heat exchanger damage or of carbon monoxide or flue gas leaks should be taken seriously and those heating systems should be immediately checked by a professional.

Current industry standards for furnace heat exchanger leak testing procedures & results are given at HEAT EXCHANGER LEAK TEST STANDARDS

Testing for carbon monoxide level in the exhaust of a Tempstar condensing gas furnace (C) In Roger Hankey

Above: Minnesota home inspector Roger Han key using a combustible gas analyzer to check for CO in the exhaust flue of a Tempstar condensing furnace suspected of having heat exchanger damage. Details are at CONDENSING GAS FURNACE INSPECTION & TESTS.

Smoke test example (C) Daniel Friedman

Photograph of a rusted out furnace

Photograph of a rusted out furnace

Photograph of a rusted out furnace heat exchanger showing flame anomaly

Visual Inspection of the Furnace Heat Exchanger Alone is Unreliable for Detecting Furnace Heat Exchanger Leaks or Damage

One of the most common methods used is a visual inspection of the heat exchanger with a strong light. The serviceman must then make a judgment as to whether any cracks or holes he finds are large enough to warrant replacement of the furnace. Generally any fault is reason for recommending that the home owner obtain a second opinion of the need for replacement of a heat exchanger.

Given that a lot of the heat exchanger surface simply cannot be seen without completely disassembling the system, we would not rely on a visual inspection alone to decide if a system was damaged or not. There are other Tests using pressure testing or more commonly, tracer gas testing, that are more reliable.

Relying on gas detection instruments: Relying on gas detection instruments alone, without a visual inspection of the system is also dangerous and can falsely indicate that no problem in present when in fact the heating system is unsafe.

We discuss the reasons for this at GAS DETECTOR WARNINGS - about relying on instruments for detection of hazardous gases in buildings.

Install CO detectors in buildings, as well as smoke detectors. Ultimately the combination of expert inspection, testing, and the use of carbon monoxide detectors and smoke detectors will make a significant improvement in the safety of any home heating system.

Smells and odors in the building could be an indicator of heat exchanger leaks - see DUCT & AIR HANDLER ODORS.

Chemical Smoke Tests of Furnace Heat Exchangers

Smoke test example (C) Daniel FriedmanThe American National Standards for Gas-Fired Central Furnaces (Z21.47-1978) outlines a test method using a fuming or smoking material such as titanium tetrachloride.

The material is introduced into the combustion chamber, and if combustion products are discharged through door cracks or other openings, their presence will be revealed by observing the smoke. A drawback to this method is that titanium tetrachloride is very corrosive.

Another widely-used procedure involves spraying a sodium salt solution into the burner flames and checking the circulating air stream with a propane torch for the presence of sodium ions. [Detection of Cracked Heat Exchangers in Warm Air Furnaces , J.F. Wunderlin, Wisconsin Gas Co., GV-6, June 1978.]

If the blue flame of the torch turns yellow, this indicates the presence of sodium ions and a leak. As the solution is sprayed into the burner flame, visual inspection of the heat exchanger surface below the burner port is needed.

If a solution of sodium chloride (table salt) is used, corrosion of the heat exchanger can be accelerated. Sodium bicarbonate salt solutions, on the other hand, are non-corrosive. The drawbacks to the sodium ion tracing method are that dust in the air can be mistaken for the sodium ion and acceptable leakages may be detected as the test is very sensitive.

A procedure similar to the sodium ion tracing is to trace the lithium ion, with the same drawbacks.

All of these methods have some undesirable drawbacks. The ideal method for detecting the leakage of flue gases into the circulating air stream would not be harmful to the furnace components of the home environment, would be performed under realistic operating conditions, and would not detect small amounts of acceptable leakage.

Smoke Bombs for Heat Exchanger Tests

The use of smoke bombs is the second most common practice. With the flue outlet and burner access opening blocked a smoke bomb or candle is ignited in the heat exchanger.

Observations for smoke are then made in the circulating air side of the heat exchanger. When the bomb explodes a positive pressure is created inside the heat exchanger forcing the smoke out of very small cracks.

This makes the test very sensitive. Precautions are also needed with this method so the smoke does not get into the house where it may stain certain paints, fabrics, and tiles.

An air analysis test may be used in addition to the smoke test. This test detects combustion products in the circulating air stream as an indirect indication of heat exchanger cracks. If high readings are obtained there is no doubt that the heat exchanger has a crack or fault.

With slight increases a more thorough examination of the heat exchanger may be required. The drawbacks of this method are that it is complicated and time consuming to perform. Also the detection device is expensive.

Editor's note: See Matzen and other articles [below] for an evaluation and survey of this equipment. Significant to Matzen et al, DeWerth does not say these tests are necessarily invalid. Odorants, such as sulphur candles and oil of wintergreen have been used for detecting leaks.

A small quantity of the odorant is introduced into the combustion side of the heat exchanger; the hot air registers in the house are then checked for the characteristic odor. Any odor in the circulating air would indicate a heat exchanger leak.

With the sulphur candle this test can be quite reliable, but very unpleasant smelling. The wintergreen is more pleasant, but the odor clings to the serviceman's hands and clothes, making the test unreliable.


Furnace Heat Exchanger Pressure Measurements for Leak Detection

The pressure drop across the furnace heat exchanger surface has a significant effect on the tendency of flue gases to pass from one side of the exchanger to the other. Only if the flue gas side is more positive than the circulating air side will there be a tendency for flue gases to leak into the circulating air through any cracks or corrosion holes in the heat exchanger.

In order to determine the actual operating pressure inside and outside the heat exchanger, measurements were made with four furnaces with typical heat exchangers. ["A Simple Test That You Can Use to Check a Furnace for Leaks," American Artisan, September 1966.]

Two of the furnaces were equipped with atmospheric burners, one with an induced draft system, and the fourth with a power burner. Flue gas pressure taps were attached at two-inch intervals up the side of the heat exchanger from the burner port to the flue outlet. Circulating air taps were located adjacent to the flue gas taps.

Pressure measurements were taken with and without the circulating air blower in operation. The results showed that with atmospheric burners the average pressure on the flue gas side with the burners operating is about 0.02 inch water column (w.c.).

The average pressure on the circulating air side of the heat exchanger is 0.3 inches w.c. when the blower is operating. Thus the air side of the heat exchanger is more positive than the flue gas side when the circulating blower is on. If there were a hole leakage would be from the air side to the flue gas side.

When the blower is off the air side pressure is less positive than the flue gas side and leakage would occur from the flue gas side to the air side. This condition exists only during the short heat up period before the blower starts.

An induced draft system has a negative pressure of about 0.2 inch w.c. or less on the flue gas side of the heat exchanger at all times. The flue products are drawn through the combustion chamber causing the flue gas side to always be under a negative pressure. A positive pressure on the air side of the heat exchanger, caused by the circulating air blower, further insures that the flue gases stay in the combustion chamber.

With a power burner system the flue gas side of the heat exchanger must have a positive pressure of as much as 0.4 inch w.c. Thus, the pressure on the flue gas side is more positive than the air side, whether the circulating air blower is on or off. The only exception is the top of the heat exchanger where the circulating air blower may impinge directly onto the heat exchanger.

Air side pressure would be more positive than the flue gas side at these points. If there were a hole or crack, flue gases would leak into the circulating air stream at all times except if the hole or crack were at the top of the heat exchanger.

Table 1 summarizes the various systems and the leakage potential due to the pressure drop across the heat exchanger. In a furnace with atmospheric burners flue gases could only leak into the circulating air system if there were a hole or crack during the time the blower was not operating.

Induced draft furnaces would not allow any leakage through the heat exchanger as they operate under a negative pressure. A power burner system is more critical in that the flue gas side of the heat exchanger is always more positive than the circulating air side when the furnace is operating. Thus, if there were a hole or crack flue gases could leak into the circulating air stream.

This analysis of flue gas leakage based on the pressure difference across the heat exchanger is true for relatively small holes. For heat exchangers with large perforations flue gases may leak regardless of the pressure differential. Large perforations would be detected by visual observation of the heat exchanger or by the flame pattern. No further test procedure would then be needed.

Test methods should not subject the heat exchanger to abnormally high pressures, and should not detect insignificant leakage which may occur from pin holes in welded seams or other manufacturing imperfections.

Current Standards & Procedures for Residential Furnace Heat Exchanger Inspection, Testing

Heater Combustion Safety & Effieciency Testing U.S. DOE at InspectApedia.comIllustration: adapted from "Combustion Appliance Safety & Efficiency Testing", U.S. DOE, cited below. [Click to enlarge any image]


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