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In the last entry I made I looked at how not to find cracks in a heat exchanger. There are ways to find cracks in heat exchangers that do work and can be learned with a little bit of skill and knowledge of where to look. Without a doubt, the only surefire and 100% best way to find a crack is visually. This way there can be no doubt. Although I don’t condone sticking your face in front of a furnace to watch the flame for disturbances, there are other ways to determine this.

The main problem with a visual inspection is access to the heat exchanger itself. Most gas furnaces have an evaporator coil that sits directly on top of them making inspection of the heat exchanger from the top extremely difficult. On the lower side of the gas furnace is the blower compartment which also creates just as much difficulty in gaining access. If the furnace is a 90% or condensing furnace, there will be a large secondary heat exchanger in the way that will block your view of the primary heat exchanger altogether. Let’s look at some of the most common test methods being used that may actually find a cracked heat exchanger.

Mirrors and Flashlights

One method that has been used for decades is the practice of using a mirror and flashlight to visually locate cracks in a heat exchanger. Depending on the type of furnace, this inspection was typically performed by removing the burners and using an inspection mirror or strips of mirror placed back inside the bottom of the heat exchanger. Once in place, a flashlight was used to help visually locate any potential cracks that may be in the heat exchanger.

This method is not without its drawbacks though, as it is dependent on the ability of the one inspecting to be able to see all the surfaces of the heat exchanger. Due to baffling and contours in heat exchanger designs it is nearly impossible to be 100% sure that the entire surface of the heat exchanger is crack free due to this method. The one inspecting might be able to get a good look at much of the heat exchanger but there may be areas that cannot be seen.


Another method that has been around for decades is hydro-testing of heat exchangers. The heat exchanger is removed from the furnace and filled up with water to see if any leaks out of the heat exchanger. Any leaking water is deemed to indicate a crack in the heat exchanger. Sounds good right?

This method isn’t recommended as it will typically show up cracks in a newer heat exchanger that are designed to be there. Yes that’s right, cracks by design. When furnace designs changed over from natural draft to induced draft (80%) and condensing furnaces (90%) so did the heat exchanger designs. The newer designs are no longer welded and sealed like the older designs were. Instead, they are mechanically fastened by crimps and flanges so a small amount of leakage in these designs is normal. This method may work on an older heat exchanger designs, but I would recommend avoiding it due to the possible side effects it may create by dumping water into a heat exchanger.

Video Inspection Cameras

Inspection cameras take the visual inspection to another level. Certain camera manufacturers make small video cameras for heat exchanger inspection that will fit into areas that cannot be seen by the naked eye and an inspection mirror. This allows for a more thorough inspection due to more angles that can be viewed from. Depending on make and model, the ability to save the image of a cracked heat exchanger is also available to show a homeowner.

Visible Defects

A newer testing method, Visible Defects is a heat exchanger inspection method that uses a concept similar to one found in detecting refrigeration leaks. Ultra-violet dye which causes an illuminating effect and a super small video inspection camera allow a technician a means of covering more surface of the heat exchanger. Due to its design, it can find small cracks that may be hidden to a video camera alone.

Visible Defects uses a bright green, florescent dye to indicate where a crack is located. A brass spray nozzle is inserted into the air side of the heat exchanger and covered with the dye. The dye is then lit up with a UV light making any existing cracks easy to find.

Pressure Testing

The use of pressure testing was made popular when Lennox started having issues out of their Pulse furnaces. Variations of this test can be used to check the integrity of induced draft and condensing furnace heat exchangers.

The inlet and exhaust openings in the heat exchanger are sealed off. Once completed a micromanometer or draft gauge like a Dwyer 460 is used to check for a change in heat exchanger pressure once the fan in the furnace is turned on. If the pressure goes up in the heat exchanger, this could indicate a crack or hole in the heat exchanger.

One drawback to this test is that leaky ductwork or windy conditions could also interfere with this test making the results suspect.

Flue Gas Testing

One of the most common tests used by technicians is to see if the carbon monoxide or oxygen numbers change in the flue gas of a furnace once the fan in the furnace energizes.

It is assumed that if the CO and/or O2 numbers begin to change that the heat exchanger is cracked. While this is a possibility, it could also be a sign of interference due to various other factors such as duct leakage and equipment leakage. Additional testing would be needed to determine if other conditions were causing the interaction in this test.

Another drawback is that a small crack may not be large enough to have any noticeable impact on the flue gas readings. In this case a crack may be present but it isn’t severe enough to impact the flue gas readings.

AHRI 2009 Guidelines for Induced Draft Heat Exchanger Inspection

In 2009 AHRI released Guidelines for induced draft heat exchanger inspection that you can download here. This document was and is referred to by many equipment manufacturers in the diagnosis of potential heat exchanger issues in their equipment as various recalls were issued over the years.

I’m not going to condone or beat up any part of this document. Instead, you can view it and come to a conclusion yourself based on other entries I’ve made in the past. Anyone who has been to a CO class from Jim Davis knows that there are many more things besides a crack in a heat exchanger that will cause CO readings in the flue gas of a furnace to be over 200 ppm.

If further investigation is necessary according to section 4.5 of this document to determine the source of the CO problem what would you do next? Kind of open ended isn’t it? Most don’t know where to go from there.

Ultimately, there is no silver bullet when it comes to finding cracks in heat exchangers. Experience and knowledge of particular equipment will help you know where the flaws typically occur. Just be sure that in the hunt for cracked heat exchangers, you don’t overlook a water heater in the corner producing thousands of parts per million of carbon monoxide waiting to cause a problem.