Visual method. Sometimes a crack or hole can be found in a heat exchanger by looking visually with the naked eye. You may look into the heat exchanger cells with the burners in place or the burners removed. A much better view can be achieved with the burners removed. Remember, metal expands when it is heated. Of course when your furnace is running, it is in an expanded state. Any cracks in the heat exchanger will "open up" larger when heated. With that being the case, you would be more successful in discovering a bad heat exchanger when it is first heated with the burners up to operating temperature. Telescoping inspection mirrors can be used to view more surface areas. Some people like to purchase strips of scrap mirrors one inch wide by twenty inches long to slide into the cells. Even with mirrors, a technician's view will be extremely limited using this method.
Light method. Another way to view a heat exchanger is to slide a powerful light (long fluorescent trouble lights work well with this method) into each heat exchanger cell while looking through the plenum or blower opening. This, of course, requires the removal of the blower or an opening to be made in the plenum (preferably both) so that more heat exchanger surface area can be viewed. With the light illuminating the interior of each heat exchanger cell independently, visual observance of any light present will indicate a bad heat exchanger.
Flame pattern method. A technician will sometimes watch the flame pattern to see if it changes shape or color when the blower turns on. If there is a hole or crack, the theory is that it will be discovered, however, many times there will be a hole or crack and it will be in an area where it won't affect the flame pattern. It is also common to see a flame pattern change color from the cooling effect of the blower when it turns on, which cools off the heat exchanger. This flame color change occurs due to the draft temperature suddenly changing. Rusted heat exchangers can also change the flame color or shape due to rust particles falling into the flame when the heat exchanger expands when it is heated or contracting when it is cooled from the blower turning on. As you can see, unless a heat exchanger has major failure, this method is not as reliable or thorough a method to use as other sources do cause flame pattern changes.
Smoke method. Ah yes, the old smoke method. A smoke bomb is placed into each heat exchanger cell, and the registers are checked for presence of smoke being emitted. This method is not very effective because the heat exchanger cells are under a lower air pressure than the blower side of the system. Even if a large hole was present, the likelihood of smoke moving into the air stream is highly unlikely.
Spray method. Some companies produce a spray, which is to be sprayed into a blower compartment while the furnace is running. The idea is that some of the spray will be forced into the burner area through any cracks or holes in the heat exchanger. When the spray comes into contact with the flame, the flame color will change. Obviously, you wouldn't want to spray into a blower compartment that has an open type blower motor or you would then be seeing the July fourth furnace fireworks when the blower motor gets shorted out.
Sulfur method. This method is similar to the smoke method except a sulfur stick is placed into the heat exchanger cells and ignited. If any sulfur is smelled in the rooms, it would indicate a bad heat exchanger.
Torch method. This method involves the use of salt along with a torch. The salt is thrown into the burner flames and a torch is taken around to each register vent to see if its flame color is changed by any salt that was sent into the air stream from holes or cracks in the heat exchanger. This is a very ineffective and dangerous method.
Oil or water method. This method requires the removal of the blower and sometimes an opening to be made in the plenum above the heat exchanger. A sprayer, such as a coil or insecticide type sprayer, is used. It is filled with very light oil, WD 40, or with water. The heat exchanger is sprayed through the blower opening, plenum opening, or both. If any water or oil were discovered inside the heat exchanger after spraying, it would indicate a leak.
Inspection camera method. This method involves using an inspection camera by inserting it into each heat exchanger cell to view the condition of the cells on a monitor. Many cameras can be used with a VCR to record the findings. Cameras come in black and white or color.
Refrigerant gas method. A non-ozone depleting refrigerant gas is injected into each heat exchanger cell. A refrigerant leak detector is then used to check for traces of gas in the blower compartment and the supply plenum area. It is important to check with EPA regulations to insure that the refrigerant you are using is allowed for this purpose.
Tracer gas method. A tracer gas consisting of 14.3% non-odorized methane gas is injected into each heat exchanger cell. A combustible gas leak detector is then inserted into a plenum opening and blower compartment to detect any traces of methane gas.
Combustion analysis method. A combustion analyzer is used to sample flue gas levels of oxygen. If the oxygen level increases when the blower turns on, it may indicate that air is being forced into the heat exchanger cell areas. Some contractors like to use a combustion analyzer that has a printer. A print out of the sample pre-blower and post-blower flue gas readings can be used to document the conditions to show to the furnace owner.
These 12 methods of finding bad heat exchangers are just some of the methods of finding bad heat exchangers. There are many more. Also, many heat exchangers are bad even though they do not leak and should be replaced. After 30 years as a contractor, what method or methods have I found most effective? This information and more is available in the audio training tape entitled "Finding Bad Heat Exchangers."