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[size=3]Types of Heat Exchanger Tests
There are three basic types of heat exchangers: clamshell, Serpentine, and tubular. This article is going to focus solely on the clamshell because it is most commonly found on the older models (typically installed before 1990) and is still in use today. The Serpentine & tubular exchangers are common on a portion of the newer furnaces (mid '80’s - today) and different tests should be applied to these.
Leak Test: **
** For the clamshell heat exchanger, the most accurate test is the leak test. **This test involves spraying the outside of the heat exchanger with a water-surfactant solution and then looking on the inside to see if it has leaked through. **If it has, anyone conducting this test is 100% certain a crack exists. This test will find 95%+ of all cracks in the clamshell heat exchangers.
For a clamshell heat exchanger, a visual only inspection is the most ineffective test. Some companies try to fool you by informing you they use “state-of-the-art” video camera systems to look for cracks. What they are admitting is they do not have proper training to inspect your heat exchanger. Using the expensive camera or mirrors, these companies will only find about 10% of the actual cracks. Why will they fail to find most of the cracks? Most furnaces at this stage of their lives have rust or soot buildup on the inside of the heat exchanger preventing anybody from seeing the cracks. Couple this with the fact that CO gas can seep through cracks not visible by the human eye and you can see their shortcomings.
CO Gas Test:
If conducted alone, the CO gas test is another test where the HVAC company is admitting to you that they do not have proper training to inspect heat exchangers. This test has nothing to do with inspecting the condition of the metal of the heat exchanger. This test consists of boring a small hole above the plenum and inserting a carbon monoxide detector. The only useful information this test tells you is if the furnace is currently blowing carbon monoxide gas throughout the home. If they fail to let the furnace run long enough, the crack may not widen to allow the CO gas to leak out. Also, conditions have to be just right for CO gas to be produced. Unless the flame is burning inefficiently and is finding its way through the crack at that specific time when the test is conducted, their CO detector may never register any levels. If a crack is found using this test, chances are a crack existed in this furnace for almost 2 years! That is a long time to chance the safety of the occupants of the home!!!
This test consists of setting smoke canisters inside the heat exchanger and seeing if the smoke leaks to the outside. Most companies that were conducting the smoke test in the past have graduated up to the leak test for the clamshells. The main reasons are the leak test is faster, has a higher probability of finding a crack, and does not set off the smoke detectors. One of the downsides to the smoke test, as with the CO Gas test, is if the heat exchanger is not warmed up enough, the test may not find the crack.