Cracked Heat Exchanger
This is one I always hated telling the customer, "You have a cracked heat exchanger and your furnace has to be replaced, today." Unless you are one of the top 3% who have all the money, replacing the furnace is going to hurt the pocket book if you have not planed for it, but it has to be done.
Corrosion can also cause the heat exchanger to leak. The corrosion normally comes from the flue gases when allowed to cool and condense inside the heat exchanger. The condensate is very corrosive to metal from the carbonic acids. Rust can work its way through the metal surfaces from the interior out. Standing pilot flames a known to cause interior rusting because the gases from the flame condense inside the heat exchanger. During the summer if the pilot is left on the pilot will rust out the furnace. The pilot does not get hot enough to dry out the heat exchanger and the main flame does not come until the cooler weather. Small rust hole can be very hard to find but they can leak large amounts of gas. Testing the heat exchanger with sulphur or a smoke bomb can relieve leaks in the heat exchanger that cannot be seen from an external inspection. The hot flue gas products of combustion are mostly harmless. Natural gas and propane produce mostly CO2, nitrogen and water when burning correctly. These are harmless unless they build up. CO2 level up to 2500 PPM may cause a headache but are not toxic. Most burner even in good running order will produce small amounts of CO and NOX. CO (Carbon Monoxide) "The Silent Killer" is toxic. If the burners are rusted or dirty and not burning properly it can produce lethal levels of CO 2000PPM+ in the flue gas. Even when the burners are in good shape, the flue gas contains NOX, which affects your lungs in very small amounts, 1/2 PPM or 500 PPB. That is why the furnace must be shut off if the heat exchanger has failed. Under the right circumstances a faulty heat exchanger can have serious health effects. In the worst-case scenarios, the CO has lulled everyone in the household into a permanent sleep. This still happens far too often. CO is colourless, and odourless and works very fast in high concentration. A concentration of 1500 PPM can kill in 15 minutes and you may not notice the effects until it's too late. This makes the furnace the most dangerous appliance you will ever own. When was the last time your dishwasher threaten you and all your family. If the heat exchanger is leaking that does not mean immediate danger, but because of the risk, the furnace need to be shut off. In order for the worst to happen, it normally requires a combination of problems * Failure in the flue gas venting or heat exchanger leaking flue gas into the supply air. * Poor Combustion from the burner, resulting in High CO production * Poor fresh air ventilation in the building or area allowing the CO to build up. Cracks or holes in the heat exchanger in small residential furnaces and rooftops cannot be repaired. On larger industrial equipment some heat exchangers can be welded safely to seal any leaks but it is a difficult job to do. The repair requires a double pass pressure welding to seal the leaks, which is not always possible due to access to the repair area. On commercial equipment and in building where there are no living quarters gas heating equipment can be left running with cracks in the heat exchanger while waiting replacement as long as no CO is detected in the air. If the equipment is leaking CO then the gas must be shut off immediately. We have used muffler cement as a short-term repair to reduce leakage until the system can be replaced. A long-term repair for commercial heat exchangers is possible but not very practical. It would cost more than a new unit or heat exchanger would cost. The metal in the heat exchanger is usually too thin to weld and impossible to access with welding equipment even with removing the heat exchanger completely. Heat exchangers will fail from age, hours of operation, and from operating conditions. A properly maintained system will last 20 years or more, but anything past 25 years is on borrowed time and should be inspected annually and tested for CO. When the heat exchanger fails the air pressure and venting inside the combustion chamber changes. Any changes to air pressure inside the heat exchanger or air patterns can have large effect how clean the flame burns. This effect will get worst as the furnace heats up from expansion and the crack gets bigger. Depending on where the crack is, the supply fan creates a positive pressure on the outside of the heat exchanger that can blow air inside the heat exchanger and on to the flame. We find some cracked heat exchangers from the pilot lights that are blown out. A negative pressure from the fan can suck flue gases out of the heat exchanger at an accelerated rate. Negative and positive pressure can be created on the heat exchanger weather the fan is upstream or downstream of the heat exchanger. If your heat exchanger is cracked, then replace your furnace ASAP. It may be possible to still run your furnace, but precautions must be taken first to ensure the safely of the occupants. * Test for CO in the supply air, in and around the furnace burners and venting. Any CO detected here and the furnace should not be used under any circumstance * Test the flue gas for CO. If readings exceed 50PPM CO then it should not be used. * Ensure that there is a source of fresh air for the building ventilation and proper combustion air supply for the burner. * Use a good quality CO detector to monitor area.
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