Heating Service Tips
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, but it has to be done.
The heat exchanger, transfers heat from the hot flue gas to your home's indoor air through the heat exchanger. Everytime the furnace heats and cools the heat exchanger will expand and contract a little and eventually cause metal fatigue and split. Just like when you take a piece of metal and bend it back and forth several times, it will eventually crack and break off. The heat exchanger, venting and safely controls and design to protect you from the products of combustion.
The hot flue gas contains the products of combustion, which are normally harmless, but under the right circumstances can have a serious health effects. In the worst case sernerio, it can lull everyone in the household into a permanent sleep. CO is colourless, and ordourless and works surprisily fast. 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.
This is the worst case sernerio and is unlikely to ever happen, but it can and does happen when the conditions are right. There are few supporting statistics on CO poisoning from heating system. Statistic in Canada and the US are mixed with suicides, automobile, and manufacturing incidents, making it difficult to identify how many actually occurred from a malfunctioning heating system. There are 3 thing needed, but they can all be a result of cracked heat exchanger.
* Failure in the flue gas venting or heat exchanger
* Poor Combustion, resulting in High CO production
* Poor Ventilation in the building or area. Lack of Fresh Air
Cracks or holes in the heat exchanger in small residential furnaces and rooftops cannot be repaired for long term and even short term repairs are likely to cost more than a new unit. It maybe possible to seal the hole temporarily until the unit can be replaced, but the metal in the heat exchanger is usually too thin to weld, impossible to access even with removing it completely, and usually for every hole or crack that can be seen there will be two more that your can't see. Heat exchangers 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 should be replaced, weather you can see a crack or not.
Patches or sealant can be used for emergency if there is a danger of freezing, but the flame must be burning cleaning and sufficient ventilation must be present before attempting to use it. Use of a good CO detector will practically eliminate any threat to life. The CO detector is set go off a 15PPM to 25 PPM and will not detect NOX. You want the CO level to be 0, short term exposure at low levels of CO is said to be harmless but long term exposure a low levels can be just as bad as a short term exposure at higher levels.
A crack can cause the pressures inside the heat exchanger to change effecting how it burns. When the supply fan comes on it can blow air onto the burners or suck flue gas out of the heat exchanger This effect will get worst as the furnace heats up from expansion and the crack gets bigger. This can cause the burners to produce high CO as high a 2500 PPM and provide the 2nd requirement to CO poisoning. High efficecnicy gas furnaces are especially bad for this, due to the tight air/gas ratio used.
When a flame burns poorly, in produces Carbon Monoxide, when it burns very poorly it will produce free carbon or soot. This can quickly block off passages inside the heat exchanger and block the venting of the flue gases. No longer venting up the chimney, but directly into the house. This is the third requirement for CO Poisoning, and unless the front wall is missing from your house or there are some big fans blowing in fresh air directly from outside, the CO level in the house will reach lethal levels.
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.