As long as a gas furnace is in good shape and working correctly, it offers a safe and highly effective method for heating a home or business. On the other hand, if a gas furnace is in poor condition, it can become a major health and safety hazard. The heat exchanger is one of the most essential components for ensuring a furnace can heat safely. In this article, we’ll discuss what role the heat exchanger serves in the heating process and why ensuring your furnace’s heat exchange remains in good shape is so important.
How a Heat Exchanger Allows a Gas Furnace to Safely Heat
The heat exchanger is one of the most important parts of a gas furnace since it’s what works to transfer the heat produced by the gas burners to the air flowing through the furnace. It consists of multiple metal tubes connecting one end to the combustion chamber and the other to the exhaust flue or chimney. When a gas furnace runs, its draft inducer continually draws the hot combustion fumes from the burner flames up through the heat exchanger tubes and then eventually into the flue so they get safely vented outside.
As the fumes flow through the heat exchanger, the metal tubes absorb a large amount of heat from the fumes. This causes the heat exchanger to get extremely hot quickly. Once it reaches a specific temperature, the furnace’s low-temperature limit switch activates, signaling the blower to run and pulling air into the furnace through the return ducts. All the heat radiating off the heat exchanger ensures that the air flowing over the exchanger increases to at least 120 to 140 degrees before the air exits the furnace and circulates the building through the supply ducts.
An important thing to note is that there is an airtight seal where the heat exchanger connects to the combustion chamber and where it connects to the exhaust flue. This is important for ensuring that the combustion fumes the burning gas produces are contained. That means the fumes can’t escape into the ductwork, where they would then get blown around the house along with the hot air produced by the furnace.
Why a Damaged Heat Exchanger Is Such a Concern
The biggest issue that a gas furnace can have is if the heat exchanger develops a hole or any of the weld joints that connect the metal tubes to the combustion chamber or the flue ends up cracking. Any holes or cracks will allow some combustion fumes to escape and get into the home’s supply ductwork. The main risk in this regard is that it creates a potential for carbon monoxide poisoning.
Gas furnaces only produce carbon monoxide if the gas flowing out of the burners doesn’t burn cleanly, which results in it not fully combusting. Under normal circumstances, carbon dioxide and water vapor are the only byproducts produced when natural gas burns. If the gas doesn’t fully combust due to the flames not receiving sufficient oxygen or because the burners are overly dirty, you’ll typically also end up with carbon monoxide being produced as well. This usually isn’t an issue if the heat exchanger is maintained.
The draft inducer fan will continually draw all the carbon monoxide straight through the metal tubes and ensure it is safely vented outside, where it immediately disperses. If the heat exchanger is damaged, you can end up with carbon monoxide escaping and getting pumped out into every part of the house through the supply ducts and vents.
What Does It Mean if Your Furnace Gets “Red-Tagged?”
If your furnace is serviced annually as it should be, a technician should always inspect the heat exchanger to ensure it’s still fully sealed and not damaged. They’ll also provide the furnace venting the combustion fumes outside and check everything else to ensure it is safe to operate. If your furnace has a damaged or defective heat exchanger or any other issues that pose an immediate risk to your health or safety, the technician must red-tag your furnace. This means they put a label or tag on the furnace stating that it cannot be safely used again until it’s been repaired or replaced.
In most cases, the technician is also legally required to contact your gas company when they red-tag your furnace. The gas company will immediately shut off your home’s gas supply so that you don’t try to run your furnace until you have it fixed or replaced. Some places have Type A and Type B red tags. A Type A tag is applied when the furnace poses an immediate risk and is unsafe to run until repaired or replaced.
A Type B tag is for when the furnace has an issue that doesn’t pose an immediate risk but is still serious enough to be repaired or replaced relatively soon. In this case, the technician will still have to notify your gas company that they put a Type B red tag on your furnace, and you’ll usually have 30 days to get your furnace repaired or replace it with a new unit. If you fail to do so within the required time, the utility company will still shut off the gas to your home.
What Can Cause a Damaged or Cracked Heat Exchanger?
Heat exchangers can crack or get damaged for several different reasons. In some cases, it simply happens due to age and normal wear and tear, which there isn’t anything you can do to prevent. It can also occur because the heat exchanger or furnace has some type of manufacturing defect, leading to the heat exchanger cracking or getting damaged. Most furnace manufacturers warranty the heat exchanger on the units they produce for a minimum of 10 years, and some provide a 15-year warranty.
If your heat exchanger fails because of a defect during the warranty period, the manufacturer should provide a new one at no cost. You’ll usually still have to pay the labor costs of replacing the heat exchanger, but the warranty will save you some serious money. One of the most common reasons a heat exchanger cracks is that the furnace frequently overheats. Overheating can happen when insufficient air is pulled into the furnace because you failed to change the air filter and continued running your furnace with an extremely dirty filter.
A furnace can sometimes overheat because the blower isn’t functioning correctly and can’t pull enough air in. Overheating is such an issue for a heat exchanger because it causes the metal to expand much more than it usually would. The metal expands a bit when it gets hot and then again contracts after the furnace shuts off, and everything cools down. Nonetheless, excessive expansion and contraction that results from overheating can weaken the metal or the weld joints and lead to cracks or holes forming.
The warranty won’t cover anything if your heat exchanger fails due to overheating or a lack of maintenance. In this case, your best option will almost always be to install a new furnace due to the extremely high cost and difficulty of replacing the heat exchanger.
Since 1965, Marr's Heating and Air Conditioning has provided homeowners in Bellingham, WA, with the best heating and air conditioning services. We specialize in installation, repairs, and maintenance to ensure your HVAC works properly and remains safe. Contact us today to schedule an inspection and furnace tune-up so you can ensure your furnace is safe to use this winter. We also offer heat pump installation!
