Secondary Heat Exchanger: The Unsung Furnace Hero Slashing Your Bills

Okay, you want to discuss the secondary heat exchanger. Are you the type to snooze through your furnace (?) sounding like it’s having a temper tantrum, your energy bills skyrocketing, and if we’re really unlucky, there’s some weird, funky smells going on? Likely, that one frequently-ignored, super-important piece of your high-efficiency furnace is the culprit. It is a bit like the unsung hero that only really gains attention when it begins to break down. But when it does, it’s not just an issue of comfort; it’s a safety concern, as well.

The secondary heat exchanger is a key part of many high-efficiency furnacesIt has a quiet presence in the long-term heating of your home efficiency. When it’s going right, you don’t even realize it’s there. But when it’s not, you need to pay attention: a faulty one can be quite dangerous to your health and home.

The Furnace Secondary Heat Exchanger Deconstructed: Your Home’s Secret Efficiency Weapon

A secondary heat exchanger. Most folks don’t deal with their furnace’s secondary heat exchanger until it goes out. When your furnace is working well, it’s easy to forget about this key piece of equipment. But the thing is, this part is a massive feature of your furnace and it essentially requires servicing. Heat exchangers can become worn and develop holes or cracks, just like any other part, with age and use.

Which brings me back to my original question: What does the secondary heat exchanger do, exactly? Its central purpose is the safe exchange of heat, and that is essential for, say, space heating. In furnaces, a heat exchanger is usually a bundle of tubes, joined together by tube plates to form a cylindrical shell, internally, of metal through which hot gases are sent so as to transfer heat to cooler gases in another metal structure. When your furnace combusts natural gas or propane, the gas travels around the heat exchanger. The super-hot flue gas warms the metal and, as it does so, the heated metal in turn warms the air that circulates around and outside the heat exchanger. It’s actually a fairly smart system, if you think about it.

How Secondary Heat Exchangers Increase the Efficiency of Your Furnace

If you’re armed with a high-efficiency furnace, you have two heat exchangers: one main and one secondary. Think of them as a cooperative tag team, who work together to wring every last drop of warmth from your fuel.

• The Primary Heat Exchanger’s Gig: This is where the first ‘heat exchange’ takes place. It carries off the warmest flue gases, right near the gas rings of your furnace. This extreme heat and flame puts a lot of stress on the metal, and over time it can cause cracks to develop. Less efficient furnaces, such as 70% efficient ones, have this one major heat exchanger.
• The Secondary Heat Exchanger’s Special Job: Here’s where the magic of efficiency starts showing off. Those combustion gases aren’t simply vented out after they exit the primary heat exchanger. They instead pounded the secondary heat exchanger. This is where that remaining heat from the exhaust gases is trapped.
• The Condensation Process: The Efficiency Cheat CodeAs the exhaust gases cool even further inside this secondary heat exchanger they begin to condense into water vapour. This transformation from gas to liquid gives off what’s known as latent heat. Then that additional heat goes back into the radiant system. This entire condensing process is the reason these are called condensing furnaces. It’s an ingenious strategy because your furnace draws more heat out of the fuel, which dramatically increases the furnace’s efficiency and reduces energy waste.
• Built to Last (Sort of): Additional heat exchangers are usually stainless steel or coated steel. Why these materials? Because they have to be able to withstand heat, moisture, and even possible acidic conditions caused by that process of condensation.

The Things You Need to know about High-Efficiency Furnaces

Upgrading to a high-efficiency furnace is about more than just displaying fancy tech; it’s a wise investment for your wallet and your peace of mind.

• Major Bump In Fuel Efficiency: If you’re looking at those with furnaces of 90%+ AFUE (Annual Fuel Utilization Efficiency) values, that is. That means they’re getting 90% or more of the fuel that they burn out as useful heat, rather than just sending it up the flue. By comparison, standard 80% furnaces send 20% of their heat up the flue. That’s essentially like tossing money out the window.
• Big Energy Savings: More heat is pulled out of the fuel; that results is drastically reduced energy bills. Some estimates are even as high as 20% energy savings over traditional furnaces. Who wouldn’t take that kind of saving?
• Greener on the Earth: Lower energy consumption means creating a smaller carbon footprint. It’s a win-win.
• Better Home Security: This is a major one. 9 Those 90% furnaces are seen as a safer option, as the system’s burning and combustion is sealed with outside air being used for burning, thereby leading the flue gases outside the house. This design discourages potentially lethal carbon monoxide (CO) from seeping into your home. Even so, it’s still a good idea to have a carbon monoxide alarm in your home. CO, as we have seen is an invisible, odorless killer, so you can never be too cautious.

Warnings: Is Your Second Heat Exchanger Crying For Help?

Alright, let’s get real. How can you tell if your secondary heat exchanger is melting down? Because a damaged one can put your home and health at risk. Look for these warning signals:

• Funky Odours: Ever come home to a funky smell? A damaged heat exchanger can produce a smell that is similar to formaldehyde, or even to pickles. If your nose is saying that something stinks, you’re asking the right questions.
• Soot: Notice any white soot around the gas rings of your furnace? That’s not a good sign. Soot is a sign that your furnace isn’t completely burning the natural gas and that your secondary heat exchanger is in danger of getting clogged.
• Obvious damage or leaks: Perform a visual check. This is where you’ll check for cracks or damage, particularly around the inducer motor; if you see either, it’s time to bring in the pros. It is a big issue, cracking. The panels are also hugely suspect if they are beginning to show signs of rust and corrosion.
• Change in Flame Colour: A good furnace flame is usually blue. If your own is yellow or orange, that’s a sign of a gas mixture problem, and a possible issue with the heat exchanger. You can see this a lot even when you’re not trying too hard.
• Spooky sounds: If your furnace is making noises like it’s haunted, pay attention. If a heat exchanger is broken, you will probably hear a rattling noise as it turns on (yes, this will also be a reference to it being louder than normal). Because it’s metal, and as it heats up, those cracks can grow and contract, causing banging or popping sounds.
• Carbon Monoxide Detector Going Off: This is the O.G., and you’ll want to take it seriously. CO is an odorless, tasteless and deadly gas. A cracked heat exchanger can cause CO to seep into your home. Though CO alarms are your friend here, don’t count on them to capture every instance of failure. If that’s what it sounds like, turn your furnace off, air out your house and call an HVAC technician right away.
• Your Furnace Keeps Cycling: Are you noticing your furnace is cycling on and off shorter intervals? This “short cycling” could be a sign of inefficiency with heat exchanger and inadequate heating demand.
• Mysterious High Bills for Heat: If your heating costs jump for no reason (because you haven’t touched your thermostat), that means it’s likely that your furnace is working too hard to function (and is doing so at a diminished output), often because its heat exchanger is conked out or struggling.
• Too much Condensation or Water: See any abnormal puddling or moisture accumulation around your furnace? This is usually indicative of damage to the secondary heat exchanger.

What Are These Things Even For? Common Culprits

Why exactly these secondary heat exchangers might bite the dust is not always clear. But there are some usual suspects:

• Clogged or Dirty Air Filters: This is one that often gets overlooked and seems so simple that homeowners will almost feel embarrassed for not thinking of it. An air filter that is clogged with dust prevents the proper amount of air flow from entering a furnace. When the flow of air becomes impeded, the heat exchanger becomes too hot and is at risk of cracking. Imagine trying to run a marathon through a pillow on your face, it gets too hot!
• Furnace Is Incorrect Size: You may not think it, but having a furnace that’s too big can also lead to issues in your house. Over-sizing the furnace: This one is simple: If a furnace is too large for the home, it’s going to constantly overheat, and you-know-what happens to the heat exchanger.
• Good Old Wear and Tear: The most common reason for cracks is good old-fashioned wear and tear. (Your furnace’s heat exchanger is made of metal, and it stretches and contracts as it heats up and cools down.) Such thermal stress can lead to cracks over the years. While most companies recommend replacing such burners after 20 years, good maintenance is a large factor in prolonging the life of your furnace.
• Missing or Eroding Baffles: This is more of a technician problem, but it’s useful to understand. On older Carrier and Bryant 90% furnaces we often find that the rear air baffle is missing. This baffle is designed to channel the air correctly on the secondary heat exchanger, so as not to blow into a high limit switch. If it is absent or corroded, the furnace can overheat.
• Flaws but not dealbreakers: Occasionally, the black paint that seals the panels into the front plate of the heat exchanger is applied in such a way that it blocks air flow. Plus, lower-quality systems can corrode more quickly from condensation over time.

What’s Your Secondary Heat Exchanger Up To?

Listen, your high-efficiency furnace is an investment. To keep it lasting and working at its best, you’ve got to give it some love. A well-maintained heat exchanger can last the lifetime of your furnace.

• Regular Professional Maintenance and Inspections: This is a must, people. You should have an HVAC service professional conduct a furnace tune-up once a year. Throughout this annual go-through you can expect a total furnace “physical,” where they look for those pesky cracks in your heat exchanger. They’re also able to clear out any obstruction due to accumulated condensation that can impede efficiency. Don’t skip this; it can prevent you from costly, unexpected repairs in the future.
• Your Job: Change the Air Filter Regularly: This is the easiest, most important thing that you can do as a homeowner. A clean air filter allows the right amount of air to flow through so your heat exchanger doesn’t overheat. Replace your filters regularly – every 30-90 days (depending on the quality of the filter and whether or not you have an air purification system). It’s to slap on a quick fix that pays dividends.
• When to Call the Pros: If you note any of the warning signs we discussed — strange smells, odd noises, your CO detector screaming — don’t act like the tough guy. GET A PRO The key is to contact a pro who is a licensed HVAC technician. There might be underlying problems that only a pro can see. For instance, a technician could take a CO reading, measured in parts per million (PPM), on your furnace exhaust. If you have an over 100 part per million reading, that’s a pretty sure shot that you’re going to have to cut open your furnace and pull that secondary heat exchanger out. It is a laborious process not for the faint of heart or homebuilder.

Repair or Replace? The Million-Dollar Question

So, your heat exchanger is cracked. Now what? Can you simply fix it, or do you need to buy a new furnace?

• The Money Issue: Theoretically, you should be able to replace a ruptured heat exchanger cell. But this is one of the priciest furnace repair situations around. It frequently requires removing the defective cell and possibly replacing the other parts at the same time. It’s a messy affair that involves a full dismantling of all the components involved, baffles and more, to make sure everything is functioning properly and to avoid overheating.
• Your Furnace’s Age: Here’s the general rule of thumb: If you have a furnace that is older than 10-15 years, it may be cheaper in the long run to replace your entire unit as opposed to just the heat exchanger. A furnace that needs replacing probably has other parts that are nearing the end of their own lives anyway. You don’t want it to be like adding new tires to a car where the engine is dying — and sometimes, the entire upgrade is the smarter play.

Conclusion: Your Furnace Secondary Heat Exchanger—Defending Your Home, and Your Budget

Let’s wrap this up. The secondary heat exchanger in your furnace is a workhorse of efficiency, quietly toiling away to keep your home warm and your energy bills (relatively) low. But it’s not invincible. Failing to heed its warning signs can result in discomfort, higher bills and, most dangerous of all, serious health and safety risks if it is emitting carbon monoxide.

Regular maintenance — as in annual professional tune-ups along with simple filter changes — reigns as the very best way to help this crucial component remain in peak condition, and to prolong the life of your entire heating system. Do not play the waiting game if you think you may have a problem. Call a certified HVAC professional right away! This service is important because it is their ability to diagnose hidden problems that keep your home safe and comfortable. It’s worth being a little uncomfortable when you consider the difference it could make for your peace of mind, and your life.

Secondary Heat Exchangers – Questions and Answers (FAQ)

Answers to Your Frequently Asked Questions About Secondary Heat Exchangers: And now for some quick answers to commonly fielded questions about secondary heat exchangers:

Q1: What is a secondary heat exchanger and what does it do? A: A secondary heat exchanger is present in high-efficiency furnaces (90%+ AFUE). Primarily, it’s supposed to recover some extra heat from the exhaust gases once they’ve left the primary heat exchanger. It achieves this by becoming the water vapour condensed, and giving off “latent heat”, which greatly improves the overall furnace efficiency.

Q2: How can I tell if my secondary heat exchanger is failing? A: There are several warning signs to watch out for: strange odours (for example, the smell of formaldehyde or pickles), white soot that begins to accumulate around the furnace’s gas rings, visible cracks or rust (or discoloration in general), any colour other than blue for your flame (ie: yellow or orange), concerning rattling or banging sounds, your carbon monoxide detector sounding its alarm, overly-frequent cycling of the furnace, higher-than-average energy bills, or too much condensation/moisture around the furnace.

Q3: How dangerous is a secondary heat exchanger that is cracked? A: Absolutely, yes. A cracked or rusted heat exchanger can release corrosive combustion by-products which can leak into the indoor air?including deadly, odourless, tasteless carbon monoxide (CO). This can result in serious illness, including death. Always have working CO detectors.

Q4: What can cause a secondary heat exchanger to fail? A: A few are dirty and clogged filters that limit airflow and result in overheating, improperly sized furnace (especially an oversized one) that may create too much heat and damage the building, and normal deterioration from the continuous expansion and contraction of the metal over time. Defective or missing baffles and manufacturing problems, such as constrictive coatings can also be the cause.

Q5: Can a secondary heat exchangers be repaired? A: In theory, a defective heat exchanger can be replaced. It is one of the most costly furnace repairs because it is a complicated breakdown and reassembly of parts. If your current furnace is over 10-15 years old, replacing the entire system will likely prove to be the more cost-effective option for the long run.