Cracked Heat Exchangers and Carbon Monoxide: What Denver Homeowners Should Know

Few words from an HVAC technician worry homeowners more than “your heat exchanger is cracked.” It sounds serious, it often comes with a recommendation to replace the furnace, and it usually happens in the middle of winter.

It is serious. But it is also a finding you are allowed to understand and verify. This guide explains what a heat exchanger is, why it cracks, whether a crack really leaks carbon monoxide, how technicians confirm it, what a “red tag” means, and how to decide between repair and replacement.

If your CO alarm is going off right now

What is a heat exchanger?

Inside your furnace, gas burns in a set of burners. The flames and hot exhaust travel through a set of sealed metal tubes or chambers called the heat exchanger. Your furnace blower pushes house air across the outside of those hot metal surfaces. The air warms up and flows through your ducts. The exhaust stays inside the heat exchanger and leaves through the flue or vent pipe.

In short: the heat exchanger is the wall between fire and the air you breathe.

High-efficiency furnaces (the ones with white PVC vent pipes) have two heat exchangers. The primary one sits closest to the flame. The secondary one pulls extra heat from the exhaust, which cools the exhaust enough to create water (condensate). That water is slightly acidic, so the secondary heat exchanger can corrode over time.

Why heat exchangers crack

Heat exchangers heat up and cool down thousands of times over the life of a furnace. Metal expands when it heats and shrinks when it cools. Over many years, that stress can cause cracks, usually at bends, seams, and welds. Several things speed it up.

1. Overheating from poor airflow

This is one of the most common causes. When not enough air moves across the heat exchanger, it runs hotter than it was designed to. Common reasons:

• Dirty filters left in too long
• Closed or blocked vents and returns
• Undersized ductwork, especially return ducts
• A weak or failing blower

Each overheating cycle trips the furnace’s high-limit switch. If you have noticed your furnace blowing warm and then cold air repeatedly, that pattern can signal overheating. See our guide on why a furnace blows cold air.

2. An oversized furnace that short-cycles

A furnace that is too big for the home heats quickly, shuts off, cools, and restarts over and over. More starts means more expansion and contraction. Oversizing is common in homes where an old furnace was replaced “same size or bigger” without a proper load calculation.

3. Poor combustion, including altitude problems

At Denver’s altitude, gas furnaces need to be adjusted for the thinner air, usually by reducing their gas input. A furnace that is not set up correctly can burn “rich,” which creates more soot and more carbon monoxide. Soot and flame problems can overheat parts of the heat exchanger. Our guide on how Denver’s altitude affects your furnace explains the adjustment.

4. Corrosion

• Condensing furnaces: acidic condensate can corrode the secondary heat exchanger, especially if the drain has clogged.
• Chemical fumes: chlorine and similar fumes from laundry products, pool supplies, paint thinners, or cleaners stored near the furnace can be drawn into the burners and speed up corrosion. Keep the furnace area clean and clear.

5. Age

Most gas furnaces last roughly 15 to 20 years. Heat exchanger problems become much more common in older furnaces. See when to replace your furnace in Denver.

Does a cracked heat exchanger always leak carbon monoxide?

Here is the honest answer: not always, and not all the time. But you should not bet on it.

When the blower is running, the air pressure on the house side of the heat exchanger is often higher than inside it. So a small crack may push house air into the heat exchanger instead of letting exhaust out. That is why some cracked heat exchangers show no CO in the house air on the day they are tested.

But that does not make a crack safe:

• Cracks grow. A hairline crack this month can open wider over the season.
• Conditions change. Before the blower starts and after it stops, the pressure difference is different. Blocked filters, blocked vents, or blower problems change it too.
• A crack can disrupt the flame. Air rushing through the crack can make the burner flame lift, roll, or burn poorly. Poor combustion produces more CO in the exhaust, which raises the risk if any of it escapes.
• Flames can roll out. In some cases, a crack lets flames escape the burner area, which is a fire risk.

That is why furnace manufacturers and the HVAC industry treat a confirmed crack or hole in a heat exchanger as a reason to take the furnace out of service.

Warning signs of a cracked heat exchanger

None of these alone proves a crack, but each one is worth a professional inspection.

• Your CO alarm sounds or shows a low-level reading
• Burner flames change when the blower starts. Healthy flames stay steady and blue. Flames that dance, lean, lift off, or turn yellow when the blower kicks on are a classic warning sign.
• Yellow, orange, or lazy flames instead of crisp blue ones
• Soot inside the furnace, around the burners, or on registers
• Rust, corrosion, or water stains inside the furnace cabinet
• The furnace keeps shutting off on its own because of tripped safety switches, including the flame rollout switch
• Unusual smells when the furnace runs, or a stale, sharp odor
• Physical symptoms at home: headaches, dizziness, nausea, or tiredness that ease when you leave the house, especially if more than one person or a pet is affected

For the full list of CO symptoms and alarm tips, see our guide on how to tell if your furnace is leaking carbon monoxide.

How technicians confirm a cracked heat exchanger

A good technician does not just say “it is cracked.” They show you. Here are the methods used, often in combination.

A standard annual tune-up includes a visual inspection of the heat exchanger surfaces that can be seen and a CO test at the furnace. It does not normally include pulling the blower or running a camera through every passage. That is why tune-ups catch many problems but can miss small hidden cracks. We explain the full tune-up checklist in furnace tune-up in Denver: what is checked and what it costs.

What does it mean if your furnace is “red tagged”?

If a technician finds a hazard, like a cracked heat exchanger, a CO problem, or a gas leak, they may:

• Shut off the gas to the furnace
• Attach a warning tag stating the furnace is unsafe to operate
• Explain the hazard and what is needed to fix it

A red tag is meant to protect you. Do not turn the furnace back on yourself.

Your rights as a homeowner

You are entitled to understand the finding. It is completely reasonable to:

• Ask to see it. Ask for photos or a video of the crack, and the CO readings.
• Ask where the crack is and how they found it.
• Get a second opinion from another licensed company, especially if the recommendation is a full replacement. Just do not run the furnace while you wait.
• Check the warranty before paying for anything. More on that below.

A trustworthy company will welcome these questions. At Comfy Cave, we show you what we found before we recommend anything.

Staying warm while you decide

If your furnace is shut down in winter, use space heaters safely (plugged directly into the wall, 3 feet from anything that can burn, and off when you sleep or leave the room). Never use a gas oven, stove, grill, or generator to heat your home. Protect pipes on outside walls by opening cabinet doors and letting faucets drip.

Repair or replace? Your options

Option 1: Replace the heat exchanger

On many furnaces, the heat exchanger can be replaced. Things to know:

• The part may be under warranty. Many manufacturers cover heat exchangers for 20 years or longer, sometimes for life, especially if the furnace was registered after installation. Check your paperwork or the model and serial number with the manufacturer.
• Labor is usually not covered, and it is a big job. The furnace has to be largely taken apart.
• Typical cost: In the Denver area, heat exchanger replacement often costs roughly $1,000 to $3,500 or more depending on the furnace, the warranty, and labor. With a warranty part, you mainly pay labor.
• It makes the most sense when the furnace is relatively young (under about 10 to 12 years), the rest of it is in good shape, and the part is under warranty.

Option 2: Replace the furnace

Replacement usually makes more sense when:

• The furnace is 15 or more years old
• The heat exchanger is not under warranty
• The repair would cost a large share of a new furnace
• The furnace has other problems, like a failing blower or control board

Keep in mind that Colorado’s ultra-low NOx standard for new gas furnaces took effect January 1, 2026. It affects which new furnaces you can buy and has pushed prices up. See the new Colorado furnace law explained.

Option 3: Consider a heat pump or dual-fuel system

If you are replacing the furnace and your AC is also getting older, it may be the right time to look at a cold-climate heat pump or a dual-fuel system. Heat pumps do not burn fuel, so there is no heat exchanger to crack and no combustion CO from the heating system. See do heat pumps work in Denver winters? and how to choose the best HVAC system for a Denver home.

How to protect your family

Carbon monoxide alarms: your most important safety device

• Install CO alarms near every sleeping area and on every level of your home.
• Colorado law requires an operational CO alarm within 15 feet of the entrance to each bedroom in homes with fuel-burning appliances, a fireplace, or an attached garage when the home is sold, and when certain permitted work is done, including replacing a fuel-fired appliance. See C.R.S. 38-45-102.
• Test alarms monthly and replace them by the date printed on the back. CO sensors wear out.
• Consider a CO alarm with a digital display, which shows low levels that do not trigger the alarm.

The stakes are real. According to the CDC, more than 400 Americans die from unintentional, non-fire carbon monoxide poisoning each year, more than 100,000 visit an emergency department, and more than 14,000 are hospitalized.

Prevent the conditions that crack heat exchangers

• Change your filter on schedule. See our furnace filter guide.
• Keep vents and returns open.
• Get an annual tune-up that includes a CO test and flame check.
• Make sure your furnace is set up for Denver’s altitude, especially after installation or major repairs.
• Keep chemicals away from the furnace.
• Make sure a new furnace is sized with a load calculation, not just matched to the old one.

Remember other CO sources

If your CO alarm goes off, the furnace is not the only possible source. Water heaters, gas ranges, fireplaces, and cars running in an attached garage can all produce CO. A technician should check every fuel-burning appliance, not just the furnace. Boiler owners can also read our boiler safety guide.

Get an honest heat exchanger inspection

Whether your CO alarm went off, you have seen warning signs, or another company told you your heat exchanger is cracked and you would like a second look, we can help. Our NATE-certified technicians will inspect your furnace, test for carbon monoxide, show you exactly what we find, and walk you through your options, including repair when it makes sense.

Comfy Cave Heating & Air is based in Arvada and serves Denver and the surrounding metro, with 24/7 emergency service. Call 303-645-4889 or schedule furnace service.

Frequently asked questions