Never Run a Gas Furnace With a Cracked Heat Exchanger

This applies to gas furnaces — natural gas or propane. Burning fuel to make heat is normal, as long as the combustion side stays sealed off from the air moving through your home. The heat exchanger is the part that keeps that seal. When it cracks, the seal breaks — one of the few furnace problems where "wait and see" isn't an option.

What a heat exchanger actually does

Inside your furnace, burning fuel creates hot combustion gases. The heat exchanger is the metal chamber that keeps those gases separate from your breathing air — room air flows around the outside, picks up heat, and gets pushed out through your ducts, while combustion gases exit a different way, out through the flue.

A furnace can run well for many years, but the heat exchanger takes the hardest beating in the system. Every heating cycle heats it up and cools it back down, and that expansion and contraction is metal fatigue in slow motion. Eventually, the metal can crack.

Why a crack is a real hazard

A crack doesn't have to be big to be dangerous — it can widen slightly when the metal heats up, exactly when combustion gases can slip through and mix into the air your ducts push into your living room.

Those gases carry carbon monoxide. Natural gas and propane both get an additive for smell, but carbon monoxide itself has no smell, no color, no warning of its own. It replaces oxygen in your blood, and low-level exposure causes symptoms easy to write off as something else: headache, dizziness, nausea, confusion, blurred vision, shortness of breath. At higher concentrations, it's fatal. That's the reason for the rule in the headline: if you have any reason to think your heat exchanger is cracked, the furnace stays off until a technician has confirmed one way or the other.

If your CO alarm sounds, you smell gas, or you notice symptoms of CO exposure

Leave the house immediately — everyone, right away. Don't stop to open windows, don't stop to shut the furnace or any other equipment off, don't flip any light switches on your way out, and don't look for the source. Once everyone is out, call 911 or Southwest Gas from outside.

That covers three triggers: a CO alarm sounding, the smell of gas, or symptoms that could mean carbon monoxide exposure — headache, dizziness, nausea, confusion, blurred vision, shortness of breath. Carbon monoxide has no smell of its own, and a quiet detector doesn't prove the air is clean — treat those symptoms the same as an alarm sounding.

Only when none of that applies — no active CO alarm, no gas smell, nobody showing symptoms, and you simply suspect a cracked heat exchanger from something like the signs below — is it fine to shut the furnace off yourself and leave it off until a technician checks it. Don't turn it back on "to see if it's still doing it" in the meantime. A technician can then test the heat exchanger and combustion setup — usually a choice between replacing the heat exchanger or the whole furnace, weighing the unit's age, condition, and warranty status. If replacement is the safer choice, see how we size and install a Tucson furnace.

Warning signs to watch for

A cracked heat exchanger doesn't always announce itself, but these are worth taking seriously:

• A carbon monoxide detector that goes off — ever, even once
• Soot buildup inside the furnace cabinet
• A burner flame that flickers, burns yellow or orange, or isn't a steady blue
• A noticeable odor when the furnace runs
• Moisture or corrosion in or around the unit

A clicking or ticking noise as the furnace cools is usually just the cabinet or ductwork contracting — normal, not a heat-exchanger sign on its own. Strange behavior in general — short cycling, uneven heat, a burning smell — is worth a look before it becomes a heat-exchanger problem. That's what a furnace repair visit is for: a technician inspects the heat exchanger and combustion system directly, rather than guessing from symptoms.

The best fix is not needing one

Working, 10-year sealed carbon monoxide alarms on every level of your home and outside sleeping areas are non-negotiable. Beyond that, annual furnace maintenance is where a combustion problem or heat-exchanger damage is most likely to get caught early. It's the idea behind our Home Wellness Club: an annual heater tune-up is one of the included visits, alongside an AC tune-up and a whole-home plumbing inspection.