Furnace Safety: Cracked Heat Exchangers, CO, and What to Check Before Winter

By Andrew Kuznetsov, a licensed Bay Area contractor (CSLB #1136642, C-20, EPA 608), for the Tri-Valley homeowner’s guide.

Here in the Tri-Valley, your furnace probably sits idle for most of the year. Pleasanton, Livermore, Dublin, San Ramon, all the inland East Bay towns get long warm stretches where the heat never kicks on. That sounds like an easy life for a furnace. It isn’t always.

A unit that runs only a handful of weeks a year still ages. Metal expands and contracts a little every time it fires. Rodents and spiders find their way in over the summer. Dust settles in places it shouldn’t. So when the first genuinely cold morning hits and the furnace runs hard for the first time in months, that’s exactly when a hidden problem shows itself. This guide walks through the safety side of that, plainly, without trying to scare you.

Why a barely-used furnace can still be unsafe

The most serious furnace hazard is carbon monoxide, and it usually traces back to the heat exchanger. That’s the metal chamber that separates the burner’s combustion gases from the air blowing into your house. When it’s intact, exhaust goes up the flue and out. When it cracks, small amounts of combustion gas, including CO, can leak into the air you breathe.

Cracks happen from years of heating and cooling cycles, from corrosion, sometimes from a furnace that’s been running with restricted airflow (a clogged filter makes the exchanger overheat). A furnace that ran lightly for fifteen years can have a cracked exchanger just like one that ran hard for eight. Low use doesn’t mean low risk. It mostly means the problem stayed quiet longer.

Carbon monoxide is colorless and odorless. You can’t smell it, you can’t see it. That’s the whole reason detectors exist, and the whole reason a visual once-over before the season matters.

Warning signs worth taking seriously

None of these guarantees a cracked exchanger. Several of them together, or any one of them plus people feeling sick, is a reason to shut the furnace off and get a licensed tech out.

• Soot or black streaks around the furnace or registers
• A yellow or flickering burner flame instead of a steady, mostly-blue one
• Odd smells when the heat runs, like a sharp, sour, or formaldehyde-ish odor that doesn’t fade after the first few minutes
• Short-cycling, where the furnace fires up and shuts off again over and over
• Headaches, dizziness, nausea, or unusual sleepiness that ease when you leave the house and come back when the heat’s been running
• Soot or moisture collecting on windows near the furnace

That last health cluster is the one to never wave off. If more than one person in the house feels it, or pets seem off too, leave, get fresh air, and call for help before going back in.

Your CO detectors do the heavy lifting

A working carbon monoxide detector is the single most important piece of furnace safety in your home, and it’s cheap. California requires them in homes with attached garages or fuel-burning appliances, so you likely already have at least one. The catch is that a detector only protects you if it actually works.

Before heating season, do this:

• Press the test button on every CO alarm and listen for the full alarm tone
• Replace the batteries if they’re more than a year old, even if the unit still beeps
• Check the manufacture date printed on the back. CO sensors wear out, and most units are rated for roughly 5 to 7 years before they need full replacement
• Put at least one on each floor, especially near sleeping areas

Smoke alarms and CO alarms aren’t the same thing, even though some combo units do both. Make sure you’ve got real CO coverage, not just smoke.

What a real pre-winter safety inspection covers

A proper inspection is more than swapping the filter. A licensed technician with combustion tools will typically look at the things you can’t safely check yourself:

You can handle the easy parts yourself between visits: change the filter on schedule, keep the area around the furnace clear, and make sure return vents aren’t blocked by furniture. Our seasonal maintenance overview goes deeper on the homeowner-level stuff. The combustion and gas side, though, is licensed-pro territory, full stop. Gas work and refrigerant work both require licensing and the right equipment, and they’re not DIY no matter how many videos suggest otherwise.

When to stop using the furnace and call someone

Some situations are “schedule a visit soon.” Others are “shut it off now.” Here’s the line.

Turn the furnace off and get a licensed tech if you see soot, a persistent yellow flame, or smell something burning or chemical that doesn’t clear. If your CO alarm goes off, treat it as real every single time: get everyone outside into fresh air, then call 911 or the fire department, and don’t go back in until it’s cleared. Don’t reset the alarm and hope.

If you’re weighing whether an older furnace is even worth fixing again, the repair-versus-replace thinking applies to heating systems too, and a lot of Tri-Valley homes are looking hard at heat pumps as the next step, partly because there’s no combustion and no CO at all. The rebates and permits page is worth a read before any big swap, since local incentives change and permitting matters for safety sign-off.

Finding the right help

Always confirm a contractor’s license before they touch a gas appliance. California’s Contractors State License Board lets you look up any license number in a minute, and the Bureau of Household Goods and Services covers appliance repair registration. For broader heating and cooling background, the main HVAC guide and our notes on how to vet a repair tech cover what to ask before you book.

If you’d rather start with a local, licensed option, ADRIUM Service Solutions is a San Ramon-based licensed company (CSLB #1136642, C-20, EPA 608). Either way, the point holds: a barely-used furnace deserves a real look before the cold sets in, because the first hard run of the season is the worst time to discover a problem you could’ve caught in October.