Q&A: Heat Exchanger Cracks and Carbon Monoxide Myths

I recently had a reader email this question.  I sent a short answer, but thought this was deserving of a longer answer in the form of a blog post.

Please read this article about why heat exchanger cracks do not allow carbon monoxide into the house. The author has been in the industry for years as a trainer for HVAC technicians. I personally think that the HVAC businesses have a great scam going by locking-out people's furnaces without any measurable evidence that the CO levels are above normal. I think they use cracks to strong-arm sales so they can provide a replacement quote on the spot. I wish the attorney general of MN would not allow this kind of thing happen. After all isn't this why we buy CO detectors?


In reading this email, I see three assertions that need to be addressed:

  1. HVAC businesses are using cracked heat exchangers to scam people.
  2. HVAC businesses in Minnesota lock out furnaces when cracked heat exchangers are found.
  3. Carbon monoxide detectors will keep people safe from carbon monoxide.

Cracked Heat Exchanger: Sales Scam?

I checked out the COmyths web site.  There's a lot of good info on there, but some of it is misleading and there are a lot of straw man arguments made.  For example, the headline at the beginning says "Myth #1 – A furnace with a cracked heat exchanger will definitely produce carbon monoxide and poses an immediate danger. (Wrong!)" .  Yes, that's an incorrect statement, but I've never actually heard anyone say that.  That headline is easily made true by rewording the sentence just slightly: a cracked heat exchanger has the potential to increase carbon monoxide levels, and has the potential to pose a danger to the occupants.

So what's a cracked heat exchanger all about and what's the big deal?  I wrote a blog post many years ago discussing that topic, and the gist of my blog post was the same as what's stated on the COmyths web site; a cracked heat exchanger probably isn't as dangerous as many folks make it out to be, but the furnace (or heat exchanger) still needs replacement.  Here's the post: How Serious Is A Cracked Heat Exchanger?

If a heating contractor finds a cracked heat exchanger and says the furnace needs to be replaced, they're not pulling a scam unless they're just outright lying to you about finding a crack.  They're simply doing their job.  Towards the bottom of the page on the COmyths web site, you'll find this text:

Text from web site

See?  This isn't a scam, even according to their web site.

Do HVAC businesses lock out furnaces?

Sometimes.  If a company considers the heating equipment to present an imminent danger to the occupants, they might disable it, but this policy varies from company to company.

If a heating company unscrupulously disabled a furnace in an attempt to strong-arm the sale of a new furnace, I would take serious issue with that.  I don't believe that happens though.  At least I hope it doesn't.

I spoke with Becca Virden, the public relations spokeswoman at CenterPoint Energy, and she said that the gas company does not lock out furnaces unless it's an extreme situation.  They may shut the equipment off by turning the gas off to the appliance, but locking out the gas due to unsafe conditions would be extremely unusual.

When CenterPoint Energy identifies equipment as unsafe, they also red-tag the equipment.  This means they affix a tag to the equipment saying it's not safe to use.  That's about all.  If someone turns the equipment back on, the gas police won't come knocking at the door later that day.  I blogged about this topic many years ago, and the information in that blog post is still accurate today, except the "red tags" that CenterPoint Energy uses are actually now white.  They still call 'em red tags though.

Here's that blog post: Red Tagged Furnaces: Is Legal Trouble Worse Than Death?

Will carbon monoxide detectors keep you safe from carbon monoxide?

In short, no.

But let's back up a step.  Are we talking about a carbon monoxide detector, or a carbon monoxide alarm?  They're not the same thing.  UL listed carbon monoxide alarms will not alert you to low levels of carbon monoxide in your home because they're designed not to.  They're life safety devices, designed to prevent people from dying from carbon monoxide poisoning.

Carbon monoxide alarms are required by law in Minnesota and throughout many parts of the country.   Most homes that I inspect in Minnesota have at least one carbon monoxide alarm somewhere.  These alarms are sold everywhere, and they're fairly inexpensive.  Click this link for more info about the rules for carbon monoxide alarms in Minnesota: Minnesota Requirements for Carbon Monoxide Alarms .

Carbon monoxide detectors, on the other hand, are far more expensive than carbon monoxide alarms, are not UL listed, and typically can't be found in retail stores.  They're designed to detect the presence of low levels of carbon monoxide.  In other words, a carbon monoxide detector will alert you to a carbon monoxide problem in the home far earlier than a carbon monoxide alarm would, but the vast majority of homes don't have carbon monoxide detectors. For more information on the difference between carbon monoxide alarms and carbon monoxide detectors, check out my blog post on that topic: Low levels of carbon monoxide will not set off UL listed CO alarms.

Again, carbon monoxide alarms are life safety devices.  We buy carbon monoxide alarms to help make sure nobody dies in their sleep, but they are absolutely not a substitute for having safe equipment.  Carbon monoxide alarms are the last line of defense.  Relying on your carbon monoxide alarm to keep you safe and ignoring a potential safety hazard like a cracked heat exchanger is like ignoring electrical fire hazards in your home because you have smoke alarms.  Smoke and carbon monoxide alarms are not there to keep you safe; they're there to keep you alive.

In other words, if you have a potential safety issue with your furnace that a professional heating contractor has identified, fix it.

Author: Reuben SaltzmanStructure Tech Home Inspections


Written By

Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

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