COVID-19 Update: Structure Tech Home Inspections is still open for business. To see what we’re doing to help keep everyone safe, please see COVID-19 and Home Inspections.

Reuben Saltzman

Carbon monoxide = CO (houses don’t need CO2 detectors)

There are many common misconceptions and misunderstandings about furnaces, water heaters, and carbon monoxide that I hear repeated regularly, and I’d like to clear a few of them up. To start, here’s a video discussion:

They’re actually selling carbon monoxide alarms, but I’m guessing they have the wrong thing listed to help people find what they’re looking for even if the wrong thing is typed in. Google is a master at this, so don’t get me wrong, I’m not complaining. I’m just sayin’.

False: Carbon monoxide alarms are needed to sell a home in Minnesota. Carbon monoxide alarms are required within 10′ of sleeping rooms, but this has nothing to do with a real estate transaction. You don’t install carbon monoxide alarms because it’s the law, you do it because it’s a $15 life safety device. See R315 for the exact rules on CO alarms.

False: Backdrafting at a furnace or water heater means CO is filling up the home. Backdrafting means that exhaust gases are spilling back into the home, rather than leaving through the vent. A properly functioning water heater or furnace will not create high levels of CO. Even if a properly functioning furnace or water heater backdrafted into a home all day long, you might not ever see an elevated level of CO. Not that this is ok. This should still be considered a hazardous situation that requires immediate correction. Backdrafting has the potential to fill the home with CO. It will always contain CO2 (carbon dioxide), which can cause sickness and headaches in higher concentrations.

Hmm… maybe Walmart ought to start selling CO2 detectors for real ;-). They do exist, after all: CO2 Meter

False: Cracked heat exchangers create CO.  CO is caused by incomplete combustion, not a cracked heat exchanger. A heat exchanger is the part of a furnace that transfers heat from the flames to the household air. A functional heat exchanger keeps the household air and the combustion gases completely separate from each other.

A cracked heat exchanger might, under the right conditions, create elevated levels of CO, but this is not typical. If a furnace has a cracked heat exchanger, the combustion gases can mix with the household air. It’s usually just a little bit, but this is still unacceptable, and it means the furnace or heat exchanger should be replaced. More on that topic here: Heat Exchanger Cracks and Carbon Monoxide Myths.

False: High CO levels = cracked heat exchanger. See above. We usually test CO levels in the flue gas at furnaces, but not with the idea that this will tell us about a cracked heat exchanger. Heat exchangers fail when the metal rusts through or when it cracks. CO does not cause this.

False: High CO levels in the flue gas mean the furnace is leaking CO. If an appliance is venting properly, all of the exhaust gases leave the home. Even if a furnace is producing extremely high levels of carbon monoxide, this carbon monoxide will not mix with household air, provided everything else is functioning properly. If the appliance backdrafts or the exhaust gas leaks into the home through a cracked heat exchanger… well, that would be a different story.

Because of these possibilities, however, high levels of carbon monoxide are still a concern. If we find high levels of CO in the flue gas, we recommend repair. It doesn’t matter if the gases are mixing with the household air at the time of the inspection or not, because this condition could potentially change at any time. The Minnesota State Fuel Gas Code (Chapter 9, Subp.6) says that gas-fired equipment shall produce not greater than 0.04 percent of carbon monoxide on an air-free basis. This equates to 400 parts per million.

Wrong Term: Hot water heater. It’s just a water heater. The heated water that comes out is hot. Yes, I know there are plenty of retailers who will sell you a hot water heater, but please re-read the beginning of this post about how you can buy a carbon dioxide alarm at Walmart.

To summarize, high levels of CO need to be fixed, cracked heat exchangers need replacement, and backdrafting is never ok. These three things are all independent, but a combination of these conditions is especially dangerous. When using these terms, make sure you have them correct. It makes a difference.

Author: Reuben SaltzmanStructure Tech Home Inspections

Subscribe button

No responses to “Carbon monoxide = CO (houses don’t need CO2 detectors)”

  1. Terrence P Blessing
    December 4, 2018, 5:49 am


  2. Robyn Deveney
    December 4, 2018, 4:16 pm

    Oh man, I think I love you. “Hot water heater” makes me want to claw my eyes/ears every time. And now I know why our CO detector doesn’t even blink when the oil-fired furnace (and/or water heater) burps exhaust gases into the basement at the beginning of the burn cycle: Wrong gas. Maybe we do need a CO2 detector. And/or a new furnace and water heater.

  3. Reuben Saltzman
    December 5, 2018, 6:08 am

    Hi Robyn,

    Your CO alarm really is the right device, it’s just not going off because there’s not enough CO in the room for a long enough period of time to set off your alarm.

  4. Chris Bolick
    December 18, 2018, 6:11 pm

    My issue is, I had to install these alarms and I have no combustion items in the house. Everything is electrical, that means $$$ for nothing.

  5. Reuben Saltzman
    December 19, 2018, 5:30 am

    Hi Chris, assuming you’re in Minnesota, you still need to have CO alarms if you have an attached garage.

    If you don’t have an attached garage and you have no fuel-fired appliances in your home, then you do not need CO alarms.

  6. Dale
    January 3, 2019, 3:22 pm

    You are basically telling Joe homeowner that all HVAC companies are walking into homes not telling the truth about a heat exchanger. When we inspect a furnace and a crack is detected we will shut it down for safety. Get a second opinion by all means. But don’t tell people it’s a scam. How about you explain to family members after death they took your advice. A non licensed HVAC contractor. Sure there are untruthful people out there and always will but you don’t blame thousands for one liar. As for CO detectors you need to go back and learn the facts. We use a detector when we walk into any home that checks the ppm. If over 34 we investigate the situation and yes they do work. 38 years in the trade and have been called into homes after the bodies have been removed. The fire department will use our reading to theirs then investigate after home has been aired out. Detectors do work and do read the ppm so you are setting yourself up for a bad fall when someone seriously gets hurt or worse. A cracked heat exchanger and the air mixed is what? Carbon monoxide and that gets pushed through the ductwork to the home. Explain again how there would be no reading. Come to Washington DC so we can school you on a whole lot you don’t know.

  7. Reuben Saltzman
    January 3, 2019, 3:51 pm

    Hi Dale,

    We’re on the same side, my friend. Nowhere in this post did I tell people about any type of scam. Did you read my post about cracked heat exchangers? See

    I can’t figure out what part of this post (or perhaps the other post?) you don’t agree with.

RSS feed for comments on this post. TrackBack URL

Leave a Reply