Risk of carbon monoxide poisoning from furnace?

The other day, the fan in our HVAC system (which I have zero knowledge about) starting going on and off in 15 minute cycles even though it was turned off. We live in San Diego and during the summer (six months ago) we had a major $2,000 repair to our A/C system, which I thought might be responsible for this.

The repair guy looked in the attic above our bedroom and announced that no, it was a $545 circuit board that had burned out and needed to be replaced, and that, oh by the way, our gas furnace was already 19 years old and most of them only last 20 years, so we should think about replacing that at a cost of an additional $2,500-$3,500. My wife immediately freaked out about this announcing that we should replace the furnace because of the imminent death that would befall us from carbon monoxide poisoning as soon as the furnace turns 20 years old.

To this I said that 1) I don’t know if modern furnaces that are gas powered even run this risk because if they did, I can’t imagine they would allow builders to put them in attics above master bedrooms in relatively new houses (ours was built in 1994) 2) I imagine that the lifespan of a furnace is related to how often you use it to crank the heat, which in San Diego is extremely rare - maybe five times a year for an hour or two. 3) I don’t trust the word of anyone who is in the business of selling furnaces that I need a new one, just like 100% of the cars that drive into a place offering ‘free brake inspections’ need new brakes.

I’m aware we should have a carbon monoxide detector in the house, which I admit we do not, and now I will buy one just to mellow out my wife, but my question is, are my assumptions correct regarding the likelihood of death by carbon monoxide? I assume these kinds of things when they occur are due to ancient furnaces that run on heating oil in cold areas with no ventilation, whereas my wife is convinced all furnaces turn into carbon monoxide spewing death machines after some length of time. What’s the straight dope?

Saying that furnaces turn into carbon monoxide spewing machines of death over time is a bit of an exaggeration. What does happen is that exhaust vents can corrode and crack, which leaks exhaust gases into the house. Also, incomplete combustion from a poorly maintained burner can produce more carbon monoxide in the exhaust gases, increasing the risk.

Having routine maintenance (cleaning, inspection, etc) done every year drastically reduces the risk of a problem. My furnace was close to 60 years old before it developed an exhaust leak, but I had it maintained every fall just before the cold weather hit. The furnace also died a couple of years after that, but hey, 60 years is a pretty good life for a furnace (to be fair the gas burner at its core was only about 35 years old at the time - it started its life as an oil burner).

A carbon monoxide detector is a good idea for a furnace of any age.

Yup, get a detector regardless. A former coworker and her family were hospitalized due to carbon monoxide poisoning from the furnace in her relatively new condo. There was some kind of issue with the venting of the exhaust and it ended up spewing carbon monoxide into one of their bedrooms.

get a detector if you burn oil/gas for space or water heating. fireplace use is a reason to have one too.

We have a 60-year-old oil system (actually, I think it was originally coal, but don’t know when they changed over); the burner was replaced 35 years ago. We get it serviced each year and they check the CO emissions, and it’s never been a problem. A detector is a good idea, but a furnace can go on for years without there being an issue.

As your furnace ages the chances of a cracked heat exchanger increase. 20 years increase the odds. But it depends. Get a CO detector and have the furnace check.

A 20 year old furnace can be perfectly safe though.

Is there any truth to the notion that a heat exchanger can get “thin” or is that just a scare tactic from people trying to sell furnaces? Unless they pull the thing out and measure it, how are they determining if it’s thin?

As for the age, I’m assuming furnaces are being made “good enough” now, compared to the furnace at my mother’ house that lasted for 40 years. After replacing the fan/limit switch twice, the blower motor, and a fistful of thermocouples over the years, what finally did it in was when the equally old air conditioner gave up, it was cost-effective to replace the whole thing.

metals corrode, there is heat, there are corrosive chemicals formed, there is oxygen.

you can see where the metal has lost.

it will depend on the unit. an old coal fired boiler converted to oil will have a different lifetime than a compact gas unit.

why do you even have a gas furnace in San Diego? rather than replacing it consider a heat pump instead

Keno - I have whatever the house came with when it was built in 1994. If it had a politician in the attic that blew hot air (and they all do), that would be good enough for San Diego where a “bad” winter would be several consecutive days in the 30s with highs in the 50s. I have never crawled up into the attic to actually see what we have.

I understand that a furnace has a usable life, and the fact people are reporting 60 year old models that work fine is encouraging to me. I do find it odd that in a world where there is a building code for everything that modern homes with furnaces of any kind wouldn’t have some kind of automatic CO detector/shutoff system built in, if for no other reason than the fact the furnace producer could be sued if someone died from CO poisoning. After all, everyone knows our state bird in California is the lawyer.

I am buying a detector as a “just in case”, but I’m wondering how the life of a furnace varies based on use and conditions. I would assume a situation like mine where the weather is nearly perfect year round and the thing really only gets used a handful of times for less than 24 hours a year is quite different than an equivalently old furnace in a home in Alaska where it is running non-stop for 9 months out of the year and the exposure of the system in general to sub-zero conditions is common. Where I live, I don’t think we have ever had a day below freezing in 20 years, so I’m inclined to think my system could last my entire life without being changed out. I understand the “average” life of a modern furnace may be 20 years, but I assume that’s for a typical mid-West home, and not one located in an ideal climate like mine.

Yep - any kind of fossil-fueled appliance mandates a CO detector. A fireplace may not be as big a concern as presumably you’d encounter smoke before the CO levels get high enough to be risky, but I could be wrong on that.

My last year of college, I shared an apartment with 2 other girls and we spent 2+ months feeling incredibly tired and wrung out. After spring break, we all felt better. We chalked it up to fatigue due to schoolwork or something, but we all saw doctors who had no suggestions.

Years later I read an article on low-level CO exposure and the lightbulb went off in my head. We got better pretty much as soon as we quit using the furnace. A