"how dangerous is it to continue to run this one."

You can very easily end up dead.
You will fall asleep one night and NOT wake up.

If the guy was licensed he probably should have red tagged the thing and turned off the gas.

You sure can't sell the house that way. If the HX is in warranty, have it changed. If not, better replace it.

Definately in agreement, the tech should have shut off the gas and red tagged the furnace to protect you and him. Besides having a new furnace will certainly help not hinder the sale of the house when the time comes.

In my area, only the gas company has the authority to red tag. It's my job to disable the system and have the customer sign off on what I found. If they are foolish enough to relight it, I'm in the clear and Darwin has struck again.

Do not use this furnace if it does have a crack. Did the tech use a infa red camera did you see the crack? a second opinion would not hurt if no camera was used. Call around most company's don't have one . Good Luck

Do you have CO detectors in your home? If so, are they placed low, rather than near the ceiling? If you've been using a cracked furnace I would think these detectors would be beeping. If you don't have one, get one with a digital display that will show you the level of CO that it detects. Just remember that CO is heavier than CO2, which is why the detectors should be low. A forced-air furnace is a perfect appliance to pump CO into the living area.

Like Brickeyee said, you will go to bed one night and never wake up if this is bad enough. In our municipality, CO detectors are required by law in all homes. We had so many people killed (entire families, actually) that the city finally had to do something.

Just out of curiosity....why is it that a cracked heat exchanger in a natural gas fired furnace is likely to be lethal, when most natural gas ranges, nautural gas ovens, and unvented natural gas fireplaces are not potentially lethal?

While I definitely think it's a bad idea to operate any furnace with a cracked heat exchanger, I can't figure out why a natural gas furnace would potentially be any more dangerous that the afore mentioned unvented natural gas appliances.

Anyone have a definitive answer?

I don't have the definitive answer, but common sense explains that gas ranges, ovens and fireplaces are not in use 24 hours a day, unlike a furnace. They are also going to be a lower BTU rating than a furnace.

There's something about the combustion characteristics. I sound so scientific! :-)

wa8b,

Any appliance that is gas fired and unvented IS potentially lethal. If a gas range is used continuously as a supplimental heating source, CO will buildup in the structure and possibly cause death. If the furnace is a draw thru type (Heat exchanger on the inlet side of fan) a negative pressure is created on the heat exchanger and CO is drawn into the airstream. Portable unvented kerosene heaters and potentially lethal. Any source of combustion results in CO. Operating an appliance for an extended amount of time causes buildup. If an owner knowingly sells a home with a cracked heat exchanger, the owner must disclose the fault or can be held liable.

While I'm no fan of unvented natural gas appliances, and as I mentioned in my earlier post, I think it's a poor idea to operate a furnace with a cracked heat exchanger -- it still seems to me that doing so wouldn't necessarily be lethal.

Natural gas furnaces don't usually run continuously. They cycle on and off. Unvented gas fireplaces, on the other hand, may be left burning continuously for hours on end. (It's worth noting that some states like California don't allow unvented gas fireplaces -- but many others do)

The burners in a large gas range or ovens probably produce as much or more combustion byproducts as a furnace would likely spew through a crack in the heat exchanger.

I've never seen a residential natural gas furnace (or any other type, for that matter), with the heat exchanger on the air inlet side of the fan. Every furnace I've ever seen pushes air through the heat exchanger, so the negative air pressure is more likely to be on the combustion exhaust side.

I can't comment on kerosene heaters, I've no experience, but they weren't a part of this discussion, anyway.

Maybe I'm just dense or lack common sense, but I still don't quite understand why a cracked heat exchanger is likely to be lethal. Undesirable, yes. Maybe even a fire hazard -- but not necessarily lethal.

And what planet did you come from,wa8b ? To quote your own words, yes you are dense. All kitchen gas top cookers are
vented. well they should be. How many gas cook tops do you see that puts out 90,000 B.T.U. ? Unvented fireplaces up here are not permitted ANYWHERE. No unvented appliance is
permitted. Gas cook tops and gas ovens have to be vented even just by an overhead hood. Maybe we know something you don't know. Heat exchangers are positive pressure from the outside. If they crack the fan will blow back into the exchanger and roll out throught the burner inlets flooding the basement or furnace room. Kerosene heaters are not made to be on 24-7. Says so on the lable. Even then there is enough air leaks and people opening and closing doors to get an air change. How can a heat exchanger be a fire hazard ? It's already on fire. If there is found a crack in the exchanger in your furnace and was left to run most of the night would you spend the night in the same house ?
I wouldn't. Even if i did wake up next morning how many
millions of brain calls did i lose ? If i still knew what a
brain was and could think on my own. I'm a licenced gas fitter and compeled by law to shut your gas off if i find a
life threatining leak or other problems with the furnace.
I then call the gas company and they come out and look. If they say yes it's bad your gas stays off. If they say no, no biggy i turn your gas on and you are on your own my hands are clean. good luck. That is very rare if not going to happen, but sometimes it does. When i say shut the gas off it stays off. The gas company trusts us and we are god
so to speak. I've only shut the gas off once in the winter
in a home because a mother called 911 when her sleeping baby, on the floor turned blue. The hospital found Co poisoning in the blood. The heat exchanger was cracked.
The baby didn't make it. I still feel sick when it think about it. Got to go.

wa8b:
Here's a couple of thoughts for you:

/an oven left on all night to provide heat in an enclosed apartement will soon use up all the regular air (oxygen) but will continue combustion. The gas input stays the same but the air drawn in to the burner lacks enough oxygen as it is full of the byproducts of the previous combustion. Now it begins to produce high levels of CO.
Saw this once and the renter was lucky. The CO was high.

A crack in the heat exchanger interfers with the normal mixture of air and gas. Watch the flames on a gas furnace when the blower comes on; the air pushed by the blower goes into the crack and combustion is now not normal. A techs instrument will show a high level of CO. Most goes up the chimney but other factors can make it enter the airspace in a basement or it goes into the ducts and into the main house. Not good.

I have seen a crack 3" long, looked like a 7 on it's side.
The blower pushed the flames back out one inshot burner and burned and melted the wires in the cabinet near the gas valve.

Incomplete combustion causes soot to form. It builds up in some heat exchangers so much that the flames actually do not rise up but will bend downward and go outwards and bring the products of combustion (CO for one) out of the furnace and into the area around the furnace. They should stay within the heat exchanger and then in the chimney pipe to the chimney and up and out. Or thru pvc pipe on high efficiency units. The blower is still running so they will be sucked in to the return air and sent into the home.

Anyone in the trade has seen cracks. Some units have most of them in certain spots. While they seem small, how do you know the metal won't just give and the crack gets larger or becomes a hole? Who wants the responsibility? I as a former tech didn't. My boss didn't. Tag and shut off the gas, get a signature, no matter what the weather.

Hope this helps

Wiley, thanks for your comments. I completely agree that a cracked heat exchanger is not a good situation. I'm sure you're correct about a crack interfering with optimal firing, and I can also understand that it might allow some amount of CO to enter the living space or the area around the furnace -- but probably not at lethal levels.

I guess my original question had to do with a crack in the heat exchanger being deadly. Several early posters on this thread posited that a cracked heat exchanger could lead to death and that the furnace should have been disabled and "red-tagged".

While I certainly agree that a cracked heat exchanger needs to be repaired, or the furnace replaced as soon as possible, I just thought it was probably a stretch to hypothesize that the situation was an eminent threat to life. Most likely, there are countless cracked heat exchangers out there that no one has yet discovered -- yet countless households aren't being asphyxiated.

That being said, I also think if someone selling a house is aware of a major problem with the heating system (like a cracked heat exchanger), they would be obligated to repair it before selling, or at the very least, disclose the situation to any potential buyer. As a matter of fact, in some states, failure to disclose a known problem is not only a major liability problem, it's a crime.

I think you should look on the web for more complete info.

You are not understanding the cumulative effect of CO. It gets in the air stream, at least you acknowledge that.

It is not a stretch but I can see how you wonder that with all the furnaces out there with cracks in them, why aren't more people dying? I don't have an answer to that cause you would have to look at each one differently. Some cracks don't allow enough air in them to interfere with combustion--yet. But at 10 below the furnace will run longer and maybe get hotter. Then the crack can eventually expand.

I was tuning up a furnace and a woman was down in an unfinished basement with me while I checked over her 1 yr old gas furnces with inshot burners, an 80% efficient one that vented up a chimney. She had a CO detector on a basement wall and I suggest moving it up to the hall by bedrooms. She did while I finished up the check and fired up the furnace. The CO detector went off within a few minutes. I took the detector outside to clear it and ran the furnace again. Same result. I checked the furnace and tho it looked like it was operating ok (no flame back, no dancing flame), but there in one chamber was a definite crack. The inshot burners are factory set for air/gas mixture and the gas manifold pressure was correct but this unit was and would have produced enough CO in a night to be deadly.

Some doctors misdiagnose CO poisoning (low levels) as the flu because the symptoms are similar.

People selling houses want to get the maximum price back. They usually don't want an old furnace replaced, let alone checked over by a professional company. House inspectors are reluctant to have a complete inspection. They just want it certified that it was turned on and it lit. Period.

Here's a link to an article written by the University of Alaska Fairbanks. (PDF file.) Its language is written more for a general audience, so it won't answer questions about how big a crack has to be before it kills. More often I'm sure it just makes people feel ill.

Here is a link that might be useful: UAF Carbon Monoxide Article

A strong contender for a Darwin award here.

One of the problems with CO poisoning is that it slowly accumulates in the blood since it bonds to hemoglobin better than oxygen.

You essentially concentrate it in your blood until the oxygen carrying capacity is not high enough to sustain consciousness.

You are now unconscious in a CO atmosphere and it continues to saturate your body.

Eventually the heart cannot beat from lack of oxygen.

Every fall when heaters get turned on there are a number of deaths from blocked flues and cracked heat exchangers.

Going to sleep in a house with a gas heater and a cracked heat exchanger is like playing Russian roulette.

While some cracks are hard to see and you might need an IR camera, I have seen plenty that have been plainly visible to the naked eye.

Technician came on routine maintenance and said the CO level was very high and shutoff the furnace and possible cause is cracked HE, my question is why my new sensors installed at the floor level in basement or first floor did not go off.

"And what planet did you come from,wa8b ? To quote your own words, yes you are dense. All kitchen gas top cookers are
vented. well they should be. How many gas cook tops do you see that puts out 90,000 B.T.U. ? Unvented fireplaces up here are not permitted ANYWHERE. No unvented appliance is
permitted. Gas cook tops and gas ovens have to be vented even just by an overhead hood."

I think you're being a little harsh. Actually, in many areas gas ranges and ovens are not or have not been required to be vented.

kq06:

I have reduced the CO level in a gas furnace by adjusting the air shutters and/or gas pressure. also, dirty burners will cause high co readings in a flue.

A decent tech will take the time to show you the crack. You may have to pay for the time to remove a burner or two but it is worth it. A tech out to make a buck or an inexperienced one may misread their insturment.

Floor level in basement? Are there heat vents down there?

Turn on the furnace and place the CO detector near a heat register or just above it. A small nail will hold it.

Some CO detectors read accumulative and some are not working properly due to age or defects caused by other products.

Did you see the instrument the tech used?

Kalining...

Greetings from planet Earth.

Unvented gas fireplaces designed specifically for this type of operation are allowed here in Rhode Island. However, neighboring Massachusetts allows no type of unvented gas heaters.

No New England state requires vent hoods or exhaust for residential gas cooking appliances. Other states may have some restrictions I am unaware of.

Plantlover2...

Unless you want to be replaced by Plantlover3, do not run that furnace.

Peace.

Marco

I would like to see full disclosure here. For a part that costs several hundred to a few thousand dollars these extreme opinions about death make it hard to believe money isn't the motivating factor behind the scare tactics. And quite frankly it sounds like a design flaw that something this "dangerous" doesn't have a built in cutoff when it's broken. How many crack in the middle of the winter 3 years ago and were used until it wouldn't fire up again in the fall?

you don't usually need ir camera to see cracked
heat exchanger. lots of cracks are visible to naked eye.
there are less expensive tools to identify gas leaks.
if heater is in use ir will show leak, but if it
is not in use..where is the temp difference?

co monitors are used to warn homeowners of leaks.

in my area hvac contractors disconnect gas
from furnace.

get a second opinion if you feel the need to OP
but don't risk your life over this.

best of luck.

Awesome, this is a 3 year old thread.

I find it interesting you have searched for a three year old thread in order to post your comment.

There is some elemement of a scare tactic in tagging and shutting down a furnace. The contractor does want you to buy a new furnace from him. But the dangers of a cracked heat exchanger are very real. There is a legal liability that an HVAC contractor shut down a furnace which show signs of being dangerous.

There thousands of furnaces in use at this moment that have cracks and the homeowners don't know it. These furnaces continue to operate until they eventually fail for some other reason. This is not a reason to ignore the issue.

Heat exchangers crack due to excessive heat. This could be caused by not changing the filter or poor installation. There is a high heat limit switch that will shut down the furnace, but it does not guarantee it will prevent the heat exchanger from cracking. There is no detector which can signal a crack in metal. I suppose a carbon monoxide detector could be built into the furnace. The detector would probably have many false positive readings as the furnace cycled on and off.

If an HVAC technician says you have a cracked heat exchanger, then get a second opinion if you doubt his findings. It is better to be safe than sorry.

"these extreme opinions about death make it hard to believe money isn't the motivating factor behind the scare tactics."

They are NOT "extreme opinions."

In many jurisdictions any licensed serviceman is required by law to shut down and red tag ANY combustion heating appliance with a cracked heat exchanger.

A number of places do NOT allow any unvented gas appliance larger than a stove or oven.

These appliances are normally run for only short periods in a residential setting, greatly reducing the possible hazard.

A central heat system runs as long as the thermostat keeps it on.
At the outdoor design temperature that will be very close to 100% of the time.

No one will even weld a heat exchanger because of the liability and safety issues.

You only die once, rushing it is not viewed as beneficial.

Diagnosing a cracked heat exchanger is a major scam tactic used by HVAC companies to scare homeowners into a quick sale!

Before you buy, please read this blog my husband wrote to address this growing scare tactic used by dishonest HVAC companies.

Here is a link that might be useful: Cracked Heat Exchangers SCAM ALERT!!!

I agree a cracked heat exchanger is dangerous. However, I also advise anyone being told they have a cracked one to get a second opinion before replacing the furnace! We were lied to this week and almost bought a new furnace. The first (and a well known local company) that came out did not even cycle the furnace to listen to the sound we had noticed. Just went to doing a video camera inspection then condemned the furnace and left us with no heat. Until the next day when we got a reputable firm out to do the same video inspection and found ZERO problem. The first firm even took a tiny jumper cable off the gas valve to make sure we thought there was a problem. ALWAYS get at least a second opinion and estimate! We did end up replacing the 23 year old furnace, but only out of an abundance of concern. At least we paid $1500 instead of $3500 that the first crooks wanted............and got a better unit I believe. After all was done, I called the first firm and told them if they would refund their service call, I would not pursue legally. He was more than happy to comply. NEVER get just one estimate.