cracked heat exchanger???

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Old 01-06-09, 01:43 PM
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cracked heat exchanger???

I have a gas fired-induced draft warm air furnace that started to emit an odor around the same time I changed the filter.(about a week ago) I called a local hvac company to check what the problem was. he came in and walked around with a CO detector from room to room and it read about 25ppm consistently throughout. without even checking the heat exchanger he said i needed a new furnace. it is about 20 yrs old and in need of replacement but can he tell without looking that that is whats wrong?? P.S. it is the only fuel burning appliance i have. Should i get a second opinion??
 
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Old 01-06-09, 01:53 PM
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If you are at 25ppm Id say yes it is bad. 9 ppm is the max for an 8 hour time.
 
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Old 01-06-09, 02:09 PM
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Yes. Frankly, the inspection as you describe it was grossly inadequate. I've responded to thousands of complaaints of possible carbon monoxide hazards when I was a gas utility employee.


The first thing to do is to inspect EVERY fuel burning appliance in and around the house for possible carbon monoxide production. Cars, water heater, gas range, furnace--- whatever.

In particular, he should have checked the CO2 in the combustion gasses of the furnace. Were that the problem, they would have been substantially elevated --- probably 200 PPM or perhaps much higher.

Were that the case, it would have indicated a likely defect with the furnace such as dirty burners or whatever that should be repaired.


An odd odor of the kind you describe can be caused by a defective gas burner. Is there a reason why you associate it with the furnace?

If you have natural gas, you might want to ask your utility what kind of inspection they might do for you. The utility I worked for typically sent a well equipped, trained and experienced repairman out to do a thorough inspection at no cost with thirty minutes of receiving the call.
 
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Old 01-06-09, 02:10 PM
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thanx for the quick response airman. I guess its time to lay her to rest anyway.
 
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Old 01-06-09, 02:12 PM
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I might add that I found numerous cases that would duplicate that 25PPM reading throughout a house that was caused by a car that had been warmed up on a cold day in the garage or even outdoors with the garage door open.

After 2-3 hours the exhust would gradually wend it's way inside the house and set off a CO alarm, triggering a service call.

Carbon monoxide is very close to the same specific gravity as air, so it tends to hang around for hours in a house rather than disspating.
 
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Old 01-06-09, 02:13 PM
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its the only fuel burning appliance i have, Seattle
 
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Old 01-06-09, 02:30 PM
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SeattlePioneer, I'm pondering your combustion gas response and I'm baffled. Why would I smell a defective burner outside of the chamber? Please dont take offense, I just dont understand it. I appreciate any advice you give.
 
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Old 01-06-09, 03:08 PM
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No problem.

The gas burns on the inside of the heat exchanger. Air for the house flows around the outside of the heat exchanger. If the heat exchanger is intact, there should be no mixing of the combustion gasses with the room air.

That's why a cracked heat exchanger can be a hazard.

Actually, in order to have an actual hazard, you must have two defects --- a cracked heat exchanger and dirty or otherwise defective burners or combustion process that produces excessive CO in the combustion gasses.

If the burners are working properly, no significant amount of CO is produced even if there is a crack.

So--- how old is the furnace and when did it get the last get the regular maintenance work manufacturers recommed be done each year?

Modern furnaces can usually get away with having that maintenance and inspection done less often than yearly, but going too long isn't good either.
 
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Old 01-06-09, 03:19 PM
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I was told the furnace is about 20 yrs. the odor does smell used fuel-like but I'm no expert. my buddy says the burner flame looks good(blue w/just a seldom whip of orange here and there, which he believes is dirt.) I bought this house 4 years ago and have not had it serviced, just changed filters. I I believe the previous owners were up on those things though.
 
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Old 01-06-09, 03:48 PM
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Do you have any sort of fresh air intake to the house or say an open basement window near where the exhaust discharges, or some other vent that where say with an atmospheric air pressure inversion could send it back into the house?

What about doing a smoke ball test dropped into the combustion chamber and run the blower? I have never done that yet, but aim to on an erroneously replaced (I believe) furnace with similar complaint as you. An HVAC expert at a HVAC-electrical-plumbing supply house told me they sell the smoke balls for like $1.29 each. Best to have exchanger up to temp when doing it in case a crack in the exchanger only opens up more with heat.
 
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Old 01-06-09, 05:04 PM
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A dirty furnace burner can produce very stinky combustion gasses and lots of CO. They easily and rapidly pegged out my CO detector above 2000 PPM.

While carbon monoxide is odorless, other intermediate products of combustion, notably aldehydes (an alcohol) are what people usually smell.

You could take a whiff of what's coming out of your chimney or vent pipe if you want to try that.

But you can see why the vent pipe should have been checked directly by the serviceman you had do the inspection.

And I usually preferred to SHOW people what I found, so they would understand and appreciate what I was showing them. If they know that the OSHA limit for 8 hour exposure is 50 PPM and they see the multimeter rapidly climb to 2000 before pegging out, customers tend to appreciate what they are being told.



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Old 01-07-09, 01:24 PM
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UPDATE:cracked heat exchanger???

there was a hole in the secondary heat exchanger. thanx to all who responded.
 
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Old 01-07-09, 02:23 PM
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BTW, a simple visual test: if a heat exchanger crack is bad enough it will sometimes cause the flame at the burners to waver momentarily when the blower motor comes on (as this causes a pressure change at the exterior of the heat exchanger).

The fact that this does not happen is not sufficient to establish that the heat exchanger is intact, but if it does occur, it's suggestive of a heat exchanger failure.
 
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