We recently had our furnace serviced by a pro and using his CO detector, he found 21ppm of carbon monoxide coming out of the registers. He promptly turned off the furnace and gas, stating he was required to do so and that we needed a new furnace. He said the heat exchange unit was cracked, though he didn't actually look at it (I think it was an assumption). The model of the furnace is a Lennox Conservator III -- about 21 years old. It is very clean inside (it has an air-bear filter) but he said that particular model had problems with the heat exchanger rusting & cracking.

Now before anyone worries, I have not turned the furnace back on (we also have a wood stove so are OK for the moment). But another HVAC friend said to get another opinion because he's seen people burned by a "supposed" heat exchange crack when they really didn't have any problem. These guys seem pretty reputable, though, and I've been pleased with other work they've done (moving a gas line for me recently). But here are some of my questions:

1. When he moved his CO detector away from the register and across the room, the values dropped back down to 0. Also, when he placed it right over a candle we were burning, it jumped back up to 20 something ppm. So can it really be *that* big of a deal? Is there an "accepted" minimum # of CO that can come from a furnace?

2. The replacement suggested was an Amana either AMV8 or AMV9. I would lean towards the AMV8 since we have a wood stove in the basement and the variable speed would help move air upstairs. Plus we wouldn't be using the furnace that much due to the wood stove (mainly as a backup) so I don't think we'd see that much difference in costs between the 80 & 96% efficiencies. They quoted a price of about $2800 to install an AMV8 (90,000 BTU). Does that sound reasonable?

Thanks for your helpful responses!

 

The furnace combustion gasses should circulate around the inside of the heat exchanger. The room air should circulate around the outside of the heat exchanger, and those two sections should be sealed away from each other some that combustion gasses can't contaminate the room air of the dwelling spaces.

Getting 20 PPM co out of a heating register is a safety hazard --- for one thing it means that you are getting the same reading from all the other heating registers as well.

You can imagine the volume of combustion gasses the candle produces. Now compare that with the volume of air being circulated around the house by the furnace.

I used to respond to complaints of carbon monoxide hazards for a gas utility --- thousands of them over the years. On several occasion I found a single dirty pilot smaller than a candle flame making enough carbon monoxide to make people sick.


On the other hand, it's quite true that cracked heat exchangers can be exaggerated, especially if the repairman doesn't bother inspecting the equipment.

And a properly burning gas burner makes only small amounts of carbon monoxide - 20-40 PPM in the combustion gasses might be typical. Dirty burners can produce several thousand PPM CO, and when that happens you might get some contamination of the circulating air even with a good heat exchanger.

In short, in my view your repairman should have inspected the furnace to determine the cause of the CO problem, and should have made an effort to actually observe a crack or defect in the heat exchangers. He didn't do a very thorough job.

So getting a second opinion by an honest and competent repairman may be worthwhile. You might find that cleaning the burners solves the CO problem and that the heat exchangers are OK. Or perhaps there are problems there that justify replacing the equipment --- that may well be the case.


Too bad the repairman didn't do a more thorough job for you. You could also ask the same company to come back out and complete the job that should have been done. If they are a good outfit, they should do that for you.

 

No CO should come from the furnace.

 

Good rule of thumb, airman. But if you have very high levels of CO, air can get contaminated even if the heat exchanger is intact. There could also be other sources of CO contaminating the air stream, from other sources.

A more thorough examination of the furnace is warrented, and frankly if a CO problem is detected, EVERY fuel burning appliance in and around the house should be inspected as a possible source of the problem.

 

Wow, thanks for the fast replies. Well, apparently I was not accurate in my description. My wife was here when the inspector came and I just talked to her again after posting this. Apparently there was quite a bit of rust on the walls of the "tubes" above the burners. He showed that to her and said it could eventually be a problem. But then after checking CO levels, he was pretty sure it was the problem. So I guess that better convinces me.. sorry for not better checking on that before posting.

So I guess at this point the question is, does that $2,800 sound pretty reasonable for an AMV8 installed (about 90,000 BTU input)?

 

Some rust on the walls of a heat exchanger isn't unusual. It's usually bare rolled steel.

I've been called out by customers of the utility I worked for to check out "holes" "found" by overeager repairmen that were not there. Sometimes they were still there, and pretty embarrassed to find that the "hole" they found was a speck of surface rust.

There may indeed be a problem. But pointing to a non hole in the heat exchanger (a speck of rust) is not the same thing at all as proving it.

At least the guy LOOKED in the heat exchanger! Did he take a read of the carbon monoxide levels in the combustion gasses to see if that was the actual problem and if so the PPM of CO in the combustion gasses?

Checking the CO levels in the combustion gasses is a routine test that ought to be done whenever there is a suspicion there is combustion problem with a furnace, water heater or other gas appliance.

The utility I worked for required that EVERY fuel burning appliance in and around a house that was inspected for CO be checked for whether and how much CO was being produced. Suppose you have a water heater next to the furnace that is actually the problem, spilling CO into the vicinity of the furnace where it's being recirculated through the house. Would your repairman have picked up on that hazard? No--- at least based on what you've reported so far.


Sorry, still not impressed by the job that was done. That's not to say there isn't a problem --- only that I don't see that it's been proven to any reasonable degree of reliability.