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.

...i'm so glad i found this forum. i had this exact thing happen to me yesterday. heating guy comes out and starts dismantling the furnace. within 3-4 minutes he takes off this little panel, maybe 3x4, think he called it 'limit control'. he goes bad news, you have a cracked heat exchanger, you need a new furnace. mind me, he is looking in a small opening at probably 1% of the heat exchanger.he said rub your finger over and feel how sharp it is. well i did and it was as smooth as a baby's bottom. after researching by googling "heat exchanger scam" i am 100% certain he was pulling a fast one. told me due to their liability he had to shut the unit down and if i used it the furnace would be sending out deadly carbon monoxide. i look at it this way, he gave me a quote of 3,000$ to replace the furnace. my guess is 1,500$ is pure profit. what would these guys rahter do, run around and make 20 calls for a furnace tune-up or replace 1 guys furnace. just google this scam and you'll get a better idea of what these guys are doing. here's 1 of my favorites: http://carbonmonoxidemyths.com/Myth-1.html. or just go to youtube and watch the fox news report. Cracked Exchanger Scam One of the most common furnace repair scams involves homeowners being led to believe their furnace has a crack in its heat exchanger. The scam unfolds when a dishonest repairman is called to inspect an idle furnace. Upon arrival, repairmen act as though they are inspecting heat exchangers, which most homeowners are unfamiliar with. After reaching around inside the exchange, workers inform the homeowner that the unit is cracked and if the furnace is turned on, it could emit carbon monoxide into the home. Whether there is a crack in the heat exchanger or not, dishonest repairmen usually tell victims they, by law, must not turn on the furnace. This often prompts scam victims to pay for a new furnace they may not need. Read more: Furnace Repair Scams : eHow.com http://www.ehow.com/list_7425898_furnace-repair-scams.html#ixzz2AzvFpkUD

crawlfish I would say you are grossly oversized. and I suspect you will continue to have problems until you get a correctly sized package unit and corresponding correctly sized/ designed ductwork system installed by a competent and reliable dealer. I suspect you are wasting your time and money in bandaid-jury rigging "fixes". sorry for your trouble. IMO

Thanks. The Carrier quote has essentially fallen through. The contractor apparently was quoting me for a 93% non-tax credit eligible model (MEC), though he never mentioned it previously. I don't think he was trying to trick me, but it means I wouldn't be eligible to save $1,000 or more on the furnace. Even if the AC qualified, I prefer getting a 2-speed furnace at 95% for the tax credit along with a lower SEER AC (not a very high SEER AC with a non-tax-credit eligible furnace). The Trane dealer will get back to me with his bottom line on the XV later tonight. I'm leaning towards going with him with either the XR or the XV because I have more faith in him and his company at this point. The Bryant controller puts the price a little beyond what we'd like to spend considering that we don't plan to be in the house past 5 years. Also, it took a week to get an estimate and it's hard to get return calls, so I don't have faith that they'd really get the items installed next week. Thanks for any additional thoughts.