Replace Heat Exchanger or Furnace
Visibly cracked heat exchanger, but no measurable CO levels. Here is a possible explanation.
This furnace appears to have an inducer fan, which draws combustion gases through the exchanger and puts the inside of the exchanger under a negative pressure. Typically these furnaces have the indoor blower upstream of the exchanger, which means the indoor air & heat exchanger exterior are under positive pressure. Both of these factors contribute to safety and a positive pressure differential between indoor air and combustion gases. In theory, any cracks or leaks will cause indoor air to leak into the heat exchanger rather than combustion products to leak to the indoor air stream. However, it may be possible these cracks can leak the wrong way, perhaps at startup when the burner can be on for a short time, but the indoor blower has not come on yet.
If cracked, I feel the exchanger or furnace should be replaced regardless of inherent safety. If it is 15-20 years old, I would recommend new. Other components will begin to fail and be costly. Glad to see you recognize there may be a long payback for higher efficiency models.
Last edited by HVAC/R Instr.; 02-22-2012 at 10:41 AM.