I have a Carrier 58MSA furnace which was installed about 7 years ago. On the previous furnace the condensate had been routed through a neutralizer. The installer of the new furnace did not consider the neutralizer necessary and left it out. I tried at the time to determine why it might be necessary but no one could explain why.

I have now found though that the condensate which is being drained into a 3 inch copper DWV stack has been corroding the stack where the condensate tube is attached (via a 1/2 inch elbow welded on to the stack).

I read through all the other posts I could find in the forum on this subject and searched elsewhere but the clearest statement I found was in http://www.hotwater.com/PDFManuals/CycloneHB.pdf

"WHAT ABOUT CONDENSATE NEUTRALIZERS? Condensate neutralizers are usually not necessary. A condensate neutralizer is easy to make by filling a short length of 2" or 3" PVC pipe with landscape marble chips, capping it and installing it in series with the condensate drain of the equipment. Most commercial neutralizers are off the market because of poor demand for the product. Condensation from the exhaust vent piping and tank internal flue way must be allowed to drain. A "blocked flue" indication will often be your first indication that condensate is not draining."

If anyone is aware of any commercial products still available could you please let me know who they are.
Also if you have a recommendation for something to use other than landscape marble chips, could you mention that also.