So far I had one estimate and the salesman did not inspect vents at all and there was no mention of vents in his estimate. I guess that means if there were a problem during installation I'd be hit with a bill for vent work, right? Are they suppose to automatically look at the vents when estimating? Or is it up to me to ask?

Please and thank you. Dee

Or, are you talking about the 'piping' to carry away your exhaust gases, and possibly to supply combustion air?

If he is replacing a furnace with one of the same efficiency, then (probably) no exhaust changes are necessary.

Or, he could be looking at a fast in and out job.

How about a heat calculation?

v

A HERS rater can test your duct system. Some utilities have raters on staff. You can find a rater below.
http://www.natresnet.org/directory/raters.aspx

Some raters can also perform Manual J load calculations. Then you have the right sized-equipment for your house,

http://www.energystar.gov/ia/new_homes/features/DuctSystems_062906.pdf

If they are simply performing a replacement of same size unit - it is common to just leave the old ducts in place and reuse them. This may or may not be a good idea. Only an on-site evaluation can answer that question.