I recently came across a posting on Trulio that caught my attention.
The seller was being asked to pay for an additional inspection cost after the home inspection report suggested the need for a more expert look at the heat exchanger in a 50 year old functioning furnace.
Sometimes these old furnaces will go on for decades, and while they may not be as efficient as newer models, it may be more budget friendly to keep them until finances permit an upgrade down the line.
Now the problem with the re-inspection, in addition to the cost associated with it is the tendency for inspections by furnace companies to emphasize the ills in an effort to get the business of making repairs and or installing a new furnace.
In some markets, such as mine in the Twin Cities of Minneapolis or St Paul, there are independent furnace inspectors who will give a good professional inspection, but who do not work for a company looking for the possible job.
I recommend you look for such an inspector should you ever encounter a similar situation.