I have a good hvac guy who has been in business a long time and I've always had great service and advice. Central air unit is 15 years old, gas furnace is 25+ years old and having some trouble with it last winter. He is very insistent that I need to replace the ductwork, which is under the house mostly, for about another $1000. I don't understand the need to replace the ductwork. It looks good to me but he said I will need to replace it soon and it's cheaper now rather than coming back later. House is 32 years old. Your expert advice is most welcome.

If there is a dual unit with both AC and the furnace outside, how do I not lose the heat from the furnace unit to the inside of the house. It's not a heat pump.

Did he tell you why the ductwork needs to be replaced? Is it corroding?

Can you see the ductwork? Have you inspected it for any cracks or air leaks? Have you had the ductwork cleaned?

If you are replacing the a/c and the furnace it's a good idea to replace the duct work with new ducting at the same time. You may have a lot of hairline cracks or air leaks and will not get the maximum cooling or heating from your new systems if the ductwork is old. Being 15 years old (or older like 25 years) your HVAC guy is giving you good advice. I don't think he's ripping you off as it just makes good sense to change out the entire system and have it properly sealed up when the new system is installed. This way you will get the maximum effect from your new system. If you can afford to change out the ductwork now, by all means do it.

In less your duct work is to small, or wrong size. Or if it rusting and falling apart I see no reason to change it. What was his exact reasoning for wanting to do so?