No neighbor has ever had a problem, you had the air tested? Is it your concern that after 45 years the ducts are going to crack and release asbestos? First in the neighborhood?

For sealing any ducts, I would use water-based mastic backed by fiberglass mesh tape (the type used drywall) or Nashua 324A tape is an extremely nice solution. :)

I understand and appreciate the concern - even if it isn't well founded.

Too bad you can't coat the inside of the existing with a sealant using a nozzle that you pull through the ductwork.

I'm going to take a different stance, not enough attention is paid to indoor residential air quality. I certainly understand the peace of mind in not having lead or asbestos anywhere near my home. And if it's going to properly be removed, the time is before anyone is occupying the building.

If you truly care about air quality, I would also recommend a whole house filter be installed. A good filter would have a low air resistance and a high filtration rate, something like an Aprilaire 5000. Good luck.

Thanks all for your replies and recommendations. The reason why I am concerned about *not* having the reducting done is because from what I have read online about transite duct, specifically in cement slab, it is taking a chance should there be a disturbance to the ducting. We get a fair amount of flash-flooding here, have had a few earthquakes, and other issues. While I understand that it's good news that to my knowledge that has not resulted in damage to the ducts in this house, all it takes is one time for something to happen. The fact is it is a 45+ year old home and so are the ducts. Could it be fine forever just because it's been fine up until now? No one could possibly predict that.

Bsmith - I do like your suggestion to get a good filter, I think that is a good idea regardless of what route I choose to go. And thanks also for the recommendation on what to use as a sealant for the ducts if I don't go the re-ducting route.

I abandoned and removed all of my duct work. I replaced my heating and cooling system with a combination of Mini split heat pumps and Rinnai Energysavers. It is "net to the space" heating/cooling with no distribution system losses. My house is 2200sqft and we are very comfortable.

If you choose to stay with a ducted system you should have the system pressure tested for air leakage on the second floor piping before investing in the first floor repair. Trying to handle this as a first floor second floor is incorrect. You need to analyze the entire system prior to deciding which way to go. When you consider the costs of the duct work for sealing the upstairs and re-piping the first floor you will likely be well on your way to new equipment in the mshp/gas DV direction. Google "DOE duct leakage" and read a bit.

A side benefit for me was that I gained two closets and other space in the living area by eliminating the ductwork.

Jackfre, thanks for the suggestion to have the system pressure tested for leakage. I'm confused about what you are saying. You said the cost to seal the upstairs and re-pipe the first floor, but that is not what would be done.

The first floor vents would be completely sealed and would not be re-piped. There is existing duct work running across the ceiling of the first floor which now supplies the second floor. That would then become the first floor vents, so they would be ceiling vents in the existing ducting for first floor.

Had a hvac guy come yesterday who said that because of the way the existing ducting is he could actually just add a little ducting in the living room and created two duct lines from what exists in the ceiling so that the second floor vents would remain as is in the floor and the first floor vents would be in the existing ceiling ducting.

I plan to get at least 2 more hvac guys to come and if all agree that that is the best way and most effective than it would be far easier than what we previously thought of having to run ducting through the attic for the second floor.

I was quoted nearly $3000 to modify, clean and seal existing ductwork. The workers would have knocked out in a day what took me over a week. However, I have a finished attic and was worried about paneling, framing and insulation being properly disassembled/reassembled. If you want it done right...

A good bit of: elbow grease, foil and fiberglass mesh tape with a sizeable helping of mastic and my unit gratifyingly passed a blower test with way better than average numbers for less than $150.

If you truly wanted you could even purchase ductboard and rent a tablesaw, rebuild all of your own ducts back to original spec. Many of these things can be purchased at your local Home Depot/Lowe's and online at places like Supplyhouse.com and Zorotools.com.

momofone23-I realize this is an old thread but we have the exact same problem and are looking for alternatives. I have been researching online and came across your post. What did you wind up doing? we thought about converting to baseboard or possibly just running ductwork in our attic too for second floor and using second floor ducting for our first floor. Our house is also a colonial about 2400 SF and also on a slab. TIA