underground duct

Please Help. I'm add a addition on my house. But new codes are forcing me to pour a slab. So i'm having to put my duct work under the slab. But my real concern is my main truck is under my house and i be running 4 new ducts about 35' off my trunk. The furnace is about 30 years old already, But had it checked by 3 guys and all said its in great shape and more the big enough for my house and addition. Just want to know if it will push 4 more ducts out 35' and keep the new rooms heated and cooled. And what is a good duct to put underground? It will be filled with sand and a slab on top.
Thanks

Just a thought...

I’m not an A/C guy, but I have been doing a lot of research on duct work, air volume, etc; and I have made some major improvements on existing duct work systems, on a lot of homes.

You may have to post a little more information as far as the size of you addition, size of the vents, if you’re going to install an air return in the addition, and what percentage will the addition be based on the total size of your house. Also it might help if you state if you will be tapping into the main run at the start, middle, or at the end of the run. Like I said, I’m not an expert by far, but this information might be helpful for the experts that read your post.
Here in Phoenix, they use PVC for underground duck work. You might see what material, (for underground duct work) is code in your area, and go from there.

Good luck,

Mike

Not familiar at all with anything underground, so we will have to wait for some more input. My first impression is ....how do you guarantee to keep out water, bugs, and roots????

I have worries also about the water/bugs. My first thought is its own system.

addition

You use large PVC pipe and fittings, but what kind of code "MAKES" you pour a slab? Usually you use any kind of foundation you want to.

Run the duct in the attic.

Here, (just for fun) is an example of poor workmanship of how not to install underground duct work. This, (concrete pipe) duct work was found when the owners of the house wanted to do a bathroom remodel.
Apparently, the previous owners of the house already did a remodel on the bathroom, where they relocated the air handler and installed the tub where the old air handler use to be.
I don’t think this system was too air tight. Notice the snake skin, and there was also a ton of rat droppings….yuck!
I would think getting a separate system for the room addition might be the way to go, (depending on a lot of other factors not know at this point). If you used the existing system, (even if the existing system will handle the addition) you then have to be concerned on trying to balance the air delivery in the existing house, and to the addition. This maybe as simple as adjusting the vents, or could be as complicated as installing dampers and diverters etc.

Mike