ducting for cook top

[CalvinHecht]

My newly installed cook top has a down draft to ducting under my concrete slab to outside wall. It replaces an equivalent downdraft cook top which used the same duct work. When blower is on a huge amouunt of dust comes into cabinet from underground duct. (The old unit also produced dust, but not as much.) Setup is as follows: from cook unit a new 5" duct goes to old 5" duct beginning at bottom of cabinet. This old 5" duct fits inside a 6" duct which goes through the slab, turns 90 deg and runs under slab about 7' to outside wall. The dust, and blower air, enters the cabinet from the annular area between the two ducts. I checked the outside register, and there is fairly good flow to outside, but apparently some air is escaping (or posibly blockage) from the inner duct to the outer duct and flowing backwards into the house.
HELP. I do not have good access to either end of the under slab duct.
Will pulling out old 5" duct, cleaning as much as possible and replacing with new 5" duct do any good, assuming I ca pull out old ducting? Other possibility is just sealing ring area where dust enters, but that seems to be covering over problem instead of fixing, and may not work.

[Ed Imeduc]

I would try and seal where the 5 goes into the 6" pipe for sure first and see what you get then.


ED ;)

[CalvinHecht]

Thank you Ed,
I may try sealing after I have exhausted other alternatives. I am concerned there may be some blockage, in combination with holes in the 5" duct, which is causing the problem. Even after I clear the duct and replace any defective parts, I will still most likely seal that area.

[CalvinHecht]

Follow-up problem:
I made a mistake in my original description of the problem. The old ducting is 4", not 5", as I originally stated. The setup is new 5" ducting from the cook top to the base of the cabinet where it attaches to old 4" duct which fits inside a 6" tube which carries it underground for about 8 feet to the outsde of the house.
The cook top manufacturer suggests a minimum of 5" ducting no longer than 10 feet.
Can I pull out the old 4" duct and replace it with 5" ducting? I also suspect the old 4" duct has holes in it which accounts for the dust blow back problem.
I have tried to obtain professional help, but have not, as of yet, found anyone willing to come and replace the ducting. Any help or recommendations for repair people will be appreciated.