posted 7 years ago
Hi Bob,
According to Erica and Ernie's book, the downdraft space should be at least 4X the cross sectional area of the main plumbing. In your case it is a 6" round pipe which is about 30 square inches of cross sectional area and your 3.5 inch gap gives roughly 205 square inches so you are between 6X and 7X. Should work great.
BTW if you want to calculate this yourself, the formula for area of a circle is pi * r^2 ( i.e. pi * radius squared). This calculation gives 30 si for the 6 in circle. Then you have a 7.5in (15 in Dia ) inner diameter which gives 177 si and for the outer diameter you have 11in (22 in Dia) which gives 380 si. We subtract the inner diameter area from the outer diameter area and divide by the system diameter area. This gives (380 - 177)/30 = 203/30 = 6.77.
I hope this helps and didn't bore you to tears.
Sincerely,
Ralph