Quote:
Originally Posted by ductworker
I am not certain about low velocity systems/domestic. But any duct fitting with a slope angle of more than 24 degrees (1:2.4) will put extra resistance in the duct. Sure at times this is impossible and not practical, so that is why the system will require a fan to overcome the duct resistance. Try and keep all rectangular, square and circular offsets and reducers max 24 degree slope and
bends to halve the duct cheek (Rect.duct)dimension, circular duct the Dia.

> I would like to offer a calculation to use to insure that you don't exceed the 24 degree slope on reducers/transitions.(this is NOT an endorsement of any friction/resistance formula. Merely a formula to easily obtain answers for the above)
> Without boring you with the Trig.calculations,it is this. Multiply the reduction by 2.25. The centerline length of the fitting must be AT LEAST this amount.
> Example: a flat on one side reducer goes from 16" to 10". That is a 6"reduction on one side. multiply the reduction by the 2.25 factor. the result is the minimum length for the fitting,13.5" ( 2.25 x 6=13.5)
>>>NOTE (this factor,of 2.25 is only for a criterion of 24 degrees)