I'm Harvey the Master Plumber.
Your question is: You took over as owner builder when your builder "took the $ and ran". You are installing kitchen cabs now and discovered the PVC vent pipe (which goes into the foundation and out the side of the house
) is in the wrong place for the location of the downdraft unit and integral blower. The vent is actually under the electrical connection panel. You were told initially( 2 yrs ago when you bought the downdraft cook'n'vent) there could be no bends in the exhaust fan route or it would change the effectiveness. Does anyone know if it is ok to put a bend in the connection to the vent? Or do you need to redo your kitchen cabinets?
I can syphathise. I have had to deal with this same problem. I have found that one ell will not degrade the effectiveness enough to notice. If you need more then you may need to adjust cabinets.
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Harvey your Master Plumber