I would expect no longevity effect from a less-than-specified-diameter duct. Pressure loss in the duct will be higher, and hence flow rate from the hood blower will be lower, but the fan curve in concert with a duct loss calculator would have to be consulted to determine how much. For a given volumetric flow rate, the velocity of the air will be higher and hence the turbulence noise from the air flow will be higher.

One might want to believe that replacing a six foot run with the correct size duct would be straightforward. Failure of the installer to measure the hood interface or read the directions is not the customer's fault. This is the type of oversight that adequate oversight would have caught.

kas

Kaseki thank you for your response. I have great confidence in and appreciation for GW-ers and informing us novices. There's so much to know and try to stay on top of that is out of my comfort zone. Being a peacemaker is not an asset when one remodels.

Thanks to your great advice Kas and the kitchen forum GW-ers, I called Zephyr and talked to a service guy. He said technically the duct should be a minimum of 7". It isn't "extremely critical" (hesitation in service person's voice) "but it would be better all the way around if specs had been followed. If it were mine, I'd have them change it out." Talk with our contractor went well. He wants us to be happy so tomorrow morning they will change out the 6" duct for the 7" duct.

Thanks to GW and your advice we now are getting what we expected and what Zephyr states is required. Thanks and thanks again!