For a 600 CFM hood, 6" round ducting will be fine and is what is spec-ed for most such hoods. My experience is that the 4 square inches difference from round to rectangular does not really make much practical difference to noise or flow in this size and capacity of hood (A bigger deal is getting a canopy hood that will do a noticably better job of collecting steam. etc. than a flat-bottomed one.)

Don't know what hood you have, but my 650 cfm 36" hood came with fittings for venting out the top with either round or rectangular ducting. I've used both with that hood. Initially, I had to use a mix of round and rectangular to make it work with my old kitchen layout but, with further progress on the remodel, I was able to make it all round. No noticeable difference in noise levels.

With your situation, my inclination would be to run the rectangular ducting along the top of the cabinets with an offset transition to round. That puts the top of the round even with the top of the rectangular ducting. I would to cut an elongated hole in the top of the cabinet for fitting the round part down into the cabinet where it can connect with a single 90° elbow and then go down to the top of the hood. That hole will be out of sight behind the crown molding and the interior part usually will be hidden behind closed cabinet doors. In theory, the transition from 6" round to 3x10 rectangular might give you a little freer flow and a bit of muffling the air flow sounds. The biggest deal is that you have only one elbow with this approach.

jwvideo, thanks for the feedback. My hood is a Kitchen Aid KXU8036YSS. It only has the rectangle output. If I understand you correctly, your suggestion would go (from the top of the hood, going up): Rectangular transition to 6" round, 6" round up to top of cabinet, offset transition/elbow from 6" round to flat rectangular, rectangular on top of cabinet out to wall. Any concerns with the rectangular to round transition right off the hood? I was under the impression that was not suggested because the air will be hitting the round transition at its highest speed.

Just checked on the Kitchen Aid KXU8036YSS for accessories and it seems to be one that is designed only for a rectangular ducting.

Jeez, you've got kind of a "rock and hard place" dilemma.

First thing that occurs to me is to ask if your cabinets will be deep enough to fit a vertical 90° elbow with a horizontal left-hand 90° elbow together and not have the ducting sticking out a couple of inches over the top of your cabinets as they runs left to the wall? Beyond that, the double elbow won't keep the venting from working but it does diminish the efficiency. Pretty much like adding an extra 10 feet of run to the ducting. Second thing is maybe this: go up vertically with a section of 3x10 rectangular duct, and a transition section to round, add a round 90° elbow, then an offset round to rectangular transition and the rectangular to the wall.

Flow rate should be the same throughout the duct (barring leaks) and thus the average air speed will vary at each location along the duct in proportion to the effective sectional area of the duct at that location. If the effects of frictional losses from the greater exposed surface area for a given length in the case of the rectangular duct relative to the round duct, the difference in speeds along the duct should be quite modest. Of greater importance to minimizing pressure loss and hence noise is having a transition shape that smoothly interfaces the rectangle to the circle.

Note that this sectional shape transition will very likely be a lesser cause of pressure loss than the duct interface to the hood -- typically a more abrupt transition. It likely will also be a lesser cause of pressure loss than the baffles.

Be sure as installation progresses that screws and transition shapes do not interfere with damper opening or closing.

Anticipate that even with low resistance make-up air, your blower will probably not move more than roughly 400 CFM at full power due to the unavoidable pressure losses of the duct, baffles, weather cap, and hood transition. The blower will only move 600 CFM when hanging in the air sans hood (zero static pressure across the blower). Without a deliberate MUA path, the achieved air flow rate will be even less such that the lower pressure inside the house (plus the pressure losses just listed) causes an air leakage flow rate that matches a point on the blower's fan curve.

The safety aspect is to not let the house pressure drop below 0.03 inches water column if there are any un-blown combustion appliances in use taking air from a volume connected to the kitchen. Otherwise, there can be back-drafting of the combustion products, likely including carbon monoxide. When in doubt, take measurements, use separate MUA for the appliance, or provide low loss MUA to the kitchen. Blown combustion appliances, such as oil burner furnaces, can tolerate somewhat higher pressure drops, but perhaps not more than 0.06 inches.

kas

Thanks again for the feedback. Yeah, the other idea the cabinet guy had was to run the 6" round inside the top cabinets. It is actually not a bad idea. The run is really not 8 ft., it is just 6, and of those 6, 3 are in the corner cabinet, which is a wasted space anyhow. I'm going to have to buy a couple of those transitions and play with it a bit and see what fits better. I'm going to come out of the hood with a rectangular duct for sure. I'll need to decide on how to make the 90 degree left turn with two rectangular transitions on top of the cabinet, or a 90 degree transition from rectangular to 6" round inside the cabinet. Cheers!

Ok, it seems like I won't be able to do it all in rectangular duct. The turn at the top of the cabinet is approx. 17" wide. The top cabinets are only 12" deep. I'll try tomorrow with a transition elbow from rectangular to 6" round at the top and see if I can hide it on top of the cabinet. My cabinet guy said he can install a 5" crown molding on the top cabinet. In addition, we can carve a round "seat" on top of the cabinet frames. That may conceal the 6" duct completely.