I think the 9" will look wimpy with your true-pro-style Bluestar. Keep the 18" size. I would be more concerned with cfms. The PRH18 (36" width) comes in two sizes of cfms, 300 and 600. Which one did you get?

You should be looking at the rated CFMs for Ventahood products-- not their marketing tactic of using "equivalent CFMs".

We have all seen hoods, so how complex could kitchen ventilation possibly be?

The answer depends on what one cooks, and what performance one expects from the ventilation. Diving into the subject will reveal that ventilation may be surpassed in complexity only by structural issues (when removing walls and overloading floors) and by the multi-element juggling exercise of choosing and laying out the kitchen counters, cabinets, and appliances.

My thought is that review of recent ventilation related threads on this
forum will provide the OP with information on cooktop ventilation
requirements and the hood features that best meet them, while limiting
the weekly number of treatises I have to write on the subject.

kas.

There is adequate ventilation and there is optimal. We work with the kitchens and budgets we have. Since you apparently have the budget and the room for it in your kitchen, the PRH18 will be more optimal than the 9" high models with your RCS range.

I'm assuming you meant the NPH9 or PRH9 models when you referred to 9" Ventahoods. Is that correct?

An NPH9 or PRH9 will not necessarily be a bad choice. They will just be less optimal than the PRH18 with your 36" RCS. To oversimplify the treatises that Kas mentioned, consider the following:

1. Hood capture design and coverage area are as important and maybe more important than sheer CFM numbers. The 18" tall PRH 18 has a decent canopy making it more optimal for your RCS stove than VAH's sleeker and shallower VAH 9" models.
2. You can get the PRH18 in a 42" width which further enhances capture from the outer sides of your 36" wide RCS stove top. I believe that the widest NPH9 and PRH9 models are a less optimal 36" wide.
3. The PRH18 is 24" deep which gives better coverage and capture over pots on the front burners than the 21" deep PRH9 and NPH9 models.
4. Whatever you get, do not overlook make-up air MUA. We are talking real-world laws of physics as much or more than code requirements. OTOH, the low speed on your 600 CFM motor/fan might be significantly quieter when run at low speeds for those times when you do not have flaming woks going simultaneously on all six burners of the RCS at once.

Had 42"X27" 600 CFM VH in very similar config to PRH18 over 36" Wolf with grill. Concur with above post for this versus 9" for increased capture hood space and larger canopy dimensions. With the range and hoods you're looking at I don't think there may be any "overkill" on the hood. Even with a wall mount and flanking cabinets a little effluent would still get out now and then while grilling or woking.

Thanks everyone. I think we'll stick with the 18". jwvideo, it was the PRH9 that we were considering. Kaseki, do you have a link to your treatise? I'll look for it.

Please do not take this as an attack on you, but Kas has been posting almost weekly for years on the subject of range hoods. He was not kidding about weekly treatises. There are dozens and dozens of his posts that could be called treatises. If you try this search string with google: "gardenweb + range hood" --- you will find enough treatise-like discussions to keep you reading for hours.

Thanks very much, jwvideo and kaseki. Sorry, I had edited my reply above to answer M. Miller's question about blowers, but it didn't seem to take. We're getting the 600 CFM model.