What height above the floor is the existing duct? AFAIK, only lower line and OTR microwave/fans use a 10" x 3.25" rear duct. If there is enough height, you could use a 90 degree angle into a 10 x 3.25 transition.

I have no idea what you are asking :-)

The people who will be moving the duct told me it is a pretty standard 6" or 7" round duct. They just didn't realize that the soffit was concrete and therefore the actual vent would have to be in the rear instead of on top through the soffit.

If height permits, you can put a 90 degree elbow in a top-venting hood and connect it to the duct. The bottom of the hood is usually anywhere from 60-66" above floor height (or 24-30" above counter). Add to this the height of the hood, and then 3"+ (duct centre) for the 90 degree elbow. That's why it is important to know how high the ducting you're planning to hook into is off the floor, or how high the contractor can install the wall duct.

You still might be able to use your Bosch hood as the height seems to be very low. The elbow is hidden by the duct shroud.

See:
https://www.bunnings.co.nz/-/media/nz/diy-advice-house/indoor%20and%20decorating/kitchen/appliances/how%20to%20install%20a%20rangehood/0315_installrangehood_29.jpg

I can check height but I don’t think that would work. When I was in my unit with the GC they were explaining why they needed a rear vent but I will take it up with them again

Vent-a-hood has some rear venting liners. This one is designed for a cabinet. Can't tell from the photo if it will work or not.

https://www.ajmadison.com/cgi-bin/ajmadison/KH34SLDSS.html

Looking at the photo and gauging the height given the ladders, my guess is that you have a really, really low ceiling (7'?) where the soffit is and the range will be.

Can you forego a liner and go for a straight hood?

Here is a real life example of hood mounting.

Below is my installation (completed a couple of days ago) - vented through the roof as the cooktop is not by an outside wall. I can put in a duct cover (custom-made, which is a lot less than the manufacturer's) any way I vent it. In this case, the hood is 18" high and mounted 30" above counter so I would need at least a 7 1/2' ceiling to exhaust through the wall.

The hood (pretty much identical) in my previous house had a 90 degree elbow vent out the back wall and it was mounted only 24" above counter so it could have worked with a 7' ceiling (but I had, IIRC, 9 or 10').

If you can forego having a decorative hood/iner combo, here is a Bosch that might work for you. The Bosch term for rear vent is horizontal discharge.

https://www.bosch-home.ca/en/productlist/cooking-baking/ventilation/under-cabinet-hoods/DUH36252UC?breadcrumb=builtundercookerhoods#/Tabs=tab-4357071/

Just looked at the Ventahood liner mentioned above. Top venting with a 10x3.25" cutout for the elbow. I suspect the GC will have a few choice words when installing it ("install the duct from outside of the home...").

Installation guide

I found these Kobe liners that seem to only need the standard 6” round duct I don’t even know what a 10” x 3” duct would look like as I’ve got a fairly small hole that ties into my building’s venting system.

Almost everyone in my building has an over the range combo microwave vent but I didn’t want the microwave over the stove. Do those vent out the back or do they even vent at all. I had one and I think all it did was make noise as it had no discernible ability to suck anything into it

http://www.koberangehoods.com/products/full-catalog/?filters=product-type[1218]|size[681]|exhaust[1228]

https://www.ventahood.com/index.php/products/liner-inserts/li-wm?id=360&model=KHSLD

Still vents initially through the top.

Needs one of this or this at the top to vent to the rear.

Yes, but it was a lot lower profile so there will be more space to make the turn.

This Zephyr insert can vent directly through the back of the liner.

https://zephyronline.com/product/monsoon-mini-insert/

I will contact the HVAC guy who is doing the duct work for me to attempt to clarify but in the meantime could someone explain what a 3" x 10.25" vent is.

Just means that it's a rectangular vent vs a round vent.

can an HVAC person construct the vent so that it is either round or rectangular?

The rectangular duct is designed to fit inside walls for N. American construction (i.e., 2x4 construction - spacing is nominally 3.5" between drywall and 14'5" between wall studs). If your building already uses or the GC has to use rectangular ducts, it may be better to stay with that with your hood insert than transition from round to rectangular.

If you let us know what floor to ceiling height is where your cooktop will go, it will let us know how limited or open your options are.

can an HVAC person construct the vent so that it is either round or rectangular?

You can buy prefab rectangular to round transition pieces.

Thank you all very much for your help. Of course, the HVAC person and GC are knowledgeable but I like to have some understanding of issues/solutions when I talk to people and I am completely out of my depth in terms of duct work - I would be the one making the ultimate choice of vent/hood based on what is possible and within those limitations, I would like to be able to concentrate on function - especially ease of taking off the filter for cleaning.

I had read instructions for removing a filter which seemed to require scrambling around on top of the counter and I had tried out removing the Bosch filters in person and found that even a non-mechanically included person could remove simply and then put back on easily without any tools or poking around in the innards of the mechanism.

I think I remember reading instructions on removing squirrel cage type of filters and they seemed daunting to me :-)

Thanks for help. FWIW, the distance from concrete soffit to floor is 7 feet as expected.

I have narrowed it down to either Zephyr or Kobe insert liner as both have the horizontal vent in the rear. The horizontal attachment will be moved slightly to the left as that will be where the new cooktop will be moved. I am assuming that there will be some kind of attachment to the current opening which vents to the building's venting system - whence it goes after it leaves my kitchen I have no cares as long as it passes inspection.

I am not familiar with either of these brands - does anyone have any recommendations as to one or the other. My venting needs frankly are minimal as I don't deep fry and will have an induction cooktop.

I am considering mid range in each brand:

KOBE CORE COLLECTION - This retails for about $850. It's ADA compliant with a remote which seems like it would be more convenient than reaching for controls under the cabinet.

Monsoon I Insert

Monsoon I takes one-piece liners to the next level. Includes dual-level halogen lights, and ADA-compliant wireless remote control, 6-speed electronic controls and either a powerful 600 CFM blower (30″ and 36″ models) or 1,200 CFM blower (42″ and 48″ models). ACT™ available on all models.

Vertical and horizontal

Zephyr - I couldn't find any functional difference in these two models except one has slightly higher CFM but I am sure the lower would be perfectly adequate for my needs

DELUXE IN26 SQB-650-5A

BUILT IN / INSERT 30" - 36"

700 CFM

$803.49

PREMIUM IN2630SQB-700-2

BUILT IN / INSERT 30" - 30"

750 CFM

Build.com $771