The damper only opens when the hood is operating .

Ideally, for hood venting there should be a damper of some type at each end of the duct. Each damper should open toward the outside, and block when there is no airflow due to gravity or springs. One damper will likely be sufficient if the duct is very short.

Make-up air systems would operate with the damper direction reversed.

I ended up removing damper on range hood as this seem to be the only viable option.

It's one thing to dig a dead critter out of a duct or chimney, I've done it several times, but good luck eating anything out of a microwave that's had a dead thing gouged out of it.

Metal coathanger and you're good to go.

@Ed Nelson: Would you mind supplying a link to this "Houzz" hood? Who actually manufactures it? Does the installation guide specify a duct size?

@Joseph Corlett, LLC: I had to reread the OP's question to realize to what your apparently droll comment referred.

@Ed Nelson: The point is that the duct needs a damper at its exterior end, not just to keep cold air out of the house, but also to keep out critters. A damper at the hood end keeps cold air that forms in the duct when not in use if the duct passes through a cold attic from settling into the kitchen, and house humidity from rising into the duct when it is off and freezing to the duct. Double dampering is also more effective against wind gusts.

Trying to read between your lines, perhaps the hood interface with your rectangular duct cannot support whatever damper you received with the hood. A rectangular duct damper could be used. In any case, get one for the hood cap or for the duct just under the hood cap.

The exterior roof cap has a damper. I just can't use the rectangular damper at the range hood end and had to remove it. The range hood is Hauslane UC-PS18-30. If cold air entering the kitchen becomes a problem then I will probably have to make my own damper and install it inside the rectangular duct as I do not see anyone selling a damper that can be mounted inside a 3 1/4 x 12 inch rectangular duct.

Every over-the-range microwave that I've ever installed had a 3 1/4" x 12" rectangular male with a damper built in.

Listed as 860 CFM, probably free air volumetric flow for the blowers, one might expect 500 - 600 CFM in the hood with good ducting and good MUA. (To comply with the IRC and to reach a reasonable fraction of the hood's rating, a deliberate means of MUA is required.)

Rectangular ducts aren't too good at acting like circular ducts, but assuming that the flow is even across the 3 1/4" x 12" sectional area, the flow rate at 500 CFM would be 1800 ft/min, 2300 ft/min at 600 CFM. These are on the high side for pressure loss and low noise.

@Joseph Corlett, LLC: We are talking about range hood not micro wave but yes the range hood came with a 3 1/4" x 10" rectangular male with a damper.

@kaseki: Assuming that the flow is even across the 3 1/4" x 12" sectional area. This is the problem. See below.

The horizontal adapter attaches at the back of the hood. The back of the hood mounts directly against the wall. The 3 1/4" x 10" rectangular male adapter with a damper stick out about 1 inch from the back of the hood. This 1 inch extends past the 1/2 inch sheetrock and into the the 3 1/2 x 12 inch VERTICAL rectangular duct. The damper flap supplied with the hood is approximately 3 inches in height. When the hood damper opens we now reduce the vertical duct open to about 1/2 x 10 inches and I think this will greatly affect the range hood exhaust capability ( I may be wrong) . If I had about 3 inches of horizontal duct before the vertical duct there would be no problem.

I am thinking of modifying the supplied adapter to have two 1 1/2 inch dampers in the same space as the the 3 inch damper but still I think the opening would then be 2 1/2" x 12" instead of 3 1/4 x 12.

Ah so. Can you go up into the cabinet a few inches and connect to the stud cavity duct farther up. Obviously, some cabinet space would have to be sacrificed and and some ravaging of the stud cavity ducting would be needed.

Alternatively, if there is room in the hood, perhaps the supplied damper can be moved into the hood mounted to a short section of duct that mates with the wall.

Better would be to gut the cabinet and go up through it with an 8-inch duct and into the attic, continuing the 8-inch to an appropriate roof cap.

@kaseki: Paragraph #1 and #3 would require major rework and if I started that my wife would want a whole kitchen remodel. :)

Paragraph #2 that was my first thought but is not possible with this range hood.

So for now will keep the damper removed at the hood and if problems arise then modify adapter two have dual dampers in place of single damper.

Thanks for your help