Can you flash a light and side the opening? Do you see any duct work? If not then base of the cabinet is being used as the duct which is a terrible practice in my opinion. The right way to do this is to a flexible for fixed duct underneath the cabinet. The toe kick ductor is one way to do it. There is probably no code for this in your area. And if there is, the inspector did not bother getting on his hands and knees to see how it was done.

Hi mike thanks for your comment, it looks like there are no ducts. i suppose adding metal ducts would help with efficiency and directing airflow better?

is there any way to fix it without Adding a vent cover?

It will be difficult to attach and secure a flexible duct with the cabinet installed. You will likely have to cut a opening in the bottom of the cabinet to gain access. If this is a cabinet of drawers then you won't see the opening and the repair. For a base cabinet you could install a thin sheet of wood veneer to cover the opening. If you put in a shelf liner on top no one will ever know.

The toe kick height is usually 3.5 inches. If the opening is half of that then it is only 1.75 inches high. That is a very small opening. My suggestion is increase the height of the opening and install a vent cover that matches the toe kick color. It will be barely noticeable and you will get improved air flow.

You stated "air vents under several cabinets". How many is several? I'm guessing one main supply duct feeding a chase in the toe kick area but vented out several openings.

It's all about square inches. A six in duct is 28 sq. in and if you add up the square inches of all the slots under the toe kick it will come close to the main duct feeding the toe kicks. Not an issue, also the heat picked up by the cabinets will transfer into the kitchen and also not an issue. Heat is energy, energy can not be created or destroyed, and can only be transferred from one form to another. They will give up the heat to the room, and from there it will transfer to the outside.

I wish I would of thought of that when I installed a heat vent coming out my toe kick. Same area as the vent supplying it(all metal duct) but a chase with slots would make all the floors warmer and not just the one in front of the vent.

I also have had a house with this situation. I wouldn't worry too much about the heat. It'll all even out like kevin mentioned. What I found was that the duct through the floor was through a poorly cut hole, and that was the entryway for mice in the kitchen, and gave them access to all of the cabinets. When we renovated kitchen I sealed everything up tight.

Thank you all, we have four different rooms with this situation. Only One of them is under a drawer, the rest are doors. the drawer is the one I am mostly worried about since it gets much more warm, but It’s also the easiest to fix if we ever wanted to.

thank you!