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I noticed an oil stain on the air intake vent cover in my kitchen ceiling which leads to the garage.

enter image description here

My theory is that this is due to the lack of ventilation in the kitchen as we don’t have a hood or pipe sucking the fumes out of the kitchen and into the outside. Hence, the air goes out into this vent and that maybe explains the oil stain?

However, it gets more interesting. When looking into that vent above in the ceiling I noticed a horizontal piece of wood was sealed which means air couldn’t escape into the garage. So I made three holes to improve the air flow.

enter image description here

Not sure if these problems are related. I’ve had multiple experts in hvac mention that the wood should not have been there and so we made the decision to make three holes ther in order to improve the air quality. If you take a look at the pic, there seems to be oil stains on the left most hole, which is also the hole closest to the stove which might explain my theory. I would love for some guidance on how to solve this.

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  • (edited to add pics and extra context, since we know each other IRL)
    – peter
    Commented Sep 2, 2023 at 20:13
  • no pics attached.
    – RMDman
    Commented Sep 2, 2023 at 20:39
  • looks like my edit is still pending review
    – peter
    Commented Sep 2, 2023 at 20:48
  • Using a joist bay as a ventilation duct it not all that unusual. But putting blocking in that bay pretty much completely defeats the purpose of that ...
    – brhans
    Commented Sep 3, 2023 at 1:22
  • Could be blocking. Do you have other similar pieces of wood in other joist bays? Still, not right to simply cut holes in it and use the joist bay as a duct. Should have run a duct.
    – Huesmann
    Commented Sep 3, 2023 at 13:17

3 Answers 3

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I have no idea what the intended use of that vent was, but venting it into your garage by drilling those holes is at best a code violation, and at worst a huge carbon monoxide safety risk.

In all likelihood that vent was fitted with the intention of connecting a fan and duct to the true exterior of the home, or it was connected to make it look appear like a functional kitchen vent. I would prefer to believe the first.

Only real solution is to have this vent properly connected to the home’s exterior, or even better have a proper range hood installed.

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Its hard to tell in the picture, but it could be oil. It would be normal for the oils and fat to condense somewhere. And they are a fire hazard.

The solution is obviously a proper range hood. They have filters, collectors for oils and fats. This removes, or drasticslly reduces the flow of oils and fats through the vent. Ours has channels in the filters for the oils to condense and collect.

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Hot moist food laden air should never come into contact with semi-porous materials (i.e. wood). That is a recipe for mold. Your photos shows white powdery substance on the top. Has that been tested. DIT vent fan installation is a common cause of mold growth. Usually this was done prior to knowing any better.

The true fix is a ducted (metal) exhaust fan discharging outside. You should not be using joist cavities to move any "conditioned air".

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