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This crawlspace grate/vent crawlspace floor grate/vent is located in the furnace/water heater closet of my rambler house outside of Seattle, WA, built ~1959. The closet is covered by two bifold doors, so not airtight, there is also a vent in the ceiling of the closet that leads to the roof. My neighbor has the same setup, so I know this is intentional and original to the house design. My neighbor claims it is for ventilation, drawing 'fresh' air in from the outside. I can confirm I occasionally feel airflow coming into the house. However all my google-fu can't find anything similar in description or purpose.

Tried doing some research here, two related posts I found here on DIY:

  • This post also has an indoor crawlspace grate. Like my neighbor, the landlord in that post claims it is for ventilation. Unlike that post, I do have a central furnace and my grate is original to the house.
  • This post has a duct that is directly heating the crawlspace, perhaps this is a passive way of doing the same? Seem like a huge heatsink though (not seen in practice).

I'm fairly certain this is also how mice found their way into my house. I am considering putting in 1/4" hardware mesh in from the crawlspace side to keep them out however not knowing the purpose I'm a little hesitant to slow down airflow. What is this for?

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  • "How common is radon in Seattle? The risk is highest in the Cascades, but a patchwork of small areas exist on the Eastside and the biggest zone, including much of the neighborhood west of Seward Park in Seattle."
    – Mazura
    Commented Sep 29, 2023 at 2:19

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Yes, your neighbor is correct. It's for admitting fresh air into that utility closet where you have two combustion appliances. Without sufficient fresh air, the two gas appliances could deplete the oxygen in the closet and producing carbon monoxide.

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    I appreciate the confirmation. Seems to me adding 1/4" hardware mesh shouldn't be a problem given the fresh air is just for the closet. Commented Sep 28, 2023 at 2:52
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    Hardware cloth should be no problem, and will help keep pests out.
    – KMJ
    Commented Sep 28, 2023 at 5:00
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    Since it's not an air-tight space, you are unlikely to 'deplete the oxygen'. What would likely happen is that the appliances would draw air from the living space which will then be replaced by air drawn in through drafts. If the rest of your house were really tight, then you could actually have bigger issues. In order for the combustion gases to vent, you need some source of air to replace their volume.
    – JimmyJames
    Commented Sep 28, 2023 at 16:44

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