Should I close crawl space foundation vents in winter with a furnace present?

Question

I am living in a house for the first time that needs proper winterization in the winter. However, the house that I am renting was built in 1941 and is rather convoluted in a lot of ways. I was looking to put foam blocks on the inside of what appeared to be foundation vents. There are 4 vents in total and each are 14.5" x 7". They are found only underneath the living room where the furnace resides.

I'm starting to suspect that these are not foundation vents and are instead vents related to the furnace considering their placement, in addition to the fact that they have a metal door that is attached to the vent (see video).

The top two corners of each vent are directly underneath the house where the brick stops and the wood starts and therefore the vents cannot be taken off completely, making it nearly impossible to stick foam underneath. The nails that were used to keep the bottom two corners in place have been painted over (same color as the vents) and are also very old and brittle, which also supports my theory that these are not necessary to insulate every year.

I'm interested in hearing any and all thoughts, as I've shown this to a few people already but are collectively stumped.

Video

Answer 1

Probably intake air for furnace which is used for combustion and then goes up the chimney. Even if that was a vent for a crawl space, blocking it up would create moisture issues.

Answer 2

I don't have a clear picture of the overall layout, but no one would install four vents for furnace makeup air. These are crawl space vents intended to reduce moisture during damp times of the year, and therefore mold, decay, and pests. They're closable to retain heat in winter.

Your winterization strategy partly depends on specific conditions, but I'd go ahead and place your foam panels^*, then keep an eye on indoor humidity. If it's very high you might keep a bit of airflow happening.

Be sure to open them in spring when you're not actively heating.

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^* Note that insulating those panels does little to improve overall insulation, assuming that your foundation is also uninsulated. What it would do is reduce airflow, which is massively more costly in terms of heating cost. If you don't already have insulation in your floor, that's where I would focus.