Crawl Space Vents

On the foundation wall of houses with crawl spaces, there are vents to allow the flow of air under the house. My question is: When should you open and close them? Presently, I close them after the first frost and open them after what is believed to be the last frost.

I am just trying to save an HVAC dollar.

“Quoth the Raven, ‘Nevermore.’”
E A Poe

The only reason to close the vents is if you fear frozen pipes. The vents are there to prevent moisture from accumulating in the crawl-space. Sure, you can close 'em and save a few bucks on your gas bill but, in the long run, you’re exposing your wood framing to possible moisture damage.

Where possible, wrap exposed pipes with insulation and place moisture barrier on bare earth. You should leave at least half of the vents open year round.

And of course you should never close them if you have kids buried in there a la John Wayne Gacy.

In California, we have them but they are always open. Ive seen new houses put in and they just use a screen that is always open 24-7. Have no idea why yours are adjustable.