What's the best way to close up this hole in the venting leaving the hvac?
Would foil tape be sufficient? Right now it bleeds out hot air into the basement in the winter, and cold air in the summer.
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If you're talking about the seam (circled in red). Then yes, foil tape is appropriate.
However, if you're talking about sealing around the pipe (circled in orange). You'll want to use Duct Seal, or an equivalent product.