If you are a cat in my house this is the single most precious piece of square footage you can find. And, now that we have a brand new blower installed in the furnace, it conveniently doubles as a giant hair dryer. This is a feature cats find surprisingly delightful. If you have nothing better to do, it pays to just sit and wait for the heater to kick on.
Otherwise you might encounter a gathering crowd, like tourists in front of the Bellagio before the big water show. Sharing the heating vent is not quite so fun. It requires tact, and diplomacy.
And, if you find yourself trying to share the vent with Piper, who has "personal-space" issues, things can get a little tense. Here Jojo is trying hard not to blink or breathe too noticeably.