Re: hvac vent under bathroom vanity?
If you do nothing, I don't think there will be any efficiency loss. Any heat absorbed by the vanity (during heating season) will eventually be radiated back into the room. During cooling season, the cold air will absorb heat from the vanity and cool it off. Then the vanity will absorb heat from the air, cooling it off.
What you may notice is that the air in the room doesn't heat or cool as quickly after you adjust the thermostat. But once the desired temperature is reached, you won't know the difference.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.