Derek Cole explains why you shouldn't block floor vents with furniture. Air needs to rise along outside walls to heat them properly. Blocked vents disrupt the whole circulation pattern and leave cold spots.
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What I have seen and what you need to be careful of is when the vents are in the floor, do not put the furniture over the vents. Do not put a couch smack dab against the wall at the return. That will cause a problem. So back up a little bit, give it a little bit of room. And don't put furniture and things like that over the floor. Grills. Because it's trying to. If it's on the floor. Okay. It's supposed to be on an outside wall. So it's trying to condition that wall. So the air is blowing up. I mean, if you were, like doing a smoke test and you can actually see it, the air would be going up, heating the ceiling, and then falling back down to condition the wall. So if you have furniture or something in the way, obviously.