It is not going to start a fire or create mold. It will reduce the air flow. You can try it and see if the room maintains the set temperature.

I think I would like to pull the woven part of the rug straight up to edge of the vent, and allow the fringe to cover it. The fringe is maybe half an inch and I don’t think fringe would actually block any air flow (it’s a Persian rug).

If you are going to do it, you should CLOSE the vent. That way the heat from the vent is NOT damaging the RUG! If it is a real Persian rug, I wouldn't let it be anywhere NEAR a heat source. The chance of heat damage to the rug would be far too high.

Remember: heat damages FIBRES and backings as well as can create a point of combustion.

can you cover vents with a rug?

https://www.airconditionersductless.com/

Vents Need 10 Inches of Space. These vents need at least 10 inches of clearance to work, but more clearance is better.

Also, is it OK to cover air vents? You could mess with the air flow. Just because you cover the vent, it doesn't mean your HVAC system knows that. It will keep pushing air to those vents, and if they're closed, you could cause a buildup of air pressure and insufficient airflow which could damage your system.

Fire, Freeze and Financial Hazard When a vent is blocked, less cool air flows across the heat exchanger, which then overheats, and expands, and cracks. It's unlikely but not impossible that blocking a heating vent and causing the furnace apparatus to overheat could cause a fire.

Here are some simple ways you can do just that: Keep couches, chairs and other large pieces of furniture at least 10 inches away from your cold air return vents. Avoid covering your cold air return vents with curtains, drapes or rugs.

thanks everyone.

Ok- I can definitely keep the chairs at least 10 inches away from the vent. I think as far as the rug goes, I can pull it back so no part of the rug is over the vent, but it’s still going to end near the edge of the wooden vent on the floor. It sounds like the concern now is actually damage to the rug?

is it an actual vent (like heat comes out of it), or a return (air flows into it to offset what gets blown out elsewhere)? often large floor vents are the latter, but that will affect the temperature or other issues.

so sorry i missed these comments- it’s an actual vent where heat and air come out. I also misspoke- it’s a rug from Pakistan. It’s not a Persian rug. It’s supposedly new but in my experience one can never really tell the exact history of textiles. It doesn’t have any stains and I don’t see any evidence of repairs (which I have found when examining “new” rugs in the past).