The short answer to this question is "No." Vents are there to provide for a necessary free flow of air. Block that flow, and your home will run sub-optimally, and you could even cause damage to your HVAC system. But sometimes the vent is in an extremely inconvenient place -- the only wall big enough for your sofa or your library-media-wall system, the best section of the room for the bed, or the dresser, or the dining room sideboard. Then you have to grit your teeth and figure out a solution.
You might be courting mildew and mold by blocking the vent with a sofa, chair or bed. A combination of warm temperatures, high humidity and stagnant air is an invitation to mustiness in organic fabrics such as linen, cotton or other natural and some synthetic fibers and fillings. Those materials are hygroscopic; they soak up moisture like sponges. Air circulation helps to lower humidity and dry the room. Park a big hunk of upholstery -- or your prized Persian carpet -- over the vent, and you might relegate that piece and other furnishings in the room to landfill status.
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Wood cells are made to contain water, and even as lumber, the wood in your furniture will shrink and swell as the humidity in the environment changes. Placing wood furniture on or too near air vents will almost certainly damage the wood. The forced air is very drying and could cause joints to loosen and boards to crack. If another piece of furniture is blocking the vent, exceptionally moist conditions in the room might cause wood furniture to warp.
Fire, Freeze and Financial Hazard
Never mind that you would love to have a toasty warm room in the dead of winter, that sofa, highboy, or bookcase over the vent could also wreck your furnace. A furnace works with a metal coil called a heat exchanger that heats the air forced across it by the system's blowers. When a vent is blocked, less cool air flows across the heat exchanger, which then overheats, and expands, and cracks. Huge bill, very chilly house, unhappy homeowner. It's unlikely but not impossible that blocking a heating vent and causing the furnace apparatus to overheat could cause a fire.
Put the furniture over the vent in summertime, and you decrease the overall air flow from a central air-conditioning system. Blocking a vent or vents creates a pressure imbalance that causes the system to work harder, produce less cool air, use more expensive energy, and potentially results in a frozen coil -- a very expensive repair and replacement job. The house temperature and the replacement charge will have you breaking out in a sweat.
Follow sensible and safe guidelines for furniture placement when you have to work around vents. Leave at least 18 inches between furniture and vents to promote free air flow. Don't park the stuffed sofa or your futon over a vent because you will end up with mildew and mold. Respect the nature of your wood pieces, especially fragile antiques, and keep them out of the line of fire of a vent to prevent excessive swelling or shrinking and the resultant damage.
As a last-ditch solution when vents and furnishings collide, invest in an air deflector. Deflectors, or vent covers, magnetically connect to the vent and redirect the air flow, pushing it out from beneath a cupboard or couch to encourage intended air circulation and distribute heated or chilled air.
If you plan to protect pets or toddlers from contact with the vents, invest in custom grilles and radiator covers that hide plain builder-grade vents, match your decor and keep curious paws and fingers safe.