If a heating vent is leaking air, you can seal it with mastic sealant. You can also use duct tape to seal the leak, but only as a temporary solution.
Seal a heating vent that is leaking air by first determining the cause of the leaks. Turn on the heating system, and locate the ducts, which are usually located in attics or below the house in basements or craw spaces. Leaks are generally marked by black cracks, but if you can't find cracks, feel around the duct with your fingers to search for leaks. Leaks occur most often at vent connections in floors and walls. Once you find the leak, apply a thick layer of mastic sealant. The sealant is a liquid that hardens but remains flexible to account for contraction and expansion of the vents.
Your heating vent is leaking if you come across hot or cold spots in your home. When dust settles on surfaces when you turn on the heater or air conditioner and your utility bills increase for no apparent reason, a leak is likely present. Hot and cold spots occur due to leakages because the heating system isn't working evenly. Where leaks occur, the house feels cold while heat collects in other areas. Leaky vents tend to absorb dust and blow it out when the heater is turned on.