Hot water heater problems

If this is a slab floor with water and heating vents in the floor, you probably have a leak in a water pipe draining into the vent. Not a good situation. If this is a wood joisted house, you probably have a leak from a pipe that is seeping into the air supply. Give us some more construction details.

Is your home older? Do you have clay pipes for heating and A/C? I leak in a pipe under your slab floor could cause water to fill the vents due to cracks in the vents that you may not have known about. Try taking a look a the water meter to determine if the leak is on the pressure side of the piping. If a small triangle or star on top of the meter is spinning, you have water currently leaking in your home. If you turn off the water heater, and the star or triangle stops, the leak is on a hot water line. With the water heater turned off, if the water level in the vents slowly lowers, the hot water line is the cause of the water in your vents.