Could the problem be that the plant is not in soil anymore?

It sounds like it got a big increase in the amount of direct sun? I don't think asparagus ferns (which are really asparagus, not ferns) like full, all-day sun. It may very well be too much. Also, having AC blowing directly on it would be very rough on the plant - very drying, and very cold.

General rule of thumb is to keep (most) plants that are resting (i.e., not growing and perhaps with reduced foliage) dryer than those in active growth. Many plants that can tolerate or even enjoy standing in water when in active growth will rot if the wetness continues when they are not sucking up lots of water to support lots of leaves and new growth. If the combination of lots more (probably too much) sun, plus cold, dry air blowing on it constantly has shocked it to the point that most of its foliage is yellowing and dying, you could be in danger of rotting the plant. The stress it is undergoing could also make it more vulnerable to rot. Is there any soil left to it, enough to let it dry out a bit without exposing naked roots to air? From what you describe, I'd want to get the plant away from that AC vent, and kept it a bit dryer (if possible) until it starts to send up some new shoots. Good luck!

I've just sprouted a few asparagus ferns and was curious if you ever found the answer to your yellowing. How is it doing these days?

Eric

Your asparagus fern needs acid.

Like dogwoods, azalas, etc., they love acid. Try adding acid to the water. I've never done anything hydroponic, but things I've used for my aspargus ferns are coffee grounds, prepared acidic fertilizer. Outdoors I use pine needle mulch. Let us know what happens!