I am stumped by the behavior of my Whirlpool AD50USR dehumidifier.

I have the dehumidifier mounted on a shelf in my basement with the
drain flowing into the washing machine drain pipe.

Everything worked fine until I took it down for a day to use in
another room.

When I put it back up, it still did all the usual
fan blowing and cycling of the compressor, but the volume of water
dripping into the container slowed by 90% or more.

Interestingly, when I put it back down on the floor in the same
basement room, it started working perfectly again.

When I took another identical unit and put it on the shelf, it too
slowed down markedly but worked perfectly again back on the floor.

Note:
1. The shelf is perfectly level
2. There is reasonable ventillation on all sides (plus it did work
before; also it works on the floor even when the ventillation there
is artificially constricted even more than when on the shelf)
3. There is plenty of humidity in the room at all heights (plus if the
humidity were too low at the higher height, the unit would have
shut off)
4. The temperature of the exhaust air is about the same whether on the
shelf or the ground, suggesting that the refrigerant is being
compressed the same. When I removed the cover, the front coil felt
cold though I did not compare the cold temperature in both cases.
5. The cycle time, the fan noise, the compressor noise, etc appear the
same in both situations.


About the only difference between the two setups is the height and the
fact that when mounted on the shelf, the shelf slides under the unit
between the wheels vs. the floor situation where it rides on the wheels
directly (however, as stated before the unit is level in both
situations).

I am stumped, particularly, since it worked before but now neither
unit works on the shelf and I can't think of anything that has
changed?

Is there anything subtle I could be missing or is my house just
haunted ?

Thanks