Is the whole system new, including the ducts, register boxes, boots, registers,...., or was a new split unit installed with an old duct system?

The entire system, outside unit , attic fan coil and ductwork is brand new. We have baseboard hot water heat from a gas furnace , so this installation is separate . My husband believes the ductwork to this particular register may not be insulated correctly.
Salbwil

It is very likely that there is hot, humid air in the wall around the back of the register. These things combine to make liquid water from the water vapor. The thing to do might be to look into the register to see if the water is in there or is forming somewhere on the "outside" surface within the wall. That is all you need to see a water stain.

In addition, it is possible that he box/boot behind the register is not sealed well to the wall. That can mix humid air from the wall with the cold air around the register and cause condensation there.

Pressure differentials powered by the AC blower and induced by unbalanced supplies and returns can aggravate air movement through leaks in walls and cause condensation.

The filter indicator probably means the static pressure is high. Ask the contractor to show you the static pressure reading in the service menu. Ideally it should be no greater than 0.5.

Duplicate post.

This post was edited by mike_home on Fri, Jul 11, 14 at 15:24