Central air not dehumidifying house

I can't figure this one out. I have noticed lately (since buying some temp/humidity gauges to put around the house) that when the air is on, the humidity in the house (3000sqft brick/masonry incl finished basement) refuses to drop below 60% no matter how long the system runs. The furnace is a Trane XV90 (installed 2006), the A/C is a very old 4 ton Rheem (mfd 1994) whose only flaw is that it is very expensive to run - otherwise it works fine. Supply plenum temp is 55 degrees, return plenum temp is 77 degrees. Outdoor temps have been in the 100's (lows in the high 70's) for the past week and a half.

The house stays where I set the thermostat, but I am sure I could raise the thermostat a couple degrees if I could just get the humidity down. The drain line is constantly dripping into the floor drain, but I did notice that the high position of the drain outlet in the pan does allow a fair amount of water to remain in the pan itself.

I have tried lowering the airflow CFM (currently 350CFM/ton, originally 450, also tried 400), and I have also set up the system with the humidistat (in the return plenum) on the R/BK terminals to work with Comfort-R profile (I have also enabled Comfort-R on the thermostat to delay the fan an additional 30 seconds at startup). Due to the humidistat constantly being above its setpoint (50%) the Comfort-R on the furnace never lets the blower go above 80%

I have even gone so far as to close off the basement registers/returns (I don't have damp basement issues but just to rule it out), and cover/seal the 'fresh air intake' eyebrow (it has a simple flapper damper right before it hits the return plenum that doesn't really seal too well) to ensure humid air isn't being pulled in from outside.

So could it be just the standing water in the pan re-evaporating? I did notice that the small hot lines (the ones that connect each individual coil circuit to the main liquid line) were sitting in the water, so I propped them up out of the water with little pieces of PVC pipe, but it did not seem to make any difference.

 

the refrigerant charge needs checked. Did you clip the jumper on the control board when you connected the humidistat?

 

Yes I clipped the jumper. The Comfort-R works properly (Thermostat closes Y to turn on compressor, delays 30 seconds then turns on G - then blower turns on 10% for 1 minute, 50% for 8 mins, then 80% as long as humidistat is above setpoint).

I can't see how low refrigerant would make it cool well but keep 60% humidity. I have 55 degree air coming off the coil. There's water condensing on the coil (and on the uninsulated part of the suction line), so it's doing its job..

 

Not low refrigerant but possibly overcharged. Also your duct work plays a role in how everything works.

 

Set the fan to auto to prevent condensed moisture from re-evaporating off the coil when the condenser shuts off.

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How old is the house? Could air leakage be the culprit?

Do you run exhaust fans when showering and cooking?

 

I guess I should've clarified - the thermostat is set to auto fan. The Comfort-R profiles let the coil cool down before ramping up the blower, then if the humidistat is still above setpoint when the cooling call ends the blower shuts down immediately. If the humidistat drops below the setpoint during the cooling call, then the blower ramps up to 100%, and it runs on for 60 seconds after the cooling call ends.

The house is 1939, all brick/masonry construction. All interior walls are mudwall (concrete over steel mesh). No wood/plaster/drywall anywhere but basement partition walls. The windows are original and single pane - they radiate heat from the outside through the glass but they were just caulked/sealed last year during an energy audit so they are not drafty.

Yes we do use exhaust fans, although the kitchen is just a micro/hood combo.. Not the best, but it does help.

 

Generally, since the system is dropping the air temp down to 55F, it must be dehumidifying well.

Once 55 degree air at 100% rh warms up to 77F, the humidity drops down to 46% which is very reasonable. The indoor coil of an a/c typically runs at 35-45F (depending on the cooling load and efficiency of the system), so the dewpoint of the supply air could be below 55.

Provided that the a/c runs a lot during the day (4 tons for an older 3000 sq ft house doesn't sound out of line), it sounds like an excessive amount of moisture is getting into the house.

Try supplementing the system with a dehumidifier.