I have recently purchased a new portable air conditioning unit. For some reason when I am using it the RH increases rapidly. The exhaust pipe goes through the wall and appears to have a good seal. The BTU’s seem to be suitable for the room size. Does anyone have any suggestions on what the issue could be?
The amount of water vapor that air can hold depends on the termperature. Warm air can hold more moisture than cold. The relative humidity is the amount of moisture in the air compared with the maximum that it could hold at that temperature.
If you rapidly cool the air, then the relative humidity will go up, even if the total amount of moisture stays the same.
Air conditioners condense moisture from the air. In a portable unit, there is probably a tank that needs to be emptied from time to time. But it may take some time for enough water to condense out to make up for the temperature drop.
If the A/C unit is cooling the room but not running long enough to remove the humidity, then YES, the % of relative humidity will rise. If by chance the A/C unit is too large for the space it is cooling then it will not run long enough to reduce the relative humidity.
Also, those type units exhaust air from the room as it is running and unconditioned air has to be migrating to that room to make up for the air that is being expelled. A better choice would be a portable unit that has 2 pipes; 1 to bring in the required air for the unit to operate properly and one to exhaust the warmed air that is produced by the units operation. I call the units with 1 hose a 1/2 A/C unit since they only satisfy 1/2 of the air requirements for proper operation. For a more comfortable room you need to install the correct unit.
To better understand humidity, you need to get a book or search online and learn about the relationship between % of relative humidity and air temperatures. A quick learn is that as air temperatures fall the % of relative humidity rises and visa-versa.