RH, or relative humidity, is just that. Relative to the air temp. If you raise the temp and the amount of moisture in the air remains exactly the same the RH will read lower at 80F than it did at 70F because the warmer air can hold more moisture so it is relatively drier. There is still the same amount of water in the air but it's relatively less at the higher temp. Relatively being the key word here.
How many times have you read about someone plunking an AC unit in the grow room then getting warmer temps that they started with? The AC will take heat from the air that passes through the cooling coils but if that heat isn't exhausted from the room all that happens is the RH goes down but no heat is actually removed from the room. Then you have the added heat produced by the refrigeration motor so you end up with a net increase in temp instead of the loss you were after.
Ever notice how warm the air is coming off the back of your fridge? That is from heat being removed from inside the fridge and radiated from the cooling coils on the back along with heat generated by the motor. A/C units, dehumidifiers, fridges and freezers all use the same engines and cooling systems to operate and all have the same heat issues. Keep those coils clean to work more efficiently and not only extend the life of the motor but save you electricity too.
Because an A/C is designed to cool a space and not to dehumidify, tho it does that too, it brings warm air through it's cooling coils and exhausts the heat removed out the back to the great outdoors. Along with the water that drips off the cooling coils and then drips outside too like under your car when the A/C is running. So you can see how putting a window or a portable A/C unit in the grow room without a way to direct it's exhaust outdoors is not going to work.
I've never owned a dehumid so I'm not sure if they are built to have an exhaust port that can be hooked up to direct the heat out of the room or not but you would have to do that if keeping the temps down is something else you want other than just reducing the RH. Then it will also cool the room just like the A/C does tho not as effectively.
I have the other problem with about 9 months of my year being too low RH and too cool temps. Cheap heaters and humidifiers from thrift stores are what I'm always keeping an eye out for.
