Humidity / Temperature Control

We are looking to develop & Install an environment controlling system. A suitable system may consist of the following 3 sub-units.

1) HUMIDIFYING UNIT, to add moisure during the winter (dry season).
2) DE-HUMIDIFER UNIT, to remove excess moisture from air during the rainy season.
3) AIR COOLING / AIR CONDITIONING UNIT, to maintain the Temperature.
4*) It would be worthwhile to inlcude the Air cleaning filter at this stage.

I am interested to know what kind of issues we could be facing, assistance in the selection of equipment.

and ofcourse the difficult part, inter-linking of the DE-Humidifier and Huidifier units, so that both of these dont turn on at the same time, it gets more tricky if more INPUTS (sensors) are used.

Comments and directions (links) are welcome.

There are lots of RH sensors on the market with analog outputs, tie this into a small PLC. I would guess that if the corret sensor is selected, you could also get the temperature from the same unit, (can't help with the selection, I've never used one). Use compares and have a small deadband.
Example: Desired RH=60
When RH=61 turn on dehumidifier until RH=60
When Rh=59 turn on humidifier until RH=60

This will give you an effective deadband of +/- 1, you can decrease the deadband if desired.
You could do something similar with the temperature, many of the programmable heat controls for the home allow deadband adjustment, you may or may not want to include this feature.
If you select a plc with a real time clock, you could also include features to change the setpoint based on time of day, or day of week. This could be used to conserve energy when the space is unoccupied, such as nights and / or weekends.

I suggest a small user interface also, this would allow setpoint changes, filter status, etc...

Ken