BillieBricks:
The problem with the temp/humid control is that there's no hysteria, so if the temp/humid swings around the setpoint the relays will turn on/off quite fast and then there's a chance that they burn out...
BillieBrick
I had this same problem in designing my aquarium/aquaponics monitor except in my case the issue was with my float switches which controlled my pumps via relay. With the water moving it would cause waves in the tank causing my float switch to go up and down turning on and off my relay constantly. The medicine to this problem is in the blink without delay example code http://arduino.cc/en/Tutorial/BlinkWithoutDelay. Basically what I did was made to variables for each pump one that records the last time the float was high and then one that records the current time the float was high and then previous - current > 5 sec turn on pump. I also included an if else that say if float is low reset previous. If you have any question or want some example i can dig it up and help you out just pm me.