These controllers make it easy to coordinate your exhaust and CO2 production. The 2 Timer CO2 Controller (CO2-2e) has 2 timers that allow for efficient control of CO2 levels, one controls how often CO2 is released and the other controls how long it is released. It also features temperature and humidity controls that will stop CO2 production and activate the exhaust outlet if the levels that you set are exceeded. A photosensor disables CO2 production while the lights are off. The 4 Timer CO2 Controller (CO2-4e) has the same features as the CO2-2e, but includes 2 extra timers. The CO2-4e features an Overall Cycle Timer that controls the maximum time between exhaust cycles. It also has an Exhaust Cycle Timer to control how long the exhaust fans will run. Both units are expandable to PPM?level CO2 control with the addition of the PPM-1c CO2 monitor/controller. They are both rated for 15 amps @ 120 volt. 5 year no hassle warranty.