Pressurized CO2 is naturally very cold and can cause the solenoid to freeze if the flow rate is over 10 CFH (Cubic Feet per Hour). Most modern CO2 controllers will use some type of fuzzy logic to learn how to add just the right amount of CO2 no matter what the flow rate is on the regulator, so a slower flow rate will still supply the garden with plenty of CO2. A setting of 5 or less will ensure that the the cold CO2 flows slow enough through the solenoid to avoid making it stick.