Hope this solves your problem, look for the humidistat, it should be located on cold air return duct near furnace, make sure it is not set in the on position, if it is, then this is why it is staying on, please set humidistat any where from 20 to 35%, never over 40%. If the humidistat is set properly and is still running, you may have a electrical problem, humidifier circuit should only receive power(24 volts) when the indoor blower motor of furnace receives power, also if you have galvanized plumbing in your home, you may have a piece of rust or lime deposit in the humidifier solenoid valve preventing it from closing, turn water off to humidifier and disconnect water supply line entering solenoid valve inspect & clean protector screen. Good luck.
Answers & Comments
Hope this solves your problem, look for the humidistat, it should be located on cold air return duct near furnace, make sure it is not set in the on position, if it is, then this is why it is staying on, please set humidistat any where from 20 to 35%, never over 40%. If the humidistat is set properly and is still running, you may have a electrical problem, humidifier circuit should only receive power(24 volts) when the indoor blower motor of furnace receives power, also if you have galvanized plumbing in your home, you may have a piece of rust or lime deposit in the humidifier solenoid valve preventing it from closing, turn water off to humidifier and disconnect water supply line entering solenoid valve inspect & clean protector screen. Good luck.